Merge pull request #51 from Moballo-LLC/codex/coap-initial-pacing

fix(protocol): pace CoAP request sends

README.md

@@ -48,6 +48,35 @@ sess.subscribe(["operational", "state", "vs", "0"],          # OBSERVE
 sess.close()
 ```
 
+`connect()` uses a 12-second monotonic DTLS handshake deadline by default. A
+caller that needs a shorter bounded attempt can pass a positive finite value
+without changing later reader timeouts. OpenSSL's DTLS timer schedules flight
+retransmissions within that same deadline:
+
+```python
+sess.connect(timeout=4.0)
+```
+
+The deadline stops further setup, retries, and network waits. If OpenSSL
+reports that the handshake completed at the deadline boundary, the completed
+session is retained rather than torn down as a timeout.
+
+Connection attempts can also use a one-way cancellation signal. The signal is
+backed by a socketpair, so setting it wakes the network wait immediately while
+OpenSSL retains control of DTLS retransmission timing:
+
+```python
+from smartthings_local.protocol.dtls_session import ConnectCancellation
+
+cancel_connect = ConnectCancellation()
+# Another thread may call cancel_connect.set().
+sess.connect(timeout=8.0, cancel=cancel_connect)
+```
+
+Setting the signal stops subscribed connection attempts and closes their
+temporary UDP sockets. It does not alter an already established session or add
+new session lifecycle methods. Interrupted attempts raise `SessionClosedError`.
+
 If the cert/key are minted at runtime and never written to disk (e.g. inside
 an HA config flow), create the provider from memory instead:
 
@@ -56,11 +85,122 @@ auth = CertificateAuth.from_memory(cert_pem, key_pem)
 sess = DtlsCoapSession("192.0.2.100", 49154, auth=auth)
 ```
 
+Some newer OCF-PKI devices require an exact Samsung DTLS offer and present a
+hardware certificate whose subject contains a certificate UUID. That UUID can
+be distinct from the runtime OCF device UUID reported by `/oic/d`, so callers
+must obtain and verify the certificate identity independently. When the caller
+already has an authorized client certificate and a previously verified
+hardware-certificate UUID, opt in to both requirements explicitly:
+
+```python
+from smartthings_local.protocol.auth import (
+    CertificateAuth,
+    SamsungServerProfile,
+)
+
+server_profile = SamsungServerProfile.bound_device(
+    expected_certificate_uuid,
+    additional_ca_pem=additional_samsung_ca_pem,
+)
+auth = CertificateAuth.from_memory(
+    cert_pem,
+    key_pem,
+    server_profile=server_profile,
+)
+sess = DtlsCoapSession("192.0.2.100", 49154, auth=auth)
+```
+
+The default profile is restricted to Samsung home-appliance leaves with
+`OU=OCF HA Device`. The profile limits the ClientHello to P-256,
+`ECDHE-ECDSA-AES128-GCM-SHA256`, and the observed SHA-256/SHA-1 RSA/ECDSA
+signature set, disables session tickets, preserves certificate-chain
+verification, and requires the exact subject role
+`C=KR, O=Samsung Electronics, OU=OCF HA Device` with a common name ending in
+the expected certificate UUID. `additional_ca_pem` is optional and accepts
+only a bounded PEM CA-certificate chain; it is applied only to this profiled
+context. Without a profile, `CertificateAuth` retains its existing verification
+behavior.
+
+Samsung VD-family devices can present the same wire profile with the distinct
+`OU=OCF VD Device` role. Select that role explicitly; profiles never fall back
+between device classes:
+
+```python
+from smartthings_local.protocol.auth import (
+    SamsungServerProfile,
+    SamsungServerRole,
+    ServerCertificateAuth,
+)
+
+server_profile = SamsungServerProfile.bound_device(
+    expected_certificate_uuid,
+    role=SamsungServerRole.VD_DEVICE,
+)
+auth = ServerCertificateAuth(server_profile=server_profile)
+sess = DtlsCoapSession("192.0.2.100", 5684, auth=auth)
+```
+
+`ServerCertificateAuth` is for a server-authenticated channel that does not
+send a client certificate, such as the initial DTLS carrier used by
+manufacturer-certificate OTM. It still verifies the CA chain, exact selected
+subject role, and pinned certificate UUID. It does not learn an identity from
+the first endpoint it reaches and cannot be combined with client credentials.
+
+This API deliberately does not discover, mint, authorize, provision, rotate,
+or persist credentials, and it performs no ownership transfer or OCF security
+resource writes. In particular, the server-only provider can authenticate the
+initial manufacturer-certificate channel, but it does not implement the OTM
+that follows. The already-owned new-PKI case in
+[issue #16](https://github.com/QuiteYellow/SmartThings-Local/issues/16) still
+requires an authorized client identity before ordinary protected resources
+can be used.
+
 For compatibility, the existing `cert_path` / `key_path` and `cert_pem` /
 `key_pem` session arguments remain supported without a deprecation warning.
 They are routed through `CertificateAuth` internally. Do not combine `auth`
 with those legacy arguments.
 
+An existing OCF PSK credential can be supplied through `PskAuth`:
+
+```python
+from smartthings_local.protocol.auth import PskAuth
+
+auth = PskAuth(identity=psk_identity, key=psk_key)
+sess = DtlsCoapSession("192.0.2.100", 49154, auth=auth)
+```
+
+The identity must be the raw 16-byte OCF UUID and cannot contain a NUL byte;
+the key must be exactly 16 or 32 bytes. `PskAuth` selects only
+`ECDHE-PSK-AES128-CBC-SHA256` and does not acquire, derive, provision, rotate,
+or persist credentials. Ownership transfer and credential discovery are
+outside this package.
+
+Code that has already completed an authenticated manufacturer-certificate
+session can derive IoTivity's 128-bit OwnerPSK from the resulting TLS state:
+
+```python
+from smartthings_local.protocol.owner_psk import derive_mfg_certificate_owner_psk
+
+owner_psk = derive_mfg_certificate_owner_psk(
+    master_secret=master_secret,
+    client_random=client_random,
+    server_random=server_random,
+    owner_uuid=owner_uuid,
+    device_uuid=device_uuid,
+    cipher_name=cipher_name,
+    oxm_label=selected_oxm_label,
+)
+```
+
+The caller must supply the exact authenticated TLS values, non-nil raw OCF
+UUIDs, negotiated cipher name, and label for the selected OXM. Use
+`STANDARD_MFG_CERTIFICATE_OXM_LABEL` for `oic.sec.doxm.mfgcert` and
+`CONFIRMED_MFG_CERTIFICATE_OXM_LABEL` for
+`x.org.iotivity.conmfgcert`; do not infer the label from the appliance model.
+The helper performs deterministic key derivation only: it does not access a
+session, discover credentials, choose an ownership method, write security
+resources, run OTM, or persist the result.
+
 ### Classified errors
 
 Runtime transport failures use the public types in
@@ -140,7 +280,7 @@ For a full worked integration, the higher-level `smartthings_local.ocf` layer (`
 
 Each appliance runs an independent bridge built around three coordinated pieces over one persistent DTLS session: a `StateCache` (single source of truth for all reps), a `PollScheduler` (tiered adaptive polling: hot/warm/cold plus a periodic `/device/0` sweep), and a `KeepaliveTask` (CoAP empty-CON ping for DTLS-layer liveness, with consecutive-failure detection for MQTT availability). Tier cadences are descriptor-declared and were calibrated against the empirically-measured per-firmware ceilings: dryer ~14 req/s, oven ~8 req/s. OBSERVE registrations (RFC 7641) are kept as an opportunistic freshness accelerator: when the appliance has internet and emits notifications, the cache absorbs them and the next-poll timer is reset for that resource; when it's air-gapped, polling alone carries the UX with no other code change. Token-stable Block2 (RFC 7959) handles multi-block reads. Writes are optimistically merged into the cache the moment the device 2.04-confirms, with the scheduler deferring that resource's next poll past the fetchback-revert window. Reconnect with exponential backoff on session errors, gated by a stateless DTLS ClientHello pre-flight (`smartthings_local/protocol/dtls_probe.py`) so a silent/rebooting device or wrong port drops into backoff in ~1 RTT instead of eating the full handshake timeout; when `OCF_PORT` is unset the same probe auto-discovers the live port across the OCF band.
 
-On the currently supported firmware families, authentication uses a client cert keyed to the UUID published in Samsung's own wildcard cloud TLS cert. Their factory ACL grants that UUID `perm=31` (full CRUDN) on `href=*`. That certificate path is not universal: the WD53 profile in issue #16 and the washer in issue #20 reject it and need separate authentication work.
+On the currently supported firmware families, authentication uses a client cert keyed to the UUID published in Samsung's own wildcard cloud TLS cert. Their factory ACL grants that UUID `perm=31` (full CRUDN) on `href=*`. That certificate path is not universal: the WD53 profile in issue #16 and the washer in issue #20 reject it. For those newer OCF-PKI devices, the `SamsungServerProfile` and `ServerCertificateAuth` providers (see Quick start) pin and verify the device's hardware certificate, but getting an authorized client credential to reach protected resources is still an open problem.
 
 ---
 
@@ -155,7 +295,7 @@ nmap -Pn -sU -p 5683,5684,49152-49160 "$APPLIANCE_IP"
 
 Read the result:
 
-- **`5684/udp` or a 4915x port with a DTLS first-flight response** → an OCF DTLS listener. Standard-port OCF-PKI firmware may still require an unsupported authentication profile.
+- **`5684/udp` or a 4915x port with a DTLS first-flight response** → an OCF DTLS listener. Standard-port OCF-PKI firmware needs the Samsung server-certificate profile (`SamsungServerProfile` / `ServerCertificateAuth`, see Quick start), and no working client credential for it exists yet.
 - **`5683/udp` responds to public OCF security/resource GETs** → use `/oic/res` to learn the device's advertised secure endpoint; do not assume that endpoint is fixed.
 - **Only `8888/tcp` open (token-based HTTPS)** → older firmware (~2018–2022). **Not supported here.**
 
@@ -494,6 +634,8 @@ smartthings_local/                   The installable library — `pip install sm
     coap.py                          CoAP wire protocol: message encode/decode, token handling
     dtls_session.py                  DTLS session: handshake, client-cert auth (file or in-memory PEM), Block2, liveness
     dtls_probe.py                    Stateless DTLS liveness + opt-in stateful diagnostic
+    dtls_handshake.py                Shared memory-BIO handshake driver, bounded by a monotonic deadline (used by session + probe)
+    owner_psk.py                     Pure manufacturer-certificate OwnerPSK derivation
     ocf_root_ca.pem                  Samsung OCF root CA, bundled for handshake verification
   ocf/                               OCF resource + state layer (reusable)
     __init__.py
@@ -520,7 +662,7 @@ mqtt_demo/                           MQTT bridge demo (consumes smartthings_loca
   .env.example                       Template — copy to .env, fill in
 setup_cert.py                        One-shot cert minting script (live-fetches AC14K_M + UUID)
 pyproject.toml                       Packaging — PyPI dist `smartthings-local`, hatch-vcs versioning
-tests/                               pytest suite (CoAP wire, state cache, import isolation, cert loading, DTLS probe, bridge port resolution, cert signing)
+tests/                               pytest suite (CoAP wire, state cache, import isolation, cert loading, DTLS probe, bridge port resolution, cert signing, certificate profiles, OwnerPSK derivation, connect deadline, session interruption)
 .github/workflows/publish.yml        Build + PyPI Trusted Publishing on `v*` tags
 ```
 

mqtt_demo/__main__.py

@@ -166,7 +166,7 @@ def main():
         stopping.set()
         logger.info("shutting down…")
         for b in bridges:
-            b.stop.set()
+            b.request_stop()
             try: b.set_availability(False)
             except Exception: pass
 

mqtt_demo/bridge.py

@@ -87,6 +87,7 @@ OCF_STANDARD_SECURE_PORT = 5684
 # fixed-source-port reconnect invariant is untouched (see session_once).
 _GATE_RETRIES = 1
 _GATE_TIMEOUT_S = 4.0
+_WORKER_JOIN_TIMEOUT_S = 2.0
 
 
 class PushBridge:
@@ -123,6 +124,8 @@ class PushBridge:
         self.last_cycle_pub = None
         self.last_avail_pub: str | None = None
         self.stop = threading.Event()
+        self._session_stop_lock = threading.Lock()
+        self._session_stop: threading.Event | None = None
         self.started_ts = time.time()
         self.session_started_ts = None
         self.last_change_ts = None
@@ -175,6 +178,13 @@ class PushBridge:
                 app.topic_prefix, shared.HA_DISCOVERY_PREFIX, app.device_name,
                 model=descriptor.name.title()))
 
+    def request_stop(self) -> None:
+        """Stop the bridge and wake workers belonging to its current session."""
+        self.stop.set()
+        with self._session_stop_lock:
+            if self._session_stop is not None:
+                self._session_stop.set()
+
     # ---- cache plumbing ---------------------------------------------
 
     def _on_cache_change(self, changed: bool, source: str) -> None:
@@ -423,25 +433,54 @@ class PushBridge:
         self.keepalive = keepalive
         self.observe_refresh = observe_refresh
 
+        # These workers belong to this DTLS session, not to the bridge
+        # process. A reconnect must retire them before the replacement
+        # session starts or they continue operating on the closed session.
+        session_stop = threading.Event()
+        with self._session_stop_lock:
+            self._session_stop = session_stop
+            # ``request_stop()`` sets the bridge event before taking this
+            # lock. Checking it while publishing the handle prevents a lost
+            # wakeup if shutdown races this session handoff.
+            if self.stop.is_set():
+                session_stop.set()
+
         sched_t = threading.Thread(
-            target=scheduler.run_forever, args=(self.stop,),
+            target=scheduler.run_forever, args=(session_stop,),
             daemon=True, name=f'{self.app.klass}-poll')
         ka_t = threading.Thread(
-            target=keepalive.run_forever, args=(self.stop,),
+            target=keepalive.run_forever, args=(session_stop,),
             daemon=True, name=f'{self.app.klass}-ping')
         ref_t = threading.Thread(
-            target=observe_refresh.run_forever, args=(self.stop,),
+            target=observe_refresh.run_forever, args=(session_stop,),
             daemon=True, name=f'{self.app.klass}-obsref')
-        sched_t.start()
-        ka_t.start()
-        ref_t.start()
+        workers = (sched_t, ka_t, ref_t)
+        started_workers = []
 
         try:
+            for worker in workers:
+                worker.start()
+                started_workers.append(worker)
             sess.join()
         finally:
+            # A worker already inside a tick can finish after the reader
+            # exits. Disable old-session reachability callbacks first so it
+            # cannot change availability after a replacement takes over.
+            keepalive.on_reachable = None
+            keepalive.on_unreachable = None
+            session_stop.set()
+            join_deadline = time.monotonic() + _WORKER_JOIN_TIMEOUT_S
+            for worker in started_workers:
+                worker.join(max(0.0, join_deadline - time.monotonic()))
+                if worker.is_alive():
+                    self.log.warning(
+                        "session worker did not stop: %s", worker.name)
             self.scheduler = None
             self.keepalive = None
             self.observe_refresh = None
+            with self._session_stop_lock:
+                if self._session_stop is session_stop:
+                    self._session_stop = None
 
     def _seed_from_device0(self, sess):
         code, pl = sess.get(self.descriptor.seed_path, timeout=15.0)

mqtt_demo/samples/oven.py

@@ -166,6 +166,14 @@ def flatten(links):
     if temps_items:
         cur_c = _int(temps_items[0].get('x.com.samsung.da.current'))
         des_c = _int(temps_items[0].get('x.com.samsung.da.desired'))
+    # With no cycle set the oven reports desired=0. That means "no
+    # setpoint", not a 0 °C target, and HA rejects it against the Number
+    # entity's 30-270 range on every publish. Anything outside the
+    # settable band is absent, not a value: null lands as unknown on both
+    # the Number and the Setpoint sensor, the way completion_minutes
+    # already reads when idle. _setpoint applies the same bounds on write.
+    if des_c is not None and not (SETPOINT_MIN_C <= des_c <= SETPOINT_MAX_C):
+        des_c = None
 
     # Door
     doors_items = g('/doors/vs/0', 'x.com.samsung.da.items') or []

smartthings_local/protocol/auth.py

@@ -2,15 +2,37 @@
 
 from __future__ import annotations
 
+import logging
 import re
+import warnings
+from enum import Enum
 from os import PathLike
 from pathlib import Path
 from typing import Protocol, runtime_checkable
+from uuid import UUID
 
-from OpenSSL import SSL, crypto
+from cryptography.x509.oid import ExtensionOID
+from OpenSSL import SSL, _util, crypto
+
+logger = logging.getLogger(__name__)
 
 _OCF_ROOT_CA = str(Path(__file__).with_name("ocf_root_ca.pem"))
 _DTLS_CIPHERS = b"ECDHE-ECDSA-AES128-GCM-SHA256:@SECLEVEL=0"
+_DTLS_PSK_CIPHERS = b"ECDHE-PSK-AES128-CBC-SHA256:@SECLEVEL=0"
+_SAMSUNG_SERVER_CURVES = b"prime256v1"
+_SAMSUNG_SERVER_SIGNATURE_ALGORITHMS = (
+    b"RSA+SHA256:ECDSA+SHA256:RSA+SHA1:ECDSA+SHA1"
+)
+_SAMSUNG_SERVER_CN_RE = re.compile(
+    r"\AOCF Device: [^()\r\n]{1,96} "
+    r"\((?P<device_identity>[0-9a-f]{8}-[0-9a-f]{4}-"
+    r"[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})\)\Z",
+    re.IGNORECASE,
+)
+_PSK_CLIENT_CALLBACK_CDEF = (
+    "unsigned int (*)(SSL *, char *, char *, unsigned int, "
+    "unsigned char *, unsigned int)"
+)
 _PEM_CERT_RE = re.compile(
     rb"-----BEGIN CERTIFICATE-----.*?-----END CERTIFICATE-----",
     re.DOTALL,
@@ -45,7 +67,300 @@ class AuthenticationProvider(Protocol):
     """Configure authentication for a newly created DTLS context."""
 
     def configure_context(self, context: SSL.Context) -> None:
-        """Configure trust, verification, ciphers, and client credentials."""
+        """Configure a context while this provider remains session-owned."""
+
+
+class SamsungServerRole(Enum):
+    """Known Samsung OCF hardware-certificate subject roles."""
+
+    HOME_APPLIANCE = "OCF HA Device"
+    VD_DEVICE = "OCF VD Device"
+
+
+class SamsungServerProfile:
+    """Opt-in Samsung hardware-certificate verification profile."""
+
+    __slots__ = (
+        "_additional_ca_certificates",
+        "_expected_certificate_identity",
+        "_role",
+    )
+
+    def __init__(
+        self,
+        *,
+        expected_certificate_identity: UUID | str,
+        role: SamsungServerRole = SamsungServerRole.HOME_APPLIANCE,
+        additional_ca_pem: str | None = None,
+    ) -> None:
+        if type(expected_certificate_identity) is UUID:
+            parsed_identity = expected_certificate_identity
+        elif type(expected_certificate_identity) is str:
+            try:
+                parsed_identity = UUID(expected_certificate_identity)
+            except ValueError:
+                raise ValueError(
+                    "expected_certificate_identity must be a canonical "
+                    "non-zero UUID"
+                ) from None
+            if expected_certificate_identity != str(parsed_identity):
+                raise ValueError(
+                    "expected_certificate_identity must be a canonical "
+                    "non-zero UUID"
+                )
+        else:
+            raise TypeError(
+                "expected_certificate_identity must be a UUID or string"
+            )
+        if parsed_identity.int == 0:
+            raise ValueError(
+                "expected_certificate_identity must be a canonical "
+                "non-zero UUID"
+            )
+        if type(role) is not SamsungServerRole:
+            raise TypeError("role must be a SamsungServerRole")
+
+        certificates: tuple[bytes, ...] = ()
+        if additional_ca_pem is not None:
+            if type(additional_ca_pem) is not str:
+                raise TypeError("additional_ca_pem must be a string")
+            try:
+                raw_ca_pem = additional_ca_pem.encode("ascii")
+            except UnicodeEncodeError:
+                raise ValueError(
+                    "additional_ca_pem must contain ASCII PEM certificates"
+                ) from None
+            parsed_certificates = tuple(_PEM_CERT_RE.findall(raw_ca_pem))
+            if (
+                not 1 <= len(parsed_certificates) <= 4
+                or len(raw_ca_pem) > 32 * 1024
+                or _PEM_CERT_RE.sub(b"", raw_ca_pem).strip()
+            ):
+                raise ValueError(
+                    "additional_ca_pem must contain one to four PEM certificates"
+                )
+            try:
+                loaded_certificates = [
+                    crypto.load_certificate(crypto.FILETYPE_PEM, certificate)
+                    for certificate in parsed_certificates
+                ]
+                basic_constraints = [
+                    [
+                        extension
+                        for extension in certificate.to_cryptography().extensions
+                        if extension.oid == ExtensionOID.BASIC_CONSTRAINTS
+                    ]
+                    for certificate in loaded_certificates
+                ]
+            except (crypto.Error, ValueError):
+                raise ValueError(
+                    "additional_ca_pem contains an invalid certificate"
+                ) from None
+            if any(
+                len(constraints) != 1 or not constraints[0].value.ca
+                for constraints in basic_constraints
+            ):
+                raise ValueError(
+                    "additional_ca_pem must contain only CA certificates"
+                )
+            fingerprints = {
+                crypto.dump_certificate(crypto.FILETYPE_ASN1, certificate)
+                for certificate in loaded_certificates
+            }
+            if len(fingerprints) != len(loaded_certificates):
+                raise ValueError(
+                    "additional_ca_pem must not contain duplicate certificates"
+                )
+            certificates = parsed_certificates
+
+        object.__setattr__(
+            self,
+            "_expected_certificate_identity",
+            parsed_identity,
+        )
+        object.__setattr__(self, "_role", role)
+        object.__setattr__(self, "_additional_ca_certificates", certificates)
+
+    def __setattr__(self, _name: str, _value: object) -> None:
+        raise AttributeError("SamsungServerProfile is immutable")
+
+    def __delattr__(self, _name: str) -> None:
+        raise AttributeError("SamsungServerProfile is immutable")
+
+    @classmethod
+    def bound_device(
+        cls,
+        expected_certificate_identity: UUID | str,
+        *,
+        role: SamsungServerRole = SamsungServerRole.HOME_APPLIANCE,
+        additional_ca_pem: str | None = None,
+    ) -> SamsungServerProfile:
+        """Bind a verified Samsung hardware leaf to its certificate UUID."""
+        return cls(
+            expected_certificate_identity=expected_certificate_identity,
+            role=role,
+            additional_ca_pem=additional_ca_pem,
+        )
+
+    def __repr__(self) -> str:
+        """Return a representation without device or trust-chain details."""
+        return "SamsungServerProfile()"
+
+    def _configure_context(self, context: SSL.Context) -> None:
+        curve_setter = getattr(_util.lib, "SSL_CTX_set1_curves_list", None)
+        if curve_setter is not None:
+            if curve_setter(context._context, _SAMSUNG_SERVER_CURVES) != 1:
+                raise RuntimeError(
+                    "OpenSSL rejected the Samsung server certificate profile"
+                )
+        else:
+            # pyOpenSSL 23.1 does not expose SSL_CTX_set1_curves_list. Its
+            # public set_tmp_ecdh fallback produces the same single P-256
+            # supported-groups ClientHello extension; wire-level tests protect
+            # that compatibility path. Newer pyOpenSSL uses the exact setter.
+            try:
+                with warnings.catch_warnings():
+                    warnings.simplefilter("ignore", DeprecationWarning)
+                    curve = crypto.get_elliptic_curve("prime256v1")
+                    context.set_tmp_ecdh(curve)
+            except (AttributeError, TypeError, ValueError, SSL.Error):
+                raise RuntimeError(
+                    "OpenSSL rejected the Samsung server certificate profile"
+                ) from None
+
+        signature_setter = getattr(
+            _util.lib,
+            "SSL_CTX_set1_sigalgs_list",
+            None,
+        )
+        if (
+            signature_setter is None
+            or signature_setter(
+                context._context,
+                _SAMSUNG_SERVER_SIGNATURE_ALGORITHMS,
+            )
+            != 1
+        ):
+            raise RuntimeError(
+                "OpenSSL rejected the Samsung server certificate profile"
+            )
+        context.set_options(SSL.OP_NO_TICKET)
+
+        if self._additional_ca_certificates:
+            store = context.get_cert_store()
+            try:
+                for certificate in self._additional_ca_certificates:
+                    store.add_cert(
+                        crypto.load_certificate(
+                            crypto.FILETYPE_PEM,
+                            certificate,
+                        )
+                    )
+            except crypto.Error:
+                raise RuntimeError(
+                    "OpenSSL rejected the Samsung server trust profile"
+                ) from None
+
+    def _verify_peer(
+        self,
+        _connection,
+        certificate,
+        _error,
+        depth,
+        ok,
+    ) -> bool:
+        if not ok or certificate is None or depth < 0:
+            return False
+        if depth > 0:
+            return True
+        try:
+            with warnings.catch_warnings():
+                # pyOpenSSL deprecates this API in favor of cryptography, but
+                # reparsing Samsung's non-DER factory leaves with cryptography
+                # rejects certificates that OpenSSL has already verified.
+                warnings.simplefilter("ignore", DeprecationWarning)
+                components = [
+                    (name.decode("ascii"), value.decode("ascii"))
+                    for name, value in certificate.get_subject().get_components()
+                ]
+        except (
+            AttributeError,
+            TypeError,
+            UnicodeDecodeError,
+            ValueError,
+            crypto.Error,
+        ):
+            logger.warning("Unable to parse Samsung server certificate subject")
+            return False
+
+        common_names = [value for name, value in components if name == "CN"]
+        organizational_units = [
+            value for name, value in components if name == "OU"
+        ]
+        organizations = [value for name, value in components if name == "O"]
+        countries = [value for name, value in components if name == "C"]
+        # This deliberately pins the complete Samsung subject role. The OCF
+        # reference implementation reads only the UUID-bearing CN, but
+        # relaxing C/O/OU here could accept a different certificate cohort.
+        if (
+            len(common_names) != 1
+            or organizational_units != [self._role.value]
+            or organizations != ["Samsung Electronics"]
+            or countries != ["KR"]
+        ):
+            return False
+        match = _SAMSUNG_SERVER_CN_RE.fullmatch(common_names[0])
+        return (
+            match is not None
+            and UUID(match.group("device_identity"))
+            == self._expected_certificate_identity
+        )
+
+
+def _configure_certificate_server(
+    context: SSL.Context,
+    server_profile: SamsungServerProfile | None,
+) -> None:
+    """Configure certificate-server verification for one DTLS context."""
+    context.load_verify_locations(_OCF_ROOT_CA)
+    if server_profile is None:
+        context.set_verify(SSL.VERIFY_PEER, _verify_peer)
+    else:
+        server_profile._configure_context(context)
+        context.set_verify(
+            SSL.VERIFY_PEER,
+            server_profile._verify_peer,
+        )
+    # @SECLEVEL=0 permits SHA-1 in Samsung's server cert chain (AC14K_M
+    # intermediate is SHA-1 signed). This is the only channel that reaches
+    # the OpenSSL instance cryptography bundles; ctypes and cffi bindings
+    # do not expose SSL_CTX_set_security_level on this build.
+    context.set_cipher_list(_DTLS_CIPHERS)
+
+
+class ServerCertificateAuth:
+    """Verify a pinned Samsung server without a client certificate."""
+
+    __slots__ = ("_server_profile",)
+
+    def __init__(self, *, server_profile: SamsungServerProfile) -> None:
+        if type(server_profile) is not SamsungServerProfile:
+            raise TypeError("server_profile must be a SamsungServerProfile")
+        object.__setattr__(self, "_server_profile", server_profile)
+
+    def __setattr__(self, _name: str, _value: object) -> None:
+        raise AttributeError("ServerCertificateAuth is immutable")
+
+    def __delattr__(self, _name: str) -> None:
+        raise AttributeError("ServerCertificateAuth is immutable")
+
+    def __repr__(self) -> str:
+        """Return a representation without server identity or trust details."""
+        return "ServerCertificateAuth()"
+
+    def configure_context(self, context: SSL.Context) -> None:
+        """Verify the selected server profile without loading client material."""
+        _configure_certificate_server(context, self._server_profile)
 
 
 class CertificateAuth:
@@ -60,6 +375,7 @@ class CertificateAuth:
         "_certificate_pem",
         "_private_key_path",
         "_private_key_pem",
+        "_server_profile",
     )
 
     def __init__(
@@ -69,6 +385,7 @@ class CertificateAuth:
         private_key_path: str | PathLike[str] | None = None,
         certificate_pem: str | None = None,
         private_key_pem: str | None = None,
+        server_profile: SamsungServerProfile | None = None,
     ) -> None:
         file_supplied = (
             certificate_path is not None or private_key_path is not None
@@ -96,6 +413,11 @@ class CertificateAuth:
                 "must pass either certificate_path/private_key_path or "
                 "certificate_pem/private_key_pem"
             )
+        if (
+            server_profile is not None
+            and type(server_profile) is not SamsungServerProfile
+        ):
+            raise TypeError("server_profile must be a SamsungServerProfile")
         object.__setattr__(
             self,
             "_certificate_path",
@@ -108,6 +430,7 @@ class CertificateAuth:
         )
         object.__setattr__(self, "_certificate_pem", certificate_pem)
         object.__setattr__(self, "_private_key_pem", private_key_pem)
+        object.__setattr__(self, "_server_profile", server_profile)
 
     def __setattr__(self, _name: str, _value: object) -> None:
         raise AttributeError("CertificateAuth is immutable")
@@ -120,11 +443,14 @@ class CertificateAuth:
         cls,
         certificate_path: str | PathLike[str],
         private_key_path: str | PathLike[str],
+        *,
+        server_profile: SamsungServerProfile | None = None,
     ) -> CertificateAuth:
         """Create a provider backed by certificate-chain and key files."""
         return cls(
             certificate_path=certificate_path,
             private_key_path=private_key_path,
+            server_profile=server_profile,
         )
 
     @classmethod
@@ -132,11 +458,14 @@ class CertificateAuth:
         cls,
         certificate_pem: str,
         private_key_pem: str,
+        *,
+        server_profile: SamsungServerProfile | None = None,
     ) -> CertificateAuth:
         """Create a provider backed by an in-memory PEM chain and key."""
         return cls(
             certificate_pem=certificate_pem,
             private_key_pem=private_key_pem,
+            server_profile=server_profile,
         )
 
     def __repr__(self) -> str:
@@ -145,13 +474,7 @@ class CertificateAuth:
 
     def configure_context(self, context: SSL.Context) -> None:
         """Apply the existing certificate authentication profile to a context."""
-        context.load_verify_locations(_OCF_ROOT_CA)
-        context.set_verify(SSL.VERIFY_PEER, _verify_peer)
-        # @SECLEVEL=0 permits SHA-1 in Samsung's server cert chain (AC14K_M
-        # intermediate is SHA-1 signed). This is the only channel that reaches
-        # the OpenSSL instance cryptography bundles; ctypes and cffi bindings
-        # do not expose SSL_CTX_set_security_level on this build.
-        context.set_cipher_list(_DTLS_CIPHERS)
+        _configure_certificate_server(context, self._server_profile)
         if self._certificate_pem is not None:
             _load_pem_chain(
                 context,
@@ -164,4 +487,85 @@ class CertificateAuth:
             context.check_privatekey()
 
 
-__all__ = ["AuthenticationProvider", "CertificateAuth"]
+class PskAuth:
+    """DTLS authentication using an existing OCF PSK credential.
+
+    The identity must be a raw 16-byte OCF UUID. The key must contain 16 or
+    32 bytes. Credential material is intentionally not exposed as public
+    attributes and is never included in this provider's representation. A
+    configured context must not outlive this provider; ``DtlsCoapSession``
+    enforces that lifetime by retaining its provider.
+    """
+
+    __slots__ = ("_callback",)
+
+    def __init__(self, *, identity: bytes, key: bytes) -> None:
+        if type(identity) is not bytes or type(key) is not bytes:
+            raise TypeError("identity and key must be bytes")
+        if len(identity) != 16:
+            raise ValueError("identity must be a raw 16-byte OCF UUID")
+        if b"\x00" in identity:
+            raise ValueError("identity cannot contain a NUL byte")
+        if len(key) not in (16, 32):
+            raise ValueError("key must be 16 or 32 bytes")
+
+        ffi = _util.ffi
+
+        @ffi.callback(_PSK_CLIENT_CALLBACK_CDEF)
+        def client_callback(
+            _ssl,
+            _identity_hint,
+            identity_buffer,
+            max_identity_length,
+            key_buffer,
+            max_key_length,
+        ):
+            # OpenSSL callbacks cannot propagate Python exceptions. Fail
+            # before touching either destination when a buffer is unavailable
+            # or too small for the complete credential.
+            if (
+                identity_buffer == ffi.NULL
+                or key_buffer == ffi.NULL
+                or len(identity) + 1 > max_identity_length
+                or len(key) > max_key_length
+            ):
+                return 0
+            ffi.memmove(
+                identity_buffer,
+                identity + b"\x00",
+                len(identity) + 1,
+            )
+            ffi.memmove(key_buffer, key, len(key))
+            return len(key)
+
+        object.__setattr__(self, "_callback", client_callback)
+
+    def __setattr__(self, _name: str, _value: object) -> None:
+        raise AttributeError("PskAuth is immutable")
+
+    def __delattr__(self, _name: str) -> None:
+        raise AttributeError("PskAuth is immutable")
+
+    def __repr__(self) -> str:
+        """Return a representation that never includes credential material."""
+        return "PskAuth()"
+
+    def configure_context(self, context: SSL.Context) -> None:
+        """Configure one context for the narrow Samsung OCF PSK profile."""
+        setter = getattr(_util.lib, "SSL_CTX_set_psk_client_callback", None)
+        if setter is None:
+            raise RuntimeError(
+                "the installed OpenSSL binding does not support DTLS PSK"
+            )
+        context.set_cipher_list(_DTLS_PSK_CIPHERS)
+        setter(context._context, self._callback)
+
+
+__all__ = [
+    "AuthenticationProvider",
+    "CertificateAuth",
+    "PskAuth",
+    "SamsungServerProfile",
+    "SamsungServerRole",
+    "ServerCertificateAuth",
+]

smartthings_local/protocol/coap.py

@@ -13,6 +13,7 @@ from ..errors import MalformedMessageError
 URI_PATH       = 11
 URI_QUERY      = 15
 OBSERVE        =  6
+ETAG           =  4
 CONTENT_FORMAT = 12
 ACCEPT         = 17
 BLOCK2         = 23
@@ -121,6 +122,14 @@ def block_value(num, more, szx):
     return struct.pack('>I', v)[1:]
 
 
+def block_fields(value):
+    """Decode a CoAP Block-N option value. Inverse of block_value().
+    Returns (num, more, szx). An empty value means block 0, no more,
+    SZX=0 — RFC 7959 §2.2 allows a zero-length option to elide it."""
+    v = int.from_bytes(value, 'big')
+    return v >> 4, (v >> 3) & 1, v & 0x07
+
+
 def fmt_code(c):
     """0x45 → '2.05', 0x84 → '4.04'. Used in log lines."""
     return f"{c >> 5}.{c & 0x1F:02d}"

smartthings_local/protocol/dtls_handshake.py

@@ -0,0 +1,99 @@
+"""Shared memory-BIO driver for bounded DTLS handshakes."""
+
+from __future__ import annotations
+
+import select
+import time
+from collections.abc import Callable
+
+from OpenSSL import SSL
+
+from .coap import split_dtls
+
+_HANDSHAKE_POLL_S = 0.5
+_MAX_DATAGRAM_SIZE = 65535
+
+
+class _HandshakeCancelled(Exception):
+    """Internal signal that a handshake wake socket became readable."""
+
+
+def _drive_dtls_handshake(
+    connection,
+    sock,
+    *,
+    deadline: float,
+    retries: int | None = None,
+    on_datagram: Callable[[bytes], None] | None = None,
+    wake_socket=None,
+) -> bool:
+    """Drive one memory-BIO DTLS handshake up to a monotonic deadline.
+
+    OpenSSL owns the retransmission schedule. ``retries`` optionally limits
+    how many expired retransmission timers are serviced; a normal session is
+    bounded only by its deadline, while the diagnostic probe retains its
+    explicit retry budget.
+
+    Return ``True`` once OpenSSL reports the handshake complete. The deadline
+    prevents another setup, retry, or network-wait iteration; it does not tear
+    down a session that completed while ``do_handshake()`` was running. TLS and
+    socket failures are left to the caller to classify.
+    """
+    retransmits = 0
+    while time.monotonic() < deadline:
+        try:
+            connection.do_handshake()
+            return True
+        except SSL.WantReadError:
+            pass
+
+        try:
+            output = connection.bio_read(_MAX_DATAGRAM_SIZE)
+        except SSL.WantReadError:
+            output = None
+        if output:
+            for record in split_dtls(output):
+                if sock.send(record) != len(record):
+                    raise OSError("incomplete UDP send")
+
+        remaining = deadline - time.monotonic()
+        if remaining <= 0:
+            break
+        wait = min(_HANDSHAKE_POLL_S, remaining)
+        timed_out = False
+        if wake_socket is None:
+            sock.settimeout(wait)
+            try:
+                datagram = sock.recv(_MAX_DATAGRAM_SIZE)
+            except TimeoutError:
+                timed_out = True
+        else:
+            readable, _, _ = select.select(
+                (sock, wake_socket),
+                (),
+                (),
+                wait,
+            )
+            if wake_socket in readable:
+                raise _HandshakeCancelled()
+            if sock not in readable:
+                timed_out = True
+            else:
+                datagram = sock.recv(_MAX_DATAGRAM_SIZE)
+
+        if timed_out:
+            timer = connection.DTLSv1_get_timeout()
+            if timer is not None and timer <= 0:
+                if retries is not None and retransmits >= retries:
+                    break
+                connection.DTLSv1_handle_timeout()
+                retransmits += 1
+            continue
+
+        if not datagram:
+            continue
+        if on_datagram is not None:
+            on_datagram(datagram)
+        connection.bio_write(datagram)
+
+    return False

smartthings_local/protocol/dtls_probe.py

@@ -39,8 +39,9 @@ from dataclasses import dataclass
 from OpenSSL import SSL
 
 from ..errors import ProbeError
+from .auth import _DTLS_CIPHERS, _OCF_ROOT_CA, _load_pem_chain
 from .coap import split_dtls
-from .dtls_session import _DTLS_CIPHERS, _OCF_ROOT_CA, _load_pem_chain
+from .dtls_handshake import _drive_dtls_handshake
 from .endpoint import open_connected_udp_socket
 
 # DTLS record content types (RFC 6347 §4.1)
@@ -603,73 +604,40 @@ def diagnose_dtls_handshake(
     started = time.monotonic()
     deadline = started + timeout
     seen = set()
-    retransmits = 0
+
+    def record_datagram(datagram):
+        if result.rtt_s is None:
+            result.rtt_s = time.monotonic() - started
+        result.datagrams.append(datagram)
+        for content_type, detail in classify_datagram(datagram):
+            if content_type == _CT_HANDSHAKE:
+                if detail not in seen:
+                    seen.add(detail)
+                    result.handshake_msgs.append(detail)
+                if result.outcome == DEAD:
+                    result.outcome = LIVE
+            elif content_type == _CT_ALERT and detail is not None:
+                level, name = detail
+                result.alert = (level, name)
+                if level == 2:  # fatal
+                    result.outcome = REJECTED
+
     try:
-        while time.monotonic() < deadline:
-            try:
-                conn.do_handshake()
-                result.outcome = COMPLETED
-                if result.rtt_s is None:
-                    result.rtt_s = time.monotonic() - started
-                break
-            except SSL.WantReadError:
-                pass
-            except SSL.Error:
-                # A fatal Alert lands here; the alert record was already
-                # captured below, so classification still works.
-                result.error = ProbeError()
-                break
-
-            try:
-                o = conn.bio_read(65535)
-                if o:
-                    for r in split_dtls(o):
-                        if sock.send(r) != len(r):
-                            raise OSError('short UDP send')
-            except SSL.WantReadError:
-                pass
-
-            remaining = deadline - time.monotonic()
-            if remaining <= 0:
-                break
-            sock.settimeout(min(0.5, remaining))
-            try:
-                d = sock.recv(65535)
-            except TimeoutError:
-                # No answer to the last flight. Service OpenSSL's DTLS
-                # retransmit timer: once it has counted down to 0,
-                # handle_timeout() re-queues the previous flight into the
-                # write BIO for the next iteration to flush. A live server
-                # answers within a flight or two; a silent/non-DTLS port
-                # never does, so we give up only after `retries`
-                # retransmits — one dropped ClientHello no longer reads as
-                # a false DEAD.
-                to = conn.DTLSv1_get_timeout()
-                if to is not None and to <= 0:
-                    if retransmits >= retries:
-                        break
-                    conn.DTLSv1_handle_timeout()
-                    retransmits += 1
-                continue
-            if not d:
-                continue
-
+        completed = _drive_dtls_handshake(
+            conn,
+            sock,
+            deadline=deadline,
+            retries=retries,
+            on_datagram=record_datagram,
+        )
+        if completed:
+            result.outcome = COMPLETED
             if result.rtt_s is None:
                 result.rtt_s = time.monotonic() - started
-            result.datagrams.append(d)
-            for ct, detail in classify_datagram(d):
-                if ct == _CT_HANDSHAKE:
-                    if detail not in seen:
-                        seen.add(detail)
-                        result.handshake_msgs.append(detail)
-                    if result.outcome == DEAD:
-                        result.outcome = LIVE
-                elif ct == _CT_ALERT and detail is not None:
-                    level, name = detail
-                    result.alert = (level, name)
-                    if level == 2:  # fatal
-                        result.outcome = REJECTED
-            conn.bio_write(d)
+    except SSL.Error:
+        # A fatal Alert lands here; record_datagram() has already classified
+        # the alert record before it is fed back into OpenSSL.
+        result.error = ProbeError()
     except OSError:
         result.error = ProbeError()
     finally:

smartthings_local/protocol/dtls_session.py

@@ -12,12 +12,21 @@ Wire-level details that matter (from local-tools/oven-findings.md §17):
     or interleaved one-shot / OBSERVE traffic mis-attributes.
   * Multi-block GET requires the SAME CoAP token across every block
     of the response ("token-stable Block2"). Fresh-token-per-block
-    is silently dropped by the server.
+    is silently dropped by the server, and a transfer that opens at
+    NUM>0 under a token the server has not seen gets no reply at all.
 
 Reader thread owns the UDP socket. Callers issue get()/post() and block
 on a per-token Event the reader signals. OBSERVE notifications are
 delivered via the on_notification callback.
+
+A notification carries only the first block of a large representation
+(RFC 7959 §2.6). Because a continuation cannot borrow the observation's
+token (§3.4) and this server will not continue a transfer it did not
+start, such a notification is withheld and the resource is re-read from
+block 0 on a fresh one-shot token by a worker thread. See #39.
 """
+import errno
+import math
 import os
 import socket
 import threading
@@ -33,11 +42,12 @@ from ..errors import (
     SessionTimeoutError,
 )
 from .coap import (
-    URI_PATH, URI_QUERY, OBSERVE, CONTENT_FORMAT, ACCEPT, BLOCK2, SIZE2,
+    URI_PATH, URI_QUERY, OBSERVE, ETAG, CONTENT_FORMAT, ACCEPT, BLOCK2, SIZE2,
     TYPE_CON, TYPE_NON, TYPE_ACK, TYPE_RST,
     METHOD_GET, METHOD_POST, CF_CBOR,
     OBSERVE_REGISTER, OBSERVE_DEREGISTER, BLOCK_SZX,
-    encode_options, parse_coap, build_coap, block_value, fmt_code,
+    encode_options, parse_coap, build_coap, block_value, block_fields,
+    fmt_code,
     split_dtls as _split_dtls,
 )
 from .auth import (
@@ -47,6 +57,11 @@ from .auth import (
     _OCF_ROOT_CA,
     _load_pem_chain,
 )
+from .dtls_handshake import (
+    _HANDSHAKE_POLL_S,
+    _HandshakeCancelled,
+    _drive_dtls_handshake,
+)
 from .endpoint import open_connected_udp_socket
 import logging
 
@@ -65,6 +80,11 @@ DEBUG_BRIDGE = os.environ.get('DEBUG_BRIDGE') == '1'
 _BLOCK_MAX_ATTEMPTS = 3
 _BLOCK_ACK_TIMEOUT  = 4.0
 
+# How often a block wait re-checks that the reader is still alive. Short
+# enough that a mid-transfer reader death fails fast instead of burning
+# the whole per-block timeout, long enough to stay off the CPU.
+_BLOCK_LIVENESS_POLL_S = 0.25
+
 # Inter-request pacing: minimum seconds between CoAP CON sends on one session.
 # Samsung's RT-OCF stacks drop requests when hit faster than their firmware
 # ceiling (dryer ~14 req/s, oven ~8 req/s, dishwasher unknown). 5 req/s
@@ -72,6 +92,106 @@ _BLOCK_ACK_TIMEOUT  = 4.0
 # once the ceiling is measured empirically.
 _DEFAULT_RATE_LIMIT_RPS = 5.0
 
+# Maximum hrefs held for OBSERVE refetch at once. A notification storm
+# on more resources than this is already past what the 5/s ceiling can
+# drain, so the excess is dropped rather than queued indefinitely.
+_MAX_PENDING_REFETCH = 16
+
+# Timeout for one notification refetch. Generous relative to a poll:
+# the resource is known large (that is why it blocked) and the worker
+# is serialized, so a slow one delays only later refetches.
+_REFETCH_TIMEOUT_S = 15.0
+
+
+class _EtagChanged(Exception):
+    """Internal: the server's ETag changed partway through a Block2
+    transfer, so the blocks in hand are from two different versions."""
+
+
+# ICMP errors a connected UDP socket surfaces on the next recv. On these
+# appliances they show up while the device is rebooting, while it holds an
+# orphaned association, or across a router blip, and the next datagram
+# usually works. UDP delivery was never guaranteed, so treat them as
+# advisory and keep reading. Unconnected sockets never see any of this,
+# which is why the reader survived them before the connected-socket change
+# in d677c72 (v0.1.3).
+_ADVISORY_ERRNOS = frozenset(
+    value for value in (
+        getattr(errno, name, None)
+        for name in ('ECONNREFUSED', 'EHOSTUNREACH', 'ENETUNREACH',
+                     'EHOSTDOWN', 'ENETDOWN')
+    ) if value is not None
+)
+
+def _validate_handshake_timeout(timeout, default):
+    """Return one finite, positive DTLS handshake timeout."""
+    value = default if timeout is None else timeout
+    if isinstance(value, bool) or not isinstance(value, (int, float)):
+        raise TypeError('timeout must be a number or None')
+    try:
+        value = float(value)
+    except OverflowError:
+        raise ValueError(
+            'timeout must be a positive finite number or None') from None
+    if not math.isfinite(value) or value <= 0:
+        raise ValueError('timeout must be a positive finite number or None')
+    return value
+
+
+class ConnectCancellation:
+    """One-way, socket-backed cancellation signal for ``connect()``.
+
+    Each active connection attempt receives its own wake socket. ``set()``
+    makes every subscribed socket readable immediately, without a polling
+    thread or a session-level abort API.
+    """
+
+    __slots__ = ("_is_set", "_lock", "_writers")
+
+    def __init__(self) -> None:
+        self._is_set = False
+        self._lock = threading.Lock()
+        self._writers: set[socket.socket] = set()
+
+    def set(self) -> None:
+        """Cancel current and future connection attempts using this signal."""
+        with self._lock:
+            if self._is_set:
+                return
+            self._is_set = True
+            for writer in self._writers:
+                try:
+                    writer.send(b"\0")
+                except OSError:
+                    pass
+
+    def is_set(self) -> bool:
+        """Return whether cancellation has been requested."""
+        with self._lock:
+            return self._is_set
+
+    def _subscribe(self) -> tuple[socket.socket, socket.socket]:
+        reader, writer = socket.socketpair()
+        reader.setblocking(False)
+        with self._lock:
+            self._writers.add(writer)
+            if self._is_set:
+                writer.send(b"\0")
+        return reader, writer
+
+    def _unsubscribe(
+        self,
+        reader: socket.socket,
+        writer: socket.socket,
+    ) -> bool:
+        with self._lock:
+            self._writers.discard(writer)
+            interrupted = self._is_set
+        reader.close()
+        writer.close()
+        return interrupted
+
+
 class DtlsCoapSession:
     """Single sustained DTLS-CoAP session.
 
@@ -150,6 +270,11 @@ class DtlsCoapSession:
         self.endpoint = None
 
         self._send_lock = threading.Lock()
+        # Guards the MID/token counters and _pending. The refetch worker
+        # makes the session its own second concurrent get() caller, so
+        # two threads can mint tokens at once; without this they can
+        # collide and one transfer silently absorbs the other's blocks.
+        self._state_lock = threading.Lock()
         # Randomize MID and token counter starting points so reconnects
         # don't reuse identifiers from previous sessions — Samsung's
         # RT-OCF appears to remember observer state across DTLS
@@ -166,8 +291,20 @@ class DtlsCoapSession:
         # token (bytes) → href (str)
         self._observe_tokens = {}
 
+        # OBSERVE refetch queue: href → sequence number of the newest
+        # notification that asked for it. Drained by a worker thread
+        # because _dispatch_coap cannot block (see _queue_refetch).
+        self._refetch_cond = threading.Condition()
+        self._refetch_pending = {}
+        self._refetch_seq = 0
+        self._refetch_thread = None
+
         self._stop = threading.Event()
         self._reader_thread = None
+        # Set while the reader owns the socket. Cleared when it exits for
+        # any reason, so callers fail fast through _check_live() instead of
+        # waiting out a request timeout against a session nobody is reading.
+        self._reader_running = threading.Event()
         self._last_send_ts = 0.0
 
     def pace(self) -> None:
@@ -179,69 +316,106 @@ class DtlsCoapSession:
 
     # ---- lifecycle ---------------------------------------------------
 
-    def connect(self):
-        """DTLS handshake. Blocks up to HANDSHAKE_TIMEOUT_S. Raises
-        ConnectionError / TimeoutError on failure."""
+    def connect(
+        self,
+        *,
+        timeout: float | None = None,
+        cancel: ConnectCancellation | None = None,
+    ):
+        """Perform a cancellable DTLS handshake using a monotonic deadline.
+
+        ``timeout`` overrides ``HANDSHAKE_TIMEOUT_S`` for this call. OpenSSL
+        owns DTLS retransmission timing while every receive is capped by the
+        remaining budget, so wall-clock adjustments cannot change the bound.
+        Once OpenSSL reports completion, that completed session is retained
+        even if the call returns just after the deadline. A
+        ``ConnectCancellation`` wakes the network wait immediately and does not
+        alter an already established session.
+        """
+        handshake_timeout = _validate_handshake_timeout(
+            timeout, self.HANDSHAKE_TIMEOUT_S)
+        if cancel is not None and not isinstance(cancel, ConnectCancellation):
+            raise TypeError("cancel must be a ConnectCancellation or None")
+        if cancel is not None and cancel.is_set():
+            raise SessionClosedError()
+        deadline = time.monotonic() + handshake_timeout
         ctx = SSL.Context(SSL.DTLS_METHOD)
         self.auth.configure_context(ctx)
+        if cancel is not None and cancel.is_set():
+            raise SessionClosedError()
 
         conn = SSL.Connection(ctx, None)
         conn.set_connect_state()
         conn.set_ciphertext_mtu(self.mtu)
+        if cancel is not None and cancel.is_set():
+            raise SessionClosedError()
 
+        remaining = deadline - time.monotonic()
+        if remaining <= 0:
+            raise SessionTimeoutError()
         sock, endpoint = open_connected_udp_socket(
             self.host,
             self.port,
             family=self.family,
             local_port=self.local_port,
-            timeout=2.0,
+            timeout=min(_HANDSHAKE_POLL_S, remaining),
         )
         dest = endpoint.sockaddr
+        if cancel is not None and cancel.is_set():
+            sock.close()
+            raise SessionClosedError()
+
+        wake_subscription = None
+        subscription_failed = False
+        if cancel is not None:
+            try:
+                wake_subscription = cancel._subscribe()
+            except OSError:
+                subscription_failed = True
+        if subscription_failed:
+            sock.close()
+            raise SessionError() from OSError(
+                "connection cancellation setup failed"
+            )
 
-        t0 = time.time()
         backend_failed = False
-        while time.time() - t0 < self.HANDSHAKE_TIMEOUT_S:
+        io_failed = False
+        cancelled = False
+        interrupted = False
+        completed = False
+        try:
             try:
-                conn.do_handshake()
-                break
-            except SSL.WantReadError:
-                pass
+                completed = _drive_dtls_handshake(
+                    conn,
+                    sock,
+                    deadline=deadline,
+                    wake_socket=(
+                        wake_subscription[0]
+                        if wake_subscription is not None
+                        else None
+                    ),
+                )
+            except _HandshakeCancelled:
+                cancelled = True
             except SSL.Error:
-                sock.close()
                 backend_failed = True
-                break
-            send_failed = False
-            try:
-                o = conn.bio_read(65535)
-                if o:
-                    for r in _split_dtls(o):
-                        if sock.send(r) != len(r):
-                            raise OSError('incomplete UDP send')
-            except SSL.WantReadError:
-                pass
             except OSError:
-                sock.close()
-                send_failed = True
-            if send_failed:
-                raise EndpointError() from OSError('UDP send failed')
-            receive_failed = False
-            try:
-                d = sock.recv(65535)
-                if d:
-                    conn.bio_write(d)
-            except socket.timeout:
-                pass
-            except OSError:
-                sock.close()
-                receive_failed = True
-            if receive_failed:
-                raise EndpointError() from OSError('UDP receive failed')
-            time.sleep(0.05)
-        else:
+                io_failed = True
+        finally:
+            if wake_subscription is not None:
+                interrupted = cancel._unsubscribe(*wake_subscription)
+        if cancelled or (interrupted and not completed):
+            sock.close()
+            raise SessionClosedError()
+        if backend_failed:
+            sock.close()
+            raise SessionError() from ConnectionError('DTLS backend failed')
+        if io_failed:
+            sock.close()
+            raise EndpointError() from OSError('UDP handshake I/O failed')
+        if not completed:
             sock.close()
             raise SessionTimeoutError()
-        if backend_failed:
-            raise SessionError() from ConnectionError('DTLS backend failed')
 
         self.sock = sock
         self.conn = conn
@@ -253,15 +427,33 @@ class DtlsCoapSession:
         """Spawn the reader thread. Must be called after connect()."""
         if self.sock is None:
             raise RuntimeError("connect() before start_reader()")
+        self._reader_running.set()
         t = threading.Thread(target=self._reader_loop,
                              daemon=True, name='dtls-reader')
         t.start()
         self._reader_thread = t
 
+    def _check_live(self):
+        """Raise if the session cannot carry a request. A dead reader is
+        as fatal as a closed connection: the socket may still accept
+        sends, but no response will ever be dispatched, so waiting out the
+        request timeout only delays the inevitable SessionClosedError.
+
+        Callers that never start a reader (config-flow style) keep the old
+        behaviour — only the conn check applies while _reader_thread is
+        None."""
+        if self.conn is None:
+            raise SessionClosedError()
+        if self._reader_thread is not None and \
+                not self._reader_running.is_set():
+            raise SessionClosedError()
+
     def join(self):
         """Block until the reader thread exits (i.e. socket dies)."""
         if self._reader_thread is not None:
             self._reader_thread.join()
+        if self._refetch_thread is not None:
+            self._refetch_thread.join()
 
     def _send_observe_dereg(self, tok, path_segs):
         """Send a single OBSERVE deregister GET (Observe option = 1)
@@ -293,6 +485,11 @@ class DtlsCoapSession:
             time.sleep(0.1)
 
         self._stop.set()
+        # Wake the refetch worker so it sees _stop instead of sitting on
+        # its condition for up to a second after the socket is gone.
+        with self._refetch_cond:
+            self._refetch_pending.clear()
+            self._refetch_cond.notify_all()
         if self.conn is not None:
             try:
                 self.conn.shutdown()
@@ -303,10 +500,12 @@ class DtlsCoapSession:
                 self.sock.close()
             except Exception:
                 pass
-        for tok, (ev, container) in list(self._pending.items()):
+        with self._state_lock:
+            pending = list(self._pending.items())
+            self._pending.clear()
+        for tok, (ev, container) in pending:
             container.setdefault('err', SessionClosedError())
             ev.set()
-        self._pending.clear()
         self._observe_tokens.clear()
         self.sock = None
         self.conn = None
@@ -316,27 +515,30 @@ class DtlsCoapSession:
     # ---- send / receive plumbing -------------------------------------
 
     def _next_mid(self):
-        self._mid = (self._mid + 1) & 0xFFFF
-        return self._mid
+        with self._state_lock:
+            self._mid = (self._mid + 1) & 0xFFFF
+            return self._mid
 
     def _next_tok(self):
-        self._tok_counter = (self._tok_counter + 1) & 0xFFFFFFFF
-        # 4-byte tokens — fits within tkl=8 cap with headroom and
-        # avoids collisions across long-running OBSERVE subscriptions.
-        return self._tok_counter.to_bytes(4, 'big')
+        with self._state_lock:
+            self._tok_counter = (self._tok_counter + 1) & 0xFFFFFFFF
+            # 4-byte tokens — fits within tkl=8 cap with headroom and
+            # avoids collisions across long-running OBSERVE subscriptions.
+            return self._tok_counter.to_bytes(4, 'big')
 
     def _next_observe_tok(self):
-        # Single-byte tokens for OBSERVE registrations. Samsung
-        # RT-OCF accepts these but silently drops TKL=4 OBSERVE
-        # registrations. Counter is randomly seeded per session so
-        # reconnects don't collide with stale observer state Samsung
-        # may still be holding from the previous run.
-        self._observe_tok_counter = (self._observe_tok_counter + 1) & 0xFF
-        # Avoid 0x00 — some CoAP stacks treat an all-zero token as
-        # equivalent to "no token" / empty (TKL=0).
-        if self._observe_tok_counter == 0:
-            self._observe_tok_counter = 1
-        return bytes([self._observe_tok_counter])
+        with self._state_lock:
+            # Single-byte tokens for OBSERVE registrations. Samsung
+            # RT-OCF accepts these but silently drops TKL=4 OBSERVE
+            # registrations. Counter is randomly seeded per session so
+            # reconnects don't collide with stale observer state Samsung
+            # may still be holding from the previous run.
+            self._observe_tok_counter = (self._observe_tok_counter + 1) & 0xFF
+            # Avoid 0x00 — some CoAP stacks treat an all-zero token as
+            # equivalent to "no token" / empty (TKL=0).
+            if self._observe_tok_counter == 0:
+                self._observe_tok_counter = 1
+            return bytes([self._observe_tok_counter])
 
     def _send_dgram(self, datagram):
         """Send a CoAP datagram. Holds the send lock for the
@@ -374,7 +576,23 @@ class DtlsCoapSession:
                     d = sock.recv(65535)
                 except socket.timeout:
                     continue
-                except (OSError, ValueError):
+                except OSError as e:
+                    if self._stop.is_set():
+                        return          # close() got here first
+                    if e.errno in _ADVISORY_ERRNOS:
+                        logger.debug("reader: advisory %s from %s, continuing",
+                                     errno.errorcode.get(e.errno, e.errno),
+                                     self.host)
+                        continue
+                    logger.warning("reader exiting: socket error %s from %s",
+                                   errno.errorcode.get(e.errno, e.errno),
+                                   self.host)
+                    return
+                except ValueError:
+                    # recv on a socket closed underneath the reader.
+                    if not self._stop.is_set():
+                        logger.warning("reader exiting: socket closed "
+                                       "underneath it")
                     return
                 if not d:
                     continue
@@ -419,10 +637,18 @@ class DtlsCoapSession:
                 if exit_reader:
                     return
         finally:
+            # Reader no longer owns the socket — callers must fail fast.
+            self._reader_running.clear()
             # Make sure pending waiters don't hang if the reader dies.
-            for tok, (ev, container) in list(self._pending.items()):
+            with self._state_lock:
+                pending = list(self._pending.items())
+            for tok, (ev, container) in pending:
                 container.setdefault('err', SessionClosedError())
                 ev.set()
+            # Nothing will answer a refetch now either.
+            with self._refetch_cond:
+                self._refetch_pending.clear()
+                self._refetch_cond.notify_all()
 
     def _dispatch_coap(self, datagram):
         try:
@@ -452,7 +678,8 @@ class DtlsCoapSession:
             return
 
         # Pending one-shot? Resolve and return.
-        rec = self._pending.get(tok)
+        with self._state_lock:
+            rec = self._pending.get(tok)
         if rec is not None:
             ev, container = rec
             container['code']    = code
@@ -470,6 +697,16 @@ class DtlsCoapSession:
                 logger.warning("observe %s: non-2.05 %s",
                                href, fmt_code(code))
                 return
+            # RFC 7959 §2.6: a notification carries only the first block
+            # of the representation. Handing the callback a partial CBOR
+            # buffer is what #39 was about, so anything with M=1 (or a
+            # block past the first) goes to the refetch worker instead.
+            b2 = [v for n, v in ropts if n == BLOCK2]
+            if b2:
+                num, more, _ = block_fields(b2[0])
+                if more or num:
+                    self._queue_refetch(href)
+                    return
             cb = self.on_notification
             if cb is not None:
                 try:
@@ -481,6 +718,110 @@ class DtlsCoapSession:
 
         # Stale token (post-reconnect or unknown) — drop quietly.
 
+    # ---- OBSERVE refetch ---------------------------------------------
+
+    @staticmethod
+    def _log_refetch(msg, *args):
+        """Refetch outcomes are debug-level in normal operation, which is
+        below the bridge's INFO default, so a healthy session stays quiet.
+        DEBUG_BRIDGE=1 promotes them to INFO for hardware validation:
+        that shows which token the re-read used and whether it completed,
+        without also turning on every per-block retransmit line."""
+        (logger.info if DEBUG_BRIDGE else logger.debug)(msg, *args)
+
+    def _queue_refetch(self, href):
+        """Queue a blockwise notification for re-reading.
+
+        Called from the reader thread, so it must not block: _dispatch_coap
+        runs there and _blockwise_get waits on an Event only that same
+        thread can set, which would deadlock the session outright. Latest
+        wins per href — a burst of notifications on one resource collapses
+        into a single re-read of its final state."""
+        with self._refetch_cond:
+            if (href not in self._refetch_pending
+                    and len(self._refetch_pending) >= _MAX_PENDING_REFETCH):
+                self._log_refetch(
+                    "refetch %s dropped: queue full (%d pending)",
+                    href, len(self._refetch_pending))
+                return
+            self._refetch_seq += 1
+            self._refetch_pending[href] = self._refetch_seq
+            self._refetch_cond.notify()
+        self._start_refetch_worker()
+
+    def _start_refetch_worker(self):
+        """Start the refetch worker on first use. Sessions that never see
+        a blockwise notification never grow the thread."""
+        if self._refetch_thread is not None or self._stop.is_set():
+            return
+        with self._state_lock:
+            if self._refetch_thread is not None or self._stop.is_set():
+                return
+            self._refetch_thread = threading.Thread(
+                target=self._refetch_loop, daemon=True,
+                name=f'stl-refetch-{self.host}')
+            self._refetch_thread.start()
+
+    def _refetch_loop(self):
+        """Re-read blockwise-notified resources, one at a time.
+
+        Serialized on purpose. Each re-read is a multi-block transfer and
+        _blockwise_get paces between blocks, so running one at a time is
+        what keeps a notification storm under the firmware's request
+        ceiling."""
+        while self._refetch_alive():
+            with self._refetch_cond:
+                while not self._refetch_pending and self._refetch_alive():
+                    self._refetch_cond.wait(1.0)
+                if not self._refetch_pending:
+                    return
+                href, seq = next(iter(self._refetch_pending.items()))
+                del self._refetch_pending[href]
+            self._refetch_one(href, seq)
+
+    def _refetch_alive(self):
+        """False once the session is closing or the reader has died. A
+        refetch needs the reader to resolve its token, so outliving it
+        would leave join() waiting on a thread with nothing to do."""
+        if self._stop.is_set():
+            return False
+        return self._reader_thread is None or self._reader_running.is_set()
+
+    def _refetch_one(self, href, seq):
+        """Re-read one href from block 0 and deliver it if it is still
+        the freshest thing we know about that resource."""
+        self.pace()
+        segs = [s for s in href.split('/') if s]
+        try:
+            code, payload, blocks, tok = self._blockwise_get(
+                segs, (), _REFETCH_TIMEOUT_S)
+        except Exception as e:
+            # Device silent, session gone, ETag never settled, block cap
+            # hit. Whatever the reason, dropping the notification is the
+            # contract: the poll tiers still carry freshness, and handing
+            # over the first block is the bug this replaced.
+            self._log_refetch("refetch %s failed: %s", href, e)
+            return
+        if code != 0x45:
+            self._log_refetch("refetch %s returned %s", href, fmt_code(code))
+            return
+        with self._refetch_cond:
+            # A newer notification landed while we were reading. That one
+            # has its own refetch queued, so this result is already stale.
+            if self._refetch_pending.get(href, 0) > seq:
+                self._log_refetch(
+                    "refetch %s tok=%s blocks=%d bytes=%d superseded",
+                    href, tok.hex(), blocks, len(payload))
+                return
+        self._log_refetch("refetch %s tok=%s blocks=%d bytes=%d ok",
+                          href, tok.hex(), blocks, len(payload))
+        cb = self.on_notification
+        if cb is not None:
+            try:
+                cb(href, payload)
+            except Exception as e:
+                logger.warning("notification callback %s: %s", href, e)
+
     # ---- request primitives ------------------------------------------
 
     def get(self, path_segs, query=(), timeout=10.0):
@@ -490,84 +831,189 @@ class DtlsCoapSession:
         response — Samsung's server keys per-transfer state on the
         token, and dropping a fresh token on block 1+ silently drops
         the request."""
-        if self.conn is None:
-            raise SessionClosedError()
+        self._check_live()
+        code, blob, _blocks, _tok = self._blockwise_get(
+            path_segs, query, timeout)
+        return code, blob
+
+    def _blockwise_get(self, path_segs, query=(), timeout=10.0):
+        """Shared token-stable Block2 reassembly (RFC 7959 §2.4).
+
+        Returns (code, payload, block_count, token). The last two are
+        diagnostics for the refetch log; get() drops them.
+
+        Mints one fresh 4-byte token and holds it across every block of
+        the transfer. Also the notification-refetch primitive: RFC 7959
+        §3.4 forbids continuing a blockwise notification on the
+        observation's token, and Samsung's RT-OCF drops a transfer that
+        opens at NUM>0 under a token it has not seen, so a truncated
+        notification is recovered by re-reading from block 0 through
+        this same path rather than by a §2.6 continuation.
+
+        Restarts once if the server's ETag changes mid-transfer, then
+        gives up: RFC 7959 §2.4 requires the client to compare ETags
+        when the server supplies them. None of the tested appliances
+        emit option 4, so on those this is inert."""
+        try:
+            return self._blockwise_get_once(path_segs, query, timeout)
+        except _EtagChanged:
+            logger.debug("GET %s /%s: ETag changed mid-transfer, restarting",
+                         self.host, '/'.join(path_segs))
+        try:
+            return self._blockwise_get_once(path_segs, query, timeout)
+        except _EtagChanged:
+            logger.debug(
+                "GET %s /%s: representation kept changing mid-transfer",
+                self.host, '/'.join(path_segs))
+            raise BlockwiseError() from None
+
+    def _blockwise_get_once(self, path_segs, query, timeout):
+        """One attempt at a full Block2 transfer. Raises _EtagChanged if
+        the server's representation changed while we were reassembling."""
         tok = self._next_tok()
         blob = b''
         num = 0
+        blocks = 0
         last_code = None
-        last_opts = []
+        etag = None
         deadline = time.time() + timeout
         szx = BLOCK_SZX   # server may negotiate down; track per-transfer
         while True:
-            if num > 0:
-                self.pace()
-            container = {}
-            for attempt in range(_BLOCK_MAX_ATTEMPTS):
-                ev = threading.Event()
-                container = {}
-                self._pending[tok] = (ev, container)
-                try:
-                    mid = self._next_mid()
-                    opts = [(URI_PATH, s.encode()) for s in path_segs]
-                    for q in query:
-                        opts.append((URI_QUERY, q.encode()))
-                    opts.append((ACCEPT, CF_CBOR))
-                    if num > 0:
-                        opts.append((BLOCK2, block_value(num, 0, szx)))
-                    self._send_dgram(
-                        build_coap(TYPE_CON, METHOD_GET, mid, tok, opts))
-                    per_wait = min(_BLOCK_ACK_TIMEOUT,
-                                   max(0.1, deadline - time.time()))
-                    if ev.wait(per_wait):
-                        break  # got a response
-                    remaining = deadline - time.time()
-                    if remaining <= 0 or attempt == _BLOCK_MAX_ATTEMPTS - 1:
-                        logger.debug(
-                            "GET %s /%s block %d: timed out after %d attempt(s)",
-                            self.host, '/'.join(path_segs), num, attempt + 1,
-                        )
-                        raise SessionTimeoutError()
-                    logger.debug(
-                        "GET %s /%s block %d: attempt %d/%d timeout, retrying",
-                        self.host, '/'.join(path_segs), num,
-                        attempt + 1, _BLOCK_MAX_ATTEMPTS,
-                    )
-                finally:
-                    self._pending.pop(tok, None)
+            self.pace()
+            self._check_live()
+            container = self._exchange_block(
+                tok, path_segs, query, num, szx, deadline)
             if 'err' in container:
                 raise container['err']
+            blocks += 1
 
             code = container['code']
             payload = container['payload']
             ropts = container['options']
             last_code = code
-            last_opts = ropts
-            blob += payload
             # 4.xx / 5.xx responses don't carry Block2 continuation —
             # bail with whatever we got. Caller decides if 4.xx is fatal.
             if code >> 5 != 2:
-                return code, blob
+                return code, blob, blocks, tok
+
+            # RFC 7959 §2.4: compare ETags across blocks, or we splice
+            # two versions of the resource into one buffer.
+            block_etag = next((v for n, v in ropts if n == ETAG), None)
+            if num == 0:
+                etag = block_etag
+            elif etag is not None and block_etag != etag:
+                raise _EtagChanged()
+
+            blob += payload
             b2 = [v for n, v in ropts if n == BLOCK2]
-            more = 0
-            if b2:
-                bv = int.from_bytes(b2[0], 'big')
-                more = (bv >> 3) & 1
-                server_szx = bv & 0x07
-                if server_szx != szx:
-                    szx = server_szx
+            if not b2:
+                break
+            _, more, server_szx = block_fields(b2[0])
             if not more:
                 break
-            num += 1
+            if server_szx != szx:
+                # Server negotiated the block size down. Block numbers
+                # are indices into the new size, so the next one has to
+                # come off the byte offset we have actually accumulated,
+                # not off num + 1.
+                szx = server_szx
+                num = len(blob) >> (szx + 4)
+            else:
+                num += 1
             if num > self.MAX_BLOCKS:
                 raise BlockwiseError()
-        return last_code, blob
+        return last_code, blob, blocks, tok
+
+    def _exchange_block(self, tok, path_segs, query, num, szx, deadline):
+        """Send one block request under `tok` and return its response
+        container, retransmitting up to _BLOCK_MAX_ATTEMPTS times.
+
+        A response whose Block2 NUM is not the one we asked for is a
+        retransmit of an earlier block, not the next one. Concatenating
+        it would corrupt the buffer, so keep waiting on the same
+        attempt budget instead."""
+        for attempt in range(_BLOCK_MAX_ATTEMPTS):
+            ev = threading.Event()
+            container = {}
+            with self._state_lock:
+                self._pending[tok] = (ev, container)
+            try:
+                mid = self._next_mid()
+                opts = [(URI_PATH, s.encode()) for s in path_segs]
+                for q in query:
+                    opts.append((URI_QUERY, q.encode()))
+                opts.append((ACCEPT, CF_CBOR))
+                if num > 0:
+                    opts.append((BLOCK2, block_value(num, 0, szx)))
+                self._send_dgram(
+                    build_coap(TYPE_CON, METHOD_GET, mid, tok, opts))
+                while True:
+                    per_wait = min(_BLOCK_ACK_TIMEOUT,
+                                   max(0.1, deadline - time.time()))
+                    if not self._wait_for_block(ev, per_wait):
+                        break  # attempt timed out
+                    if 'err' in container or self._block_num_matches(
+                            container, num):
+                        return container
+                    logger.debug(
+                        "GET %s /%s block %d: stale block, still waiting",
+                        self.host, '/'.join(path_segs), num)
+                    ev.clear()
+                    container.clear()
+                    if deadline - time.time() <= 0:
+                        break
+                remaining = deadline - time.time()
+                if remaining <= 0 or attempt == _BLOCK_MAX_ATTEMPTS - 1:
+                    logger.debug(
+                        "GET %s /%s block %d: timed out after %d attempt(s)",
+                        self.host, '/'.join(path_segs), num, attempt + 1,
+                    )
+                    raise SessionTimeoutError()
+                logger.debug(
+                    "GET %s /%s block %d: attempt %d/%d timeout, retrying",
+                    self.host, '/'.join(path_segs), num,
+                    attempt + 1, _BLOCK_MAX_ATTEMPTS,
+                )
+            finally:
+                with self._state_lock:
+                    self._pending.pop(tok, None)
+        raise SessionTimeoutError()
+
+    def _wait_for_block(self, ev, per_wait):
+        """Wait for one block response, giving up early if the reader
+        dies underneath us.
+
+        Only the reader thread can resolve a token, so once it is gone
+        the wait can never succeed. Polling in slices turns what would
+        be a full per-block timeout into an immediate SessionClosedError,
+        which is the same fail-fast contract get() gets from _check_live()
+        at entry — it just has to hold for every block, not only the
+        first."""
+        deadline = time.time() + per_wait
+        while True:
+            slice_s = min(_BLOCK_LIVENESS_POLL_S, deadline - time.time())
+            if slice_s <= 0:
+                return False
+            if ev.wait(slice_s):
+                return True
+            self._check_live()
+
+    @staticmethod
+    def _block_num_matches(container, num):
+        """True if this response carries the block we asked for. A
+        response with no Block2 option is the whole representation, so
+        it only answers block 0."""
+        if container.get('code', 0) >> 5 != 2:
+            return True     # error responses end the transfer either way
+        b2 = [v for n, v in container.get('options', ()) if n == BLOCK2]
+        if not b2:
+            return num == 0
+        return block_fields(b2[0])[0] == num
 
     def post(self, path_segs, body_cbor, timeout=8.0):
         """Single-frame POST with a CBOR-encoded body. Returns
         (code, payload_bytes). body_cbor must already be encoded."""
-        if self.conn is None:
-            raise SessionClosedError()
+        self._check_live()
         tok = self._next_tok()
         mid = self._next_mid()
         opts = [(URI_PATH, s.encode()) for s in path_segs]
@@ -577,8 +1023,11 @@ class DtlsCoapSession:
                               body_cbor)
         ev = threading.Event()
         container = {}
-        self._pending[tok] = (ev, container)
+        with self._state_lock:
+            self._pending[tok] = (ev, container)
         try:
+            self.pace()
+            self._check_live()
             self._send_dgram(datagram)
             if not ev.wait(timeout):
                 raise SessionTimeoutError()
@@ -586,7 +1035,8 @@ class DtlsCoapSession:
                 raise container['err']
             return container['code'], container['payload']
         finally:
-            self._pending.pop(tok, None)
+            with self._state_lock:
+                self._pending.pop(tok, None)
 
     def ping(self):
         """RFC 7252 §4.4 CoAP Ping — empty CON, no token, no payload.
@@ -600,8 +1050,7 @@ class DtlsCoapSession:
         Real half-open-session detection lives in PollScheduler's
         `last_success_ts`, surfaced through KeepaliveTask's
         `liveness_fn`."""
-        if self.conn is None:
-            raise SessionClosedError()
+        self._check_live()
         mid = self._next_mid()
         self._send_dgram(build_coap(TYPE_CON, 0, mid, b'', []))
         return mid
@@ -618,8 +1067,7 @@ class DtlsCoapSession:
         tokens via subscribe. Brief race window where a notify on the
         old token gets dropped as 'stale' — acceptable for a 6h-scale
         safety net."""
-        if self.conn is None:
-            raise SessionClosedError()
+        self._check_live()
         for tok, href in list(self._observe_tokens.items()):
             segs = [s for s in href.split('/') if s]
             try:
@@ -642,8 +1090,9 @@ class DtlsCoapSession:
 
         Returns the token used (in case the caller wants to deregister
         later)."""
-        if self.conn is None:
-            raise SessionClosedError()
+        self._check_live()
+        self.pace()
+        self._check_live()
         tok = self._next_observe_tok()
         href = '/' + '/'.join(path_segs)
         # Register the token BEFORE sending — otherwise the device

smartthings_local/protocol/owner_psk.py

@@ -0,0 +1,128 @@
+"""Pure IoTivity manufacturer-certificate OwnerPSK derivation."""
+
+from __future__ import annotations
+
+from collections.abc import Mapping
+import hashlib
+import hmac
+from types import MappingProxyType
+from typing import Final
+
+
+CONFIRMED_MFG_CERTIFICATE_OXM_LABEL: Final = b"x.org.iotivity.conmfgcert"
+STANDARD_MFG_CERTIFICATE_OXM_LABEL: Final = b"oic.sec.doxm.mfgcert"
+
+# OpenSSL cipher names mapped to the key-block lengths used by IoTivity's
+# CAGenerateOwnerPSK implementation.
+MFG_CERTIFICATE_KEY_BLOCK_LENGTHS: Final[Mapping[str, int]] = MappingProxyType(
+    {
+        "ECDHE-ECDSA-AES128-SHA256": 96,
+        "ECDHE-ECDSA-AES128-CCM": 40,
+        "ECDHE-ECDSA-AES128-CCM8": 40,
+        "ECDHE-ECDSA-AES128-GCM-SHA256": 120,
+        "AES256-SHA256": 128,
+        "ECDHE-ECDSA-AES256-SHA384": 160,
+        "ECDHE-ECDSA-AES256-GCM-SHA384": 184,
+        "AES128-GCM-SHA256": 120,
+    }
+)
+
+_TLS_MASTER_SECRET_BYTES: Final = 48
+_TLS_RANDOM_BYTES: Final = 32
+_OCF_UUID_BYTES: Final = 16
+_OWNER_PSK_BYTES: Final = 16
+
+
+def _require_bytes(name: str, value: bytes, length: int) -> bytes:
+    if not isinstance(value, bytes):
+        raise TypeError(f"{name} must be bytes")
+    if len(value) != length:
+        raise ValueError(f"{name} must be exactly {length} bytes")
+    return value
+
+
+def _require_uuid(name: str, value: bytes) -> bytes:
+    value = _require_bytes(name, value, _OCF_UUID_BYTES)
+    if not any(value):
+        raise ValueError(f"{name} must not be the nil UUID")
+    return value
+
+
+def _tls12_p_hash_sha256(
+    key: bytes,
+    label: bytes,
+    random1: bytes,
+    random2: bytes,
+    length: int,
+) -> bytes:
+    seed = label + random1 + random2
+    a_value = hmac.new(key, seed, hashlib.sha256).digest()
+    output = bytearray()
+    while len(output) < length:
+        output.extend(hmac.new(key, a_value + seed, hashlib.sha256).digest())
+        a_value = hmac.new(key, a_value, hashlib.sha256).digest()
+    return bytes(output[:length])
+
+
+def derive_mfg_certificate_owner_psk(
+    *,
+    master_secret: bytes,
+    client_random: bytes,
+    server_random: bytes,
+    owner_uuid: bytes,
+    device_uuid: bytes,
+    cipher_name: str,
+    oxm_label: bytes,
+) -> bytes:
+    """Derive a 128-bit OwnerPSK from caller-supplied DTLS state.
+
+    This implements IoTivity's two-stage TLS 1.2 SHA-256 P_hash operation.
+    It performs no session access, network I/O, ownership writes, or storage.
+    The caller must supply state from an authenticated manufacturer-certificate
+    session and explicitly select the OXM label used by that transaction.
+    IoTivity's other 96-byte ECDH_ANON, ECDHE_PSK, and ECDHE_RSA mappings are
+    intentionally outside this helper's manufacturer-certificate allowlist.
+    """
+
+    if not isinstance(cipher_name, str):
+        raise TypeError("cipher_name must be a string")
+    key_block_bytes = MFG_CERTIFICATE_KEY_BLOCK_LENGTHS.get(cipher_name)
+    if key_block_bytes is None:
+        raise ValueError("unexpected manufacturer-certificate DTLS cipher")
+
+    master_secret = _require_bytes(
+        "master_secret", master_secret, _TLS_MASTER_SECRET_BYTES
+    )
+    client_random = _require_bytes(
+        "client_random", client_random, _TLS_RANDOM_BYTES
+    )
+    server_random = _require_bytes(
+        "server_random", server_random, _TLS_RANDOM_BYTES
+    )
+    owner_uuid = _require_uuid("owner_uuid", owner_uuid)
+    device_uuid = _require_uuid("device_uuid", device_uuid)
+    if not isinstance(oxm_label, bytes):
+        raise TypeError("oxm_label must be bytes")
+    if oxm_label not in {
+        CONFIRMED_MFG_CERTIFICATE_OXM_LABEL,
+        STANDARD_MFG_CERTIFICATE_OXM_LABEL,
+    }:
+        raise ValueError("unexpected manufacturer-certificate OXM label")
+
+    key_block = _tls12_p_hash_sha256(
+        master_secret,
+        b"key expansion",
+        server_random,
+        client_random,
+        key_block_bytes,
+    )
+    # IoTivity's OTM callers pass the owner UUID first and the target device
+    # UUID second. The lower adapter's historical rsrc/prov parameter names
+    # describe those arguments inconsistently, so preserve the caller order.
+    return _tls12_p_hash_sha256(
+        key_block,
+        oxm_label,
+        owner_uuid,
+        device_uuid,
+        _OWNER_PSK_BYTES,
+    )

tests/test_coap_wire.py

@@ -2,7 +2,8 @@ import pytest
 
 from smartthings_local.errors import MalformedMessageError
 from smartthings_local.protocol.coap import (
-    build_coap, parse_coap, encode_options, block_value, fmt_code,
+    build_coap, parse_coap, encode_options, block_value, block_fields,
+    fmt_code,
     TYPE_CON, METHOD_GET, URI_PATH, ACCEPT, CF_CBOR, BLOCK2,
 )
 
@@ -43,6 +44,23 @@ def test_block_value_promotes_to_two_bytes_when_num_is_large():
     assert len(v) == 2
 
 
+@pytest.mark.parametrize('num, more, szx', [
+    (0, 0, 0),
+    (0, 1, 6),
+    (2, 1, 6),
+    (1, 0, 4),
+    (0xFFF, 0, 0),
+    (0xFFFFF, 1, 7),
+])
+def test_block_fields_inverts_block_value(num, more, szx):
+    assert block_fields(block_value(num, more, szx)) == (num, more, szx)
+
+
+def test_block_fields_treats_empty_value_as_block_zero():
+    # RFC 7959 §2.2: a zero-length Block option means num=0, m=0, szx=0.
+    assert block_fields(b'') == (0, 0, 0)
+
+
 def test_fmt_code_formats_class_dot_detail():
     assert fmt_code(0x45) == '2.05'
     assert fmt_code(0x84) == '4.04'

tests/test_dtls_session_reader_death.py

@@ -0,0 +1,173 @@
+"""Reader-thread death visibility (QuiteYellow/SmartThings-Local#37).
+
+A connected UDP socket surfaces ICMP errors on recv; before this the
+reader exited silently on the first one and every later request waited
+out its full timeout against a session nobody was reading. These tests
+pin the three behaviours that fixed it: advisory ICMP errnos keep the
+reader alive, a real socket error exits with a WARNING and clears
+_reader_running, and callers then fail fast with SessionClosedError.
+"""
+import errno
+import logging
+import socket
+import threading
+import time
+
+import pytest
+from OpenSSL import SSL
+
+from smartthings_local.errors import SessionClosedError
+from smartthings_local.protocol.dtls_session import DtlsCoapSession
+
+_LOGGER_NAME = "smartthings_local.protocol.dtls_session"
+
+
+class _NullAuth:
+    """Structural AuthenticationProvider — never configured, we skip connect()."""
+
+    def configure_context(self, _context):
+        return None
+
+
+class _FakeConn:
+    """Minimal stand-in for SSL.Connection: each datagram written to the
+    BIO surfaces as one decrypted packet on the next recv(), then
+    WantReadError like a drained DTLS record buffer."""
+
+    def __init__(self):
+        self._decrypted = []
+
+    def bio_write(self, datagram):
+        self._decrypted.append(datagram)
+
+    def recv(self, _n):
+        if self._decrypted:
+            return self._decrypted.pop(0)
+        raise SSL.WantReadError()
+
+    def bio_read(self, _n):
+        return b""
+
+    def send(self, _datagram):
+        return None
+
+    def shutdown(self):
+        return None
+
+
+class _FakeSock:
+    """Scripted UDP socket. Each step is bytes to return, an exception to
+    raise, or a callable to run (then a timeout, so the loop re-checks
+    _stop). An exhausted script blocks like a real recv timeout; once
+    close()d it raises EBADF the way a closed fd does."""
+
+    def __init__(self, steps=()):
+        self._steps = list(steps)
+        self.closed = False
+        self.timeout = None
+
+    def settimeout(self, value):
+        self.timeout = value
+
+    def recv(self, _n):
+        if self.closed:
+            raise OSError(errno.EBADF, "bad file descriptor")
+        if self._steps:
+            step = self._steps.pop(0)
+            if callable(step):
+                step()
+                raise socket.timeout()
+            if isinstance(step, BaseException):
+                raise step
+            return step
+        time.sleep(0.01)
+        raise socket.timeout()
+
+    def send(self, data):
+        return len(data)
+
+    def close(self):
+        self.closed = True
+
+
+def _make_session():
+    sess = DtlsCoapSession("host", 1234, auth=_NullAuth())
+    sess.conn = _FakeConn()
+    return sess
+
+
+def _run_reader(sess, steps, timeout=2.0):
+    sess.sock = _FakeSock(steps)
+    sess.start_reader()
+    sess._reader_thread.join(timeout)
+    assert not sess._reader_thread.is_alive(), "reader thread did not exit"
+
+
+def test_advisory_icmp_error_does_not_kill_reader(caplog):
+    sess = _make_session()
+    dispatched = []
+    sess._dispatch_coap = dispatched.append
+
+    with caplog.at_level(logging.DEBUG, logger=_LOGGER_NAME):
+        _run_reader(sess, [
+            OSError(errno.ECONNREFUSED, "connection refused"),
+            b"\x60\x00\x00\x00",              # survives, gets dispatched
+            lambda: sess._stop.set(),         # end the loop cleanly
+        ])
+
+    assert dispatched == [b"\x60\x00\x00\x00"]
+    assert not sess._reader_running.is_set()
+    assert any(r.levelno == logging.DEBUG and "advisory" in r.getMessage()
+               for r in caplog.records)
+    # An advisory errno is not a real exit — no WARNING.
+    assert not any(r.levelno >= logging.WARNING for r in caplog.records)
+
+
+def test_fatal_socket_error_exits_with_warning(caplog):
+    sess = _make_session()
+
+    with caplog.at_level(logging.WARNING, logger=_LOGGER_NAME):
+        _run_reader(sess, [OSError(errno.EBADF, "bad file descriptor")])
+
+    assert not sess._reader_running.is_set()
+    warnings = [r for r in caplog.records if r.levelno == logging.WARNING]
+    assert len(warnings) == 1
+    assert "reader exiting" in warnings[0].getMessage()
+
+
+def test_request_fails_fast_after_reader_death():
+    sess = _make_session()
+    _run_reader(sess, [OSError(errno.EBADF, "bad file descriptor")])
+    assert not sess._reader_running.is_set()
+
+    start = time.monotonic()
+    with pytest.raises(SessionClosedError):
+        sess.get(["oic", "d"], timeout=10.0)
+    elapsed = time.monotonic() - start
+    # The whole point: no waiting out the request timeout.
+    assert elapsed < 1.0, f"get() waited {elapsed:.2f}s instead of failing fast"
+
+
+def test_close_does_not_log_warning_on_teardown(caplog):
+    sess = _make_session()
+    sess.sock = _FakeSock()          # empty script: blocks on recv
+    sess.start_reader()
+    time.sleep(0.05)                 # let the reader reach recv
+
+    with caplog.at_level(logging.WARNING, logger=_LOGGER_NAME):
+        sess.close()
+        sess._reader_thread.join(2.0)
+
+    assert not sess._reader_thread.is_alive()
+    assert not sess._reader_running.is_set()
+    assert not any(r.levelno >= logging.WARNING for r in caplog.records)
+
+
+def test_check_live_without_reader_matches_old_conn_guard():
+    sess = _make_session()          # conn set, reader never started
+    assert sess._reader_thread is None
+    sess._check_live()              # must not raise — config-flow behaviour
+
+    sess.conn = None
+    with pytest.raises(SessionClosedError):
+        sess._check_live()

tests/test_endpoint.py

@@ -376,7 +376,7 @@ def test_session_uses_connected_socket_send_and_recv(monkeypatch):
     assert open_calls == [(('device.example', 5684), {
         'family': socket.AF_INET6,
         'local_port': None,
-        'timeout': 2.0,
+        'timeout': 0.5,
     })]
 
     session.close()

tests/test_observe_block2.py

@@ -0,0 +1,508 @@
+"""Blockwise OBSERVE notifications (QuiteYellow/SmartThings-Local#39).
+
+A notification carries only the first block of the representation
+(RFC 7959 §2.6). Before this, _dispatch_coap handed that first block
+straight to on_notification and the consumer decoded a truncated CBOR
+buffer. These tests pin the replacement: a truncated notification is
+withheld, the resource is re-read from block 0 under a fresh one-shot
+token, and only the reassembled representation reaches the callback.
+
+The re-read starts at block 0 rather than continuing at NUM=1 for two
+reasons, both recorded on #39: RFC 7959 §3.4 forbids continuing on the
+observation's token, and Samsung's RT-OCF drops a transfer that opens
+at NUM>0 under a token it has not seen.
+"""
+import logging
+import socket
+import threading
+import time
+
+import pytest
+from OpenSSL import SSL
+
+from smartthings_local.errors import BlockwiseError
+from smartthings_local.protocol import dtls_session
+from smartthings_local.protocol.coap import (
+    BLOCK2, ETAG, METHOD_GET, OBSERVE, TYPE_ACK, TYPE_NON,
+    block_fields, block_value, build_coap, parse_coap,
+)
+from smartthings_local.protocol.dtls_session import DtlsCoapSession
+
+SZX = 6  # 1024-byte blocks, the only size these appliances honour
+_LOGGER_NAME = "smartthings_local.protocol.dtls_session"
+
+
+class _NullAuth:
+    """Structural AuthenticationProvider — never configured, we skip connect()."""
+
+    def configure_context(self, _context):
+        return None
+
+
+class _LoopbackConn:
+    """SSL.Connection stand-in that answers requests from a script.
+
+    `responder` is called with each parsed request and returns a list of
+    CoAP datagrams to hand back (possibly empty, to model a silent
+    device). Responses surface on recv() the way decrypted records do.
+    """
+
+    def __init__(self, responder):
+        self._responder = responder
+        self._inbox = []
+        self._lock = threading.Lock()
+        self.sent = []
+
+    # -- client -> device
+    def send(self, datagram):
+        self.sent.append(parse_coap(datagram))
+        for reply in self._responder(parse_coap(datagram)):
+            with self._lock:
+                self._inbox.append(reply)
+        return len(datagram)
+
+    def bio_read(self, _n):
+        return b""
+
+    # -- device -> client
+    def inject(self, datagram):
+        """Push a device-initiated frame (an OBSERVE notification)."""
+        with self._lock:
+            self._inbox.append(datagram)
+
+    def bio_write(self, _datagram):
+        return None
+
+    def recv(self, _n):
+        with self._lock:
+            if self._inbox:
+                return self._inbox.pop(0)
+        raise SSL.WantReadError()
+
+    def shutdown(self):
+        return None
+
+    def pending(self):
+        with self._lock:
+            return len(self._inbox)
+
+
+class _PumpSock:
+    """UDP socket stand-in. recv() returns a dummy datagram whenever the
+    connection has something decrypted waiting, so the reader loop keeps
+    pumping; otherwise it times out like a real socket."""
+
+    def __init__(self, conn):
+        self._conn = conn
+        self.closed = False
+
+    def settimeout(self, _value):
+        return None
+
+    def recv(self, _n):
+        for _ in range(20):
+            if self.closed:
+                raise OSError("closed")
+            if self._conn.pending():
+                return b"\x00"
+            time.sleep(0.005)
+        raise socket.timeout()
+
+    def send(self, data):
+        return len(data)
+
+    def close(self):
+        self.closed = True
+
+
+def _make_session(responder, **kwargs):
+    calls = []
+    sess = DtlsCoapSession(
+        "host", 1234, auth=_NullAuth(),
+        on_notification=lambda href, payload: calls.append((href, payload)),
+        **kwargs)
+    sess.conn = _LoopbackConn(responder)
+    sess.sock = _PumpSock(sess.conn)
+    sess.start_reader()
+    return sess, calls
+
+
+def _notification(tok, payload, *, block2=None, mtype=TYPE_NON, obs=1):
+    opts = [(OBSERVE, bytes([obs]))]
+    if block2 is not None:
+        opts.append((BLOCK2, block2))
+    return build_coap(mtype, 0x45, 0x1234, tok, opts, payload)
+
+
+def _content(tok, mid, payload, *, block2=None, etag=None):
+    opts = []
+    if etag is not None:
+        opts.append((ETAG, etag))
+    if block2 is not None:
+        opts.append((BLOCK2, block2))
+    return build_coap(TYPE_ACK, 0x45, mid, tok, opts, payload)
+
+
+def _requested_block(request):
+    """(num, szx) the request asked for, or (0, None) with no Block2."""
+    _, _, _, _, opts, _ = request
+    b2 = [v for n, v in opts if n == BLOCK2]
+    if not b2:
+        return 0, None
+    num, _, szx = block_fields(b2[0])
+    return num, szx
+
+
+def _wait_for(predicate, timeout=3.0):
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        if predicate():
+            return True
+        time.sleep(0.01)
+    return False
+
+
+def _close(sess):
+    sess.close()
+    sess.join()
+
+
+# --------------------------------------------------------------------
+# The no-regression case
+
+
+def test_single_block_notification_is_delivered_inline():
+    sess, calls = _make_session(lambda request: [])
+    try:
+        tok = sess.subscribe(["oven", "vs", "0"])
+        sess.conn.inject(_notification(tok, b"\xa1\x01\x02"))
+
+        assert _wait_for(lambda: calls)
+        assert calls == [("/oven/vs/0", b"\xa1\x01\x02")]
+        # The subscribe GET is the only thing we sent — no refetch.
+        assert len(sess.conn.sent) == 1
+    finally:
+        _close(sess)
+
+
+def test_complete_block_zero_notification_is_delivered_inline():
+    """Block2 present but M=0 and NUM=0 means the whole representation
+    fit in one block. Nothing to fetch back."""
+    sess, calls = _make_session(lambda request: [])
+    try:
+        tok = sess.subscribe(["oven", "vs", "0"])
+        sess.conn.inject(
+            _notification(tok, b"\xa1\x01\x02",
+                          block2=block_value(0, 0, SZX)))
+
+        assert _wait_for(lambda: calls)
+        assert calls == [("/oven/vs/0", b"\xa1\x01\x02")]
+        assert len(sess.conn.sent) == 1
+    finally:
+        _close(sess)
+
+
+# --------------------------------------------------------------------
+# The fix
+
+
+def test_truncated_notification_is_refetched_and_reassembled():
+    blocks = [b"A" * 1024, b"B" * 40]
+
+    def responder(request):
+        _mtype, code, mid, tok, opts, _ = request
+        if code != METHOD_GET or any(n == OBSERVE for n, _ in opts):
+            return []           # the subscribe registration itself
+        num, _ = _requested_block(request)
+        more = 1 if num + 1 < len(blocks) else 0
+        return [_content(tok, mid, blocks[num],
+                         block2=block_value(num, more, SZX))]
+
+    sess, calls = _make_session(responder)
+    try:
+        tok = sess.subscribe(["mode", "vs", "0"])
+        sess.conn.inject(
+            _notification(tok, blocks[0],
+                          block2=block_value(0, 1, SZX)))
+
+        assert _wait_for(lambda: calls), "callback never fired"
+        assert calls == [("/mode/vs/0", b"".join(blocks))]
+    finally:
+        _close(sess)
+
+
+def test_refetch_uses_a_fresh_one_shot_token_not_the_observe_token():
+    """RFC 7959 §3.4: the requests for additional blocks cannot use the
+    token of the observation relationship."""
+    blocks = [b"A" * 1024, b"B" * 40]
+
+    def responder(request):
+        _mtype, _code, mid, tok, opts, _ = request
+        if any(n == OBSERVE for n, _ in opts):
+            return []
+        num, _ = _requested_block(request)
+        more = 1 if num + 1 < len(blocks) else 0
+        return [_content(tok, mid, blocks[num],
+                         block2=block_value(num, more, SZX))]
+
+    sess, calls = _make_session(responder)
+    try:
+        observe_tok = sess.subscribe(["mode", "vs", "0"])
+        assert len(observe_tok) == 1, "OBSERVE registrations use 1-byte tokens"
+        sess.conn.inject(
+            _notification(observe_tok, blocks[0],
+                          block2=block_value(0, 1, SZX)))
+        assert _wait_for(lambda: calls)
+
+        refetch = [r for r in sess.conn.sent
+                   if not any(n == OBSERVE for n, _ in r[4])]
+        assert refetch, "no refetch request was sent"
+        tokens = {r[3] for r in refetch}
+        assert observe_tok not in tokens
+        assert all(len(t) == 4 for t in tokens), "one-shot tokens are 4-byte"
+        assert len(tokens) == 1, "the transfer must hold one token throughout"
+
+        # And the transfer restarts at block 0 rather than continuing at 1.
+        assert _requested_block(refetch[0])[0] == 0
+        assert [_requested_block(r)[0] for r in refetch] == [0, 1]
+        # No Observe option on a continuation request.
+        assert not any(n == OBSERVE for r in refetch for n, _ in r[4])
+    finally:
+        _close(sess)
+
+
+def test_silent_device_drops_the_notification_without_delivering_a_partial():
+    sess, calls = _make_session(lambda request: [])
+    try:
+        tok = sess.subscribe(["mode", "vs", "0"])
+        sess.conn.inject(
+            _notification(tok, b"A" * 1024,
+                          block2=block_value(0, 1, SZX)))
+        # Give the worker a chance to try and fail. _BLOCK_ACK_TIMEOUT is
+        # 4s per attempt, so we only need to see that nothing partial got
+        # through in the meantime.
+        assert not _wait_for(lambda: calls, timeout=0.6)
+        assert sess._reader_thread.is_alive(), "reader must survive"
+    finally:
+        _close(sess)
+
+
+def test_non_2xx_refetch_is_dropped_rather_than_delivered():
+    def responder(request):
+        _mtype, _code, mid, tok, opts, _ = request
+        if any(n == OBSERVE for n, _ in opts):
+            return []
+        return [build_coap(TYPE_ACK, 0x84, mid, tok, [], b"")]
+
+    sess, calls = _make_session(responder)
+    try:
+        tok = sess.subscribe(["mode", "vs", "0"])
+        sess.conn.inject(
+            _notification(tok, b"A" * 1024,
+                          block2=block_value(0, 1, SZX)))
+        assert not _wait_for(lambda: calls, timeout=0.6)
+    finally:
+        _close(sess)
+
+
+def test_notification_burst_collapses_to_one_refetch_per_resource():
+    blocks = [b"A" * 1024, b"B" * 40]
+    gate = threading.Event()
+
+    def responder(request):
+        _mtype, _code, mid, tok, opts, _ = request
+        if any(n == OBSERVE for n, _ in opts):
+            return []
+        gate.wait(2.0)          # hold the first transfer open
+        num, _ = _requested_block(request)
+        more = 1 if num + 1 < len(blocks) else 0
+        return [_content(tok, mid, blocks[num],
+                         block2=block_value(num, more, SZX))]
+
+    sess, calls = _make_session(responder)
+    try:
+        tok = sess.subscribe(["mode", "vs", "0"])
+        for seq in range(5):
+            sess.conn.inject(
+                _notification(tok, blocks[0], obs=seq + 1,
+                              block2=block_value(0, 1, SZX)))
+        # Five notifications, one queue entry: latest wins per href.
+        assert _wait_for(lambda: sess._refetch_pending or sess.conn.sent[1:])
+        assert len(sess._refetch_pending) <= 1
+        gate.set()
+
+        assert _wait_for(lambda: calls)
+        assert _wait_for(
+            lambda: not sess._refetch_pending and len(calls) >= 1)
+        time.sleep(0.2)
+        # Two transfers at most: the one in flight when the burst landed,
+        # plus one for the final state.
+        starts = [r for r in sess.conn.sent
+                  if not any(n == OBSERVE for n, _ in r[4])
+                  and _requested_block(r)[0] == 0]
+        assert len(starts) <= 2, f"{len(starts)} refetches for one burst"
+        assert calls[-1] == ("/mode/vs/0", b"".join(blocks))
+    finally:
+        gate.set()
+        _close(sess)
+
+
+def test_close_during_a_queued_refetch_stops_the_worker():
+    sess, _calls = _make_session(lambda request: [])
+    tok = sess.subscribe(["mode", "vs", "0"])
+    sess.conn.inject(
+        _notification(tok, b"A" * 1024, block2=block_value(0, 1, SZX)))
+    assert _wait_for(lambda: sess._refetch_thread is not None)
+
+    sess.close()
+    sess.join()     # hangs if the worker outlives the session
+    assert not sess._refetch_thread.is_alive()
+
+
+def test_refetch_worker_exits_when_the_reader_dies():
+    sess, _calls = _make_session(lambda request: [])
+    tok = sess.subscribe(["mode", "vs", "0"])
+    sess.conn.inject(
+        _notification(tok, b"A" * 1024, block2=block_value(0, 1, SZX)))
+    assert _wait_for(lambda: sess._refetch_thread is not None)
+
+    # Kill the reader the way a socket error does, without close().
+    sess.sock.closed = True
+    assert _wait_for(lambda: not sess._reader_running.is_set(), timeout=5.0)
+    sess._refetch_thread.join(6.0)
+    assert not sess._refetch_thread.is_alive()
+    sess.close()
+
+
+def test_debug_bridge_promotes_the_refetch_outcome_to_info(monkeypatch, caplog):
+    """The hardware validation for #39 reads this line to confirm which
+    token the re-read used, so it has to survive the bridge's INFO
+    default. Without DEBUG_BRIDGE it stays at debug."""
+    monkeypatch.setattr(dtls_session, "DEBUG_BRIDGE", True)
+    blocks = [b"A" * 1024, b"B" * 40]
+
+    def responder(request):
+        _mtype, _code, mid, tok, opts, _ = request
+        if any(n == OBSERVE for n, _ in opts):
+            return []
+        num, _ = _requested_block(request)
+        more = 1 if num + 1 < len(blocks) else 0
+        return [_content(tok, mid, blocks[num],
+                         block2=block_value(num, more, SZX))]
+
+    sess, calls = _make_session(responder)
+    try:
+        with caplog.at_level(logging.INFO, logger=_LOGGER_NAME):
+            tok = sess.subscribe(["mode", "vs", "0"])
+            sess.conn.inject(
+                _notification(tok, blocks[0], block2=block_value(0, 1, SZX)))
+            assert _wait_for(lambda: calls)
+
+        line = next((r.getMessage() for r in caplog.records
+                     if r.getMessage().startswith("refetch /mode/vs/0")), None)
+        assert line is not None, "no refetch line at INFO"
+        assert "blocks=2" in line
+        assert f"bytes={sum(len(b) for b in blocks)}" in line
+        assert line.endswith("ok")
+        # The token in the line is the one-shot token, not the observe one.
+        assert f"tok={tok.hex()} " not in line
+    finally:
+        _close(sess)
+
+
+# --------------------------------------------------------------------
+# Shared Block2 loop hardening
+
+
+def test_stale_block_number_is_not_concatenated():
+    """A retransmit of block 0 arriving while we wait for block 1 must
+    not be appended as if it were block 1."""
+    served = []
+
+    def responder(request):
+        _mtype, _code, mid, tok, _opts, _ = request
+        num, _ = _requested_block(request)
+        served.append(num)
+        if num == 0:
+            return [_content(tok, mid, b"A" * 1024,
+                             block2=block_value(0, 1, SZX))]
+        # Answer the block-1 request with a duplicate of block 0 first.
+        return [
+            _content(tok, mid, b"A" * 1024, block2=block_value(0, 1, SZX)),
+            _content(tok, mid, b"B" * 40, block2=block_value(1, 0, SZX)),
+        ]
+
+    sess, _calls = _make_session(responder)
+    try:
+        code, payload = sess.get(["mode", "vs", "0"], timeout=5.0)
+        assert code == 0x45
+        assert payload == b"A" * 1024 + b"B" * 40
+    finally:
+        _close(sess)
+
+
+def test_etag_change_mid_transfer_restarts_then_fails():
+    etags = [b"\x01", b"\x02", b"\x03", b"\x04"]
+
+    def responder(request):
+        _mtype, _code, mid, tok, _opts, _ = request
+        num, _ = _requested_block(request)
+        # A different ETag on every single response: the representation
+        # never settles, so reassembly can never be consistent.
+        etag = etags.pop(0) if etags else b"\xff"
+        payload = b"A" * 1024 if num == 0 else b"B" * 40
+        more = 1 if num == 0 else 0
+        return [_content(tok, mid, payload, etag=etag,
+                         block2=block_value(num, more, SZX))]
+
+    sess, _calls = _make_session(responder)
+    try:
+        with pytest.raises(BlockwiseError):
+            sess.get(["mode", "vs", "0"], timeout=5.0)
+    finally:
+        _close(sess)
+
+
+def test_stable_etag_across_blocks_reassembles():
+    def responder(request):
+        _mtype, _code, mid, tok, _opts, _ = request
+        num, _ = _requested_block(request)
+        payload = b"A" * 1024 if num == 0 else b"B" * 40
+        more = 1 if num == 0 else 0
+        return [_content(tok, mid, payload, etag=b"\x77",
+                         block2=block_value(num, more, SZX))]
+
+    sess, _calls = _make_session(responder)
+    try:
+        code, payload = sess.get(["mode", "vs", "0"], timeout=5.0)
+        assert code == 0x45
+        assert payload == b"A" * 1024 + b"B" * 40
+    finally:
+        _close(sess)
+
+
+def test_szx_downshift_asks_for_the_block_after_what_we_have():
+    """Server answers block 0 at SZX=6 (1024B) then drops to SZX=4
+    (256B). Block numbers index the new size, so the next request is
+    block 4, not block 1."""
+    requested = []
+
+    def responder(request):
+        _mtype, _code, mid, tok, _opts, _ = request
+        num, szx = _requested_block(request)
+        requested.append((num, szx))
+        if num == 0:
+            return [_content(tok, mid, b"A" * 1024,
+                             block2=block_value(0, 1, 4))]
+        return [_content(tok, mid, b"B" * 100,
+                         block2=block_value(num, 0, 4))]
+
+    sess, _calls = _make_session(responder)
+    try:
+        code, payload = sess.get(["mode", "vs", "0"], timeout=5.0)
+        assert code == 0x45
+        assert payload == b"A" * 1024 + b"B" * 100
+        # 1024 bytes in hand at 256B blocks = blocks 0..3 done, ask for 4.
+        assert requested[1] == (4, 4)
+    finally:
+        _close(sess)

tests/test_oven_flatten.py

@@ -0,0 +1,86 @@
+"""Oven descriptor flatten() contracts for the HA Number entity's range.
+
+The oven reports ``x.com.samsung.da.desired = 0`` whenever no cycle is
+set. That is "no setpoint", not a 0 °C target, and publishing it as one
+makes Home Assistant reject every state message against the Number
+entity's declared 30-270 range.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from mqtt_demo.samples import oven
+
+
+def _links(desired, current=180):
+    """A /temperatures/vs/0 link tree carrying one desired/current pair."""
+    return {
+        '/temperatures/vs/0': {
+            'x.com.samsung.da.items': [{
+                'x.com.samsung.da.current': str(current),
+                'x.com.samsung.da.desired': str(desired),
+            }],
+        },
+    }
+
+
+@pytest.mark.parametrize('desired', [
+    oven.SETPOINT_MIN_C,
+    oven.SETPOINT_MIN_C + oven.SETPOINT_STEP_C,
+    180,
+    oven.SETPOINT_MAX_C,
+])
+def test_settable_setpoints_are_published_unchanged(desired):
+    assert oven.flatten(_links(desired))['target_temp_c'] == desired
+
+
+@pytest.mark.parametrize('desired', [
+    0,                              # the idle oven; see module docstring
+    oven.SETPOINT_MIN_C - 1,
+    oven.SETPOINT_MAX_C + 1,
+])
+def test_unsettable_setpoints_are_published_as_absent(desired):
+    assert oven.flatten(_links(desired))['target_temp_c'] is None
+
+
+def test_out_of_range_setpoint_does_not_suppress_current_temperature():
+    """The guard applies to the setpoint alone. A cooling oven still
+    reports its cavity temperature after the cycle ends."""
+    sensors = oven.flatten(_links(0, current=210))
+
+    assert sensors['target_temp_c'] is None
+    assert sensors['current_temp_c'] == 210
+
+
+def test_missing_temperature_resource_leaves_both_absent():
+    sensors = oven.flatten({})
+
+    assert sensors['target_temp_c'] is None
+    assert sensors['current_temp_c'] is None
+
+
+def test_every_committed_write_is_a_value_flatten_will_publish():
+    """The write path snaps to the step grid *before* bounds-checking, so
+    it accepts more than flatten() publishes: 29 commits as 30, and 271 as
+    270. That is fine for a slider, but it means the two range checks are
+    not symmetric. What has to hold is the weaker invariant: any setpoint
+    the oven is actually told to adopt is one flatten() will show back,
+    otherwise a write appears to succeed and then reads as unknown."""
+    handler = oven.command_handlers()[oven.CMD_SETPOINT]
+
+    for requested in range(-20, oven.SETPOINT_MAX_C + 40):
+        write = handler(str(requested), _links(180))
+        if write is None:
+            continue
+        _path, body = write
+        committed = int(body['x.com.samsung.da.items'][0][
+            'x.com.samsung.da.desired'])
+        assert oven.flatten(_links(committed))['target_temp_c'] == committed
+
+
+def test_zero_is_rejected_on_the_write_path_too():
+    """0 is the one value that neither snaps into range nor publishes."""
+    handler = oven.command_handlers()[oven.CMD_SETPOINT]
+
+    assert handler('0', _links(180)) is None

tests/test_owner_psk.py

@@ -0,0 +1,144 @@
+"""IoTivity manufacturer-certificate OwnerPSK derivation contracts."""
+
+from __future__ import annotations
+
+import pytest
+
+from smartthings_local.protocol.owner_psk import (
+    CONFIRMED_MFG_CERTIFICATE_OXM_LABEL,
+    MFG_CERTIFICATE_KEY_BLOCK_LENGTHS,
+    STANDARD_MFG_CERTIFICATE_OXM_LABEL,
+    derive_mfg_certificate_owner_psk,
+)
+
+
+_VALID_INPUTS = {
+    "master_secret": bytes(range(48)),
+    "client_random": bytes(range(32)),
+    "server_random": bytes(range(32, 64)),
+    "owner_uuid": bytes.fromhex("00112233445566778899aabbccddeeff"),
+    "device_uuid": bytes.fromhex("ffeeddccbbaa99887766554433221100"),
+    "cipher_name": "ECDHE-ECDSA-AES128-GCM-SHA256",
+    "oxm_label": CONFIRMED_MFG_CERTIFICATE_OXM_LABEL,
+}
+
+
+# These synthetic expected values were generated from the fixed inputs above;
+# they were not captured from IoTivity or a device. They lock deterministic
+# output for the selected mappings, while the table test below states the
+# key-block-length contract explicitly.
+def test_fixed_synthetic_gcm_regression_vector():
+    assert derive_mfg_certificate_owner_psk(**_VALID_INPUTS).hex() == (
+        "ccd6c618a91290dee8c106544ed79a33"
+    )
+
+
+def test_owner_then_device_uuid_order_matches_iotivity_callers():
+    reversed_context = derive_mfg_certificate_owner_psk(
+        **{
+            **_VALID_INPUTS,
+            "owner_uuid": _VALID_INPUTS["device_uuid"],
+            "device_uuid": _VALID_INPUTS["owner_uuid"],
+        }
+    )
+
+    assert reversed_context.hex() == "8f0f2416c483546dc1806db769b21b68"
+    assert reversed_context != derive_mfg_certificate_owner_psk(**_VALID_INPUTS)
+
+
+def test_fixed_synthetic_ccm8_regression_vector():
+    inputs = {
+        **_VALID_INPUTS,
+        "cipher_name": "ECDHE-ECDSA-AES128-CCM8",
+    }
+    assert derive_mfg_certificate_owner_psk(**inputs).hex() == (
+        "ddd3d945e266ee3dc27ff3a2c4321d32"
+    )
+
+
+def test_standard_and_confirmed_labels_derive_distinct_keys():
+    confirmed = derive_mfg_certificate_owner_psk(**_VALID_INPUTS)
+    standard = derive_mfg_certificate_owner_psk(
+        **{**_VALID_INPUTS, "oxm_label": STANDARD_MFG_CERTIFICATE_OXM_LABEL}
+    )
+
+    assert standard.hex() == "26ee1fe4c3e74509a2f5db5ab41b1e47"
+    assert standard != confirmed
+
+
+def test_iotivity_cipher_key_block_lengths_are_immutable():
+    assert dict(MFG_CERTIFICATE_KEY_BLOCK_LENGTHS) == {
+        "ECDHE-ECDSA-AES128-SHA256": 96,
+        "ECDHE-ECDSA-AES128-CCM": 40,
+        "ECDHE-ECDSA-AES128-CCM8": 40,
+        "ECDHE-ECDSA-AES128-GCM-SHA256": 120,
+        "AES256-SHA256": 128,
+        "ECDHE-ECDSA-AES256-SHA384": 160,
+        "ECDHE-ECDSA-AES256-GCM-SHA384": 184,
+        "AES128-GCM-SHA256": 120,
+    }
+    with pytest.raises(TypeError):
+        MFG_CERTIFICATE_KEY_BLOCK_LENGTHS["new-cipher"] = 1
+
+
+@pytest.mark.parametrize(
+    ("field", "length"),
+    [
+        ("master_secret", 48),
+        ("client_random", 32),
+        ("server_random", 32),
+        ("owner_uuid", 16),
+        ("device_uuid", 16),
+    ],
+)
+def test_binary_inputs_require_exact_bytes_and_lengths(field, length):
+    with pytest.raises(TypeError, match=f"{field} must be bytes"):
+        derive_mfg_certificate_owner_psk(
+            **{**_VALID_INPUTS, field: bytearray(length)}
+        )
+    for invalid_length in (length - 1, length + 1):
+        with pytest.raises(ValueError, match=f"exactly {length} bytes"):
+            derive_mfg_certificate_owner_psk(
+                **{**_VALID_INPUTS, field: b"x" * invalid_length}
+            )
+
+
+@pytest.mark.parametrize("field", ["owner_uuid", "device_uuid"])
+def test_nil_uuid_is_rejected(field):
+    with pytest.raises(ValueError, match="must not be the nil UUID"):
+        derive_mfg_certificate_owner_psk(
+            **{**_VALID_INPUTS, field: bytes(16)}
+        )
+
+
+def test_cipher_and_label_must_be_explicit_supported_values():
+    with pytest.raises(TypeError, match="cipher_name must be a string"):
+        derive_mfg_certificate_owner_psk(
+            **{**_VALID_INPUTS, "cipher_name": b"cipher"}
+        )
+    with pytest.raises(ValueError, match="unexpected.*cipher"):
+        derive_mfg_certificate_owner_psk(
+            **{**_VALID_INPUTS, "cipher_name": "ECDHE-RSA-AES128-GCM-SHA256"}
+        )
+    with pytest.raises(TypeError, match="oxm_label must be bytes"):
+        derive_mfg_certificate_owner_psk(
+            **{**_VALID_INPUTS, "oxm_label": "oic.sec.doxm.mfgcert"}
+        )
+    with pytest.raises(ValueError, match="unexpected.*label"):
+        derive_mfg_certificate_owner_psk(
+            **{**_VALID_INPUTS, "oxm_label": b"unsupported"}
+        )
+
+
+def test_failures_do_not_include_key_material():
+    key_material = b"private-master-secret"
+    with pytest.raises(ValueError) as raised:
+        derive_mfg_certificate_owner_psk(
+            **{
+                **_VALID_INPUTS,
+                "master_secret": key_material,
+                "cipher_name": "unsupported",
+            }
+        )
+    assert key_material.hex() not in str(raised.value)
+    assert "private-master-secret" not in str(raised.value)

tests/test_psk_auth.py

@@ -0,0 +1,380 @@
+from __future__ import annotations
+
+import gc
+import traceback
+import weakref
+from concurrent.futures import ThreadPoolExecutor
+from dataclasses import asdict
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+import pytest
+from OpenSSL import SSL
+
+from smartthings_local.errors import SessionError
+from smartthings_local.protocol import auth as auth_module
+from smartthings_local.protocol import dtls_session as session_module
+from smartthings_local.protocol.auth import PskAuth
+from smartthings_local.protocol.dtls_session import DtlsCoapSession
+
+_IDENTITY = b"i" * 16
+_KEY = b"k" * 16
+_OTHER_IDENTITY = b"j" * 16
+_OTHER_KEY = b"l" * 32
+
+
+class _BytesSubclass(bytes):
+    pass
+
+
+def _fake_openssl_util(setter):
+    return SimpleNamespace(
+        ffi=auth_module._util.ffi,
+        lib=SimpleNamespace(SSL_CTX_set_psk_client_callback=setter),
+    )
+
+
+def _invoke_callback(callback, identity_size: int, key_size: int):
+    ffi = auth_module._util.ffi
+    identity_buffer = ffi.new("char[]", max(identity_size, 1))
+    key_buffer = ffi.new("unsigned char[]", max(key_size, 1))
+    copied = callback(
+        ffi.NULL,
+        ffi.NULL,
+        identity_buffer,
+        identity_size,
+        key_buffer,
+        key_size,
+    )
+    return (
+        copied,
+        bytes(ffi.buffer(identity_buffer, max(identity_size, 1))),
+        bytes(ffi.buffer(key_buffer, max(key_size, 1))),
+    )
+
+
+@pytest.mark.parametrize("key_length", [16, 32])
+def test_psk_auth_accepts_exact_supported_credential_lengths(key_length):
+    provider = PskAuth(identity=_IDENTITY, key=b"k" * key_length)
+
+    assert repr(provider) == "PskAuth()"
+
+
+@pytest.mark.parametrize(
+    ("identity", "key"),
+    [
+        ("i" * 16, _KEY),
+        (bytearray(_IDENTITY), _KEY),
+        (memoryview(_IDENTITY), _KEY),
+        (_BytesSubclass(_IDENTITY), _KEY),
+        (_IDENTITY, "k" * 16),
+        (_IDENTITY, bytearray(_KEY)),
+        (_IDENTITY, memoryview(_KEY)),
+        (_IDENTITY, _BytesSubclass(_KEY)),
+    ],
+)
+def test_psk_auth_rejects_non_bytes_credentials(identity, key):
+    with pytest.raises(TypeError, match="identity and key must be bytes"):
+        PskAuth(identity=identity, key=key)
+
+
+@pytest.mark.parametrize("identity_length", [0, 15, 17])
+def test_psk_auth_rejects_invalid_identity_lengths(identity_length):
+    with pytest.raises(ValueError, match="raw 16-byte OCF UUID"):
+        PskAuth(identity=b"i" * identity_length, key=_KEY)
+
+
+def test_psk_auth_rejects_identity_with_nul_byte():
+    with pytest.raises(ValueError, match="cannot contain a NUL"):
+        PskAuth(identity=b"i" * 15 + b"\x00", key=_KEY)
+
+
+@pytest.mark.parametrize("key_length", [0, 15, 17, 31, 33])
+def test_psk_auth_rejects_invalid_key_lengths(key_length):
+    with pytest.raises(ValueError, match="16 or 32 bytes"):
+        PskAuth(identity=_IDENTITY, key=b"k" * key_length)
+
+
+def test_psk_auth_is_immutable_and_has_no_public_credential_surface():
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+
+    rendered = repr(provider)
+    assert rendered == "PskAuth()"
+    assert str(provider) == rendered
+    assert _IDENTITY.decode() not in rendered
+    assert _KEY.decode() not in rendered
+    assert not hasattr(provider, "identity")
+    assert not hasattr(provider, "key")
+    assert not hasattr(provider, "_identity")
+    assert not hasattr(provider, "_key")
+    with pytest.raises(TypeError):
+        vars(provider)
+    with pytest.raises(TypeError):
+        asdict(provider)
+    with pytest.raises(AttributeError, match="immutable"):
+        provider.identity = _OTHER_IDENTITY
+    with pytest.raises(AttributeError, match="immutable"):
+        del provider._callback
+
+
+def test_psk_auth_identity_equality_does_not_compare_credentials():
+    first = PskAuth(identity=_IDENTITY, key=_KEY)
+    second = PskAuth(identity=_IDENTITY, key=_KEY)
+
+    assert first != second
+    assert len({first, second}) == 2
+
+
+def test_psk_callback_copies_exact_identity_and_key():
+    installed = {}
+
+    def setter(context_handle, callback):
+        installed["context"] = context_handle
+        installed["callback"] = callback
+
+    context_handle = object()
+    context = MagicMock()
+    context._context = context_handle
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+
+    with patch.object(auth_module, "_util", _fake_openssl_util(setter)):
+        provider.configure_context(context)
+
+    assert installed["context"] is context_handle
+    callback = installed["callback"]
+    copied, identity_bytes, key_bytes = _invoke_callback(callback, 17, 16)
+    assert copied == 16
+    assert identity_bytes == _IDENTITY + b"\x00"
+    assert key_bytes == _KEY
+    context.set_cipher_list.assert_called_once_with(
+        b"ECDHE-PSK-AES128-CBC-SHA256:@SECLEVEL=0"
+    )
+    context.load_verify_locations.assert_not_called()
+    context.set_verify.assert_not_called()
+
+
+@pytest.mark.parametrize(
+    ("identity_size", "key_size"),
+    [(16, 16), (17, 15)],
+)
+def test_psk_callback_rejects_short_buffers_without_partial_copy(
+    identity_size,
+    key_size,
+):
+    installed = {}
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    context = MagicMock()
+    context._context = object()
+
+    with patch.object(
+        auth_module,
+        "_util",
+        _fake_openssl_util(
+            lambda _context, callback: installed.setdefault(
+                "callback", callback
+            )
+        ),
+    ):
+        provider.configure_context(context)
+
+    ffi = auth_module._util.ffi
+    identity_buffer = ffi.new("char[]", 17)
+    key_buffer = ffi.new("unsigned char[]", 16)
+    ffi.memmove(identity_buffer, b"I" * 17, 17)
+    ffi.memmove(key_buffer, b"K" * 16, 16)
+    copied = installed["callback"](
+        ffi.NULL,
+        ffi.NULL,
+        identity_buffer,
+        identity_size,
+        key_buffer,
+        key_size,
+    )
+
+    assert copied == 0
+    assert bytes(ffi.buffer(identity_buffer, 17)) == b"I" * 17
+    assert bytes(ffi.buffer(key_buffer, 16)) == b"K" * 16
+
+
+@pytest.mark.parametrize("null_buffer", ["identity", "key"])
+def test_psk_callback_rejects_null_buffers(null_buffer):
+    installed = {}
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    context = MagicMock()
+    context._context = object()
+
+    with patch.object(
+        auth_module,
+        "_util",
+        _fake_openssl_util(
+            lambda _context, callback: installed.setdefault(
+                "callback", callback
+            )
+        ),
+    ):
+        provider.configure_context(context)
+
+    ffi = auth_module._util.ffi
+    identity_buffer = ffi.new("char[]", 17)
+    key_buffer = ffi.new("unsigned char[]", 16)
+    ffi.memmove(identity_buffer, b"I" * 17, 17)
+    ffi.memmove(key_buffer, b"K" * 16, 16)
+    if null_buffer == "identity":
+        identity_buffer = ffi.NULL
+    else:
+        key_buffer = ffi.NULL
+
+    copied = installed["callback"](
+        ffi.NULL,
+        ffi.NULL,
+        identity_buffer,
+        17,
+        key_buffer,
+        16,
+    )
+    assert copied == 0
+    if null_buffer == "identity":
+        assert bytes(ffi.buffer(key_buffer, 16)) == b"K" * 16
+    else:
+        assert bytes(ffi.buffer(identity_buffer, 17)) == b"I" * 17
+
+
+def test_psk_auth_unsupported_binding_error_contains_no_credentials():
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    context = MagicMock()
+    context._context = object()
+    unsupported_util = SimpleNamespace(
+        ffi=auth_module._util.ffi,
+        lib=SimpleNamespace(),
+    )
+
+    with (
+        patch.object(auth_module, "_util", unsupported_util),
+        pytest.raises(RuntimeError) as captured,
+    ):
+        provider.configure_context(context)
+
+    rendered = (
+        str(captured.value)
+        + repr(captured.value)
+        + "".join(traceback.format_exception(captured.value))
+    )
+    assert _IDENTITY.decode() not in rendered
+    assert _KEY.decode() not in rendered
+    context.set_cipher_list.assert_not_called()
+
+
+def test_psk_auth_configures_real_openssl_context():
+    context = SSL.Context(SSL.DTLS_METHOD)
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+
+    assert provider.configure_context(context) is None
+
+
+def test_distinct_psk_providers_do_not_share_callback_credentials():
+    callbacks = []
+
+    def setter(_context, callback):
+        callbacks.append(callback)
+
+    first = PskAuth(identity=_IDENTITY, key=_KEY)
+    second = PskAuth(identity=_OTHER_IDENTITY, key=_OTHER_KEY)
+    first_context = MagicMock()
+    first_context._context = object()
+    second_context = MagicMock()
+    second_context._context = object()
+
+    with patch.object(auth_module, "_util", _fake_openssl_util(setter)):
+        first.configure_context(first_context)
+        second.configure_context(second_context)
+
+    assert callbacks[0] is not callbacks[1]
+    with ThreadPoolExecutor(max_workers=2) as executor:
+        first_future = executor.submit(_invoke_callback, callbacks[0], 17, 16)
+        second_future = executor.submit(
+            _invoke_callback,
+            callbacks[1],
+            17,
+            32,
+        )
+        first_result = first_future.result()
+        second_result = second_future.result()
+    assert first_result == (16, _IDENTITY + b"\x00", _KEY)
+    assert second_result == (32, _OTHER_IDENTITY + b"\x00", _OTHER_KEY)
+
+
+def test_session_retains_psk_callback_only_with_provider_lifetime():
+    callback_reference = None
+
+    def setter(_context, callback):
+        nonlocal callback_reference
+        callback_reference = weakref.ref(callback)
+
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    session = DtlsCoapSession(
+        "appliance.invalid",
+        49154,
+        auth=provider,
+    )
+    context = MagicMock()
+    context._context = object()
+    with patch.object(auth_module, "_util", _fake_openssl_util(setter)):
+        session.auth.configure_context(context)
+
+    del provider
+    gc.collect()
+    assert callback_reference is not None
+    assert callback_reference() is not None
+    assert _invoke_callback(callback_reference(), 17, 16) == (
+        16,
+        _IDENTITY + b"\x00",
+        _KEY,
+    )
+
+    del session
+    gc.collect()
+    assert callback_reference() is None
+
+
+def test_session_accepts_psk_provider_without_legacy_certificate_material():
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    session = DtlsCoapSession("appliance.invalid", 49154, auth=provider)
+
+    assert session.auth is provider
+    assert session.cert_path is None
+    assert session.key_path is None
+    assert session.cert_pem is None
+    assert session.key_pem is None
+
+
+def test_psk_handshake_rejection_does_not_expose_credentials():
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    session = DtlsCoapSession("appliance.invalid", 49154, auth=provider)
+    context = MagicMock()
+    context._context = object()
+    connection = MagicMock()
+    connection.do_handshake.side_effect = SSL.Error()
+    udp_socket = MagicMock()
+    endpoint = SimpleNamespace(sockaddr=("192.0.2.100", 49154))
+
+    with (
+        patch.object(auth_module, "_util", _fake_openssl_util(lambda *_: None)),
+        patch.object(session_module.SSL, "Context", return_value=context),
+        patch.object(session_module.SSL, "Connection", return_value=connection),
+        patch.object(
+            session_module,
+            "open_connected_udp_socket",
+            return_value=(udp_socket, endpoint),
+        ),
+        pytest.raises(SessionError) as captured,
+    ):
+        session.connect()
+
+    rendered = (
+        str(captured.value)
+        + repr(captured.value)
+        + "".join(traceback.format_exception(captured.value))
+    )
+    assert _IDENTITY.decode() not in rendered
+    assert _KEY.decode() not in rendered
+    udp_socket.close.assert_called_once_with()

tests/test_public_api_contract.py

@@ -6,8 +6,19 @@ import inspect
 
 from smartthings_local.ocf.observe_refresh import ObserveRefreshTask
 from smartthings_local.ocf.state_cache import StateCache
-from smartthings_local.protocol.auth import AuthenticationProvider, CertificateAuth
-from smartthings_local.protocol.dtls_session import DtlsCoapSession
+from smartthings_local.protocol.auth import (
+    AuthenticationProvider,
+    CertificateAuth,
+    PskAuth,
+    SamsungServerProfile,
+    SamsungServerRole,
+    ServerCertificateAuth,
+)
+from smartthings_local.protocol.dtls_session import (
+    ConnectCancellation,
+    DtlsCoapSession,
+)
+from smartthings_local.protocol.owner_psk import derive_mfg_certificate_owner_psk
 
 
 def _assert_compatible_signature(callable_object, expected: list[str]) -> None:
@@ -50,6 +61,79 @@ def test_certificate_auth_is_a_public_authentication_provider():
     provider = CertificateAuth.from_files("/synthetic/cert.pem", "/synthetic/key")
     assert isinstance(provider, AuthenticationProvider)
 
+    for factory in (CertificateAuth.from_files, CertificateAuth.from_memory):
+        profile_parameter = inspect.signature(factory).parameters["server_profile"]
+        assert profile_parameter.kind is inspect.Parameter.KEYWORD_ONLY
+        assert profile_parameter.default is None
+
+
+def test_samsung_server_profile_is_public_and_explicitly_bound():
+    parameters = inspect.signature(SamsungServerProfile.bound_device).parameters
+    assert list(parameters) == [
+        "expected_certificate_identity",
+        "role",
+        "additional_ca_pem",
+    ]
+    assert (
+        parameters["expected_certificate_identity"].default
+        is inspect.Parameter.empty
+    )
+    assert parameters["role"].kind is inspect.Parameter.KEYWORD_ONLY
+    assert parameters["role"].default is SamsungServerRole.HOME_APPLIANCE
+    assert parameters["additional_ca_pem"].kind is inspect.Parameter.KEYWORD_ONLY
+    assert parameters["additional_ca_pem"].default is None
+
+
+def test_server_certificate_auth_is_a_public_authentication_provider():
+    profile = SamsungServerProfile.bound_device(
+        "abababab-abab-abab-abab-abababababab",
+        role=SamsungServerRole.VD_DEVICE,
+    )
+    provider = ServerCertificateAuth(server_profile=profile)
+    assert isinstance(provider, AuthenticationProvider)
+    session = DtlsCoapSession("device.example", 5684, auth=provider)
+    assert session.auth is provider
+    assert session.cert_path is None
+    assert session.key_path is None
+    assert session.cert_pem is None
+    assert session.key_pem is None
+    parameters = inspect.signature(ServerCertificateAuth).parameters
+    assert list(parameters) == ["server_profile"]
+    assert parameters["server_profile"].kind is inspect.Parameter.KEYWORD_ONLY
+    assert parameters["server_profile"].default is inspect.Parameter.empty
+
+
+def test_psk_auth_is_a_public_authentication_provider():
+    provider = PskAuth(identity=b"i" * 16, key=b"k" * 16)
+    assert isinstance(provider, AuthenticationProvider)
+    parameters = inspect.signature(PskAuth).parameters
+    assert list(parameters) == ["identity", "key"]
+    assert all(
+        parameter.kind is inspect.Parameter.KEYWORD_ONLY
+        and parameter.default is inspect.Parameter.empty
+        for parameter in parameters.values()
+    )
+
+
+def test_owner_psk_derivation_keeps_every_security_input_explicit():
+    parameters = inspect.signature(
+        derive_mfg_certificate_owner_psk
+    ).parameters
+    assert list(parameters) == [
+        "master_secret",
+        "client_random",
+        "server_random",
+        "owner_uuid",
+        "device_uuid",
+        "cipher_name",
+        "oxm_label",
+    ]
+    assert all(
+        parameter.kind is inspect.Parameter.KEYWORD_ONLY
+        and parameter.default is inspect.Parameter.empty
+        for parameter in parameters.values()
+    )
+
 
 def test_dtls_session_keeps_current_consumer_methods():
     expected = {
@@ -65,6 +149,20 @@ def test_dtls_session_keeps_current_consumer_methods():
         "subscribe",
     }
     assert expected <= set(dir(DtlsCoapSession))
+    assert "abort" not in DtlsCoapSession.__dict__
+    assert "quiesce_for_close" not in DtlsCoapSession.__dict__
+    _assert_compatible_signature(DtlsCoapSession.connect, ["self"])
+    connect_timeout = inspect.signature(DtlsCoapSession.connect).parameters[
+        "timeout"
+    ]
+    assert connect_timeout.kind is inspect.Parameter.KEYWORD_ONLY
+    assert connect_timeout.default is None
+    connect_cancel = inspect.signature(DtlsCoapSession.connect).parameters[
+        "cancel"
+    ]
+    assert connect_cancel.kind is inspect.Parameter.KEYWORD_ONLY
+    assert connect_cancel.default is None
+    assert callable(ConnectCancellation().set)
     _assert_compatible_signature(
         DtlsCoapSession.get,
         [

tests/test_request_pacing.py

@@ -0,0 +1,169 @@
+"""Session-owned pacing for request sends."""
+
+from __future__ import annotations
+
+from unittest.mock import Mock
+
+import pytest
+
+from smartthings_local.errors import SessionClosedError
+from smartthings_local.protocol import dtls_session
+from smartthings_local.protocol.coap import (
+    METHOD_GET,
+    METHOD_POST,
+    OBSERVE,
+    TYPE_ACK,
+    TYPE_CON,
+    build_coap,
+    parse_coap,
+)
+from smartthings_local.protocol.dtls_session import DtlsCoapSession
+
+
+class _NullAuth:
+    def configure_context(self, _context):
+        return None
+
+
+def _session():
+    session = DtlsCoapSession(
+        "device.example",
+        5684,
+        auth=_NullAuth(),
+        rate_limit_rps=1_000_000,
+    )
+    session.conn = object()
+    return session
+
+
+def test_first_get_post_and_subscribe_are_paced_before_send():
+    session = _session()
+    order = []
+    requests = []
+
+    def pace():
+        order.append("pace")
+
+    def send(datagram):
+        order.append("send")
+        request = parse_coap(datagram)
+        requests.append(request)
+        _mtype, _code, mid, token, options, _payload = request
+        if any(number == OBSERVE for number, _value in options):
+            assert session._observe_tokens[token] == "/mode/vs/0"
+            return
+        session._dispatch_coap(
+            build_coap(TYPE_ACK, 0x45, mid, token, [], b"ok")
+        )
+
+    session.pace = pace
+    session._send_dgram = send
+
+    assert session.get(["device", "0"]) == (0x45, b"ok")
+    assert session.post(["mode", "vs", "0"], b"payload") == (0x45, b"ok")
+    observe_token = session.subscribe(["mode", "vs", "0"])
+
+    assert session._observe_tokens[observe_token] == "/mode/vs/0"
+    assert order == ["pace", "send", "pace", "send", "pace", "send"]
+    assert [request[1] for request in requests] == [
+        METHOD_GET,
+        METHOD_POST,
+        METHOD_GET,
+    ]
+
+
+def test_every_subscribe_in_registration_burst_honors_rate_limit(monkeypatch):
+    session = _session()
+    now = [100.0]
+    waits = []
+    sends = []
+
+    class StopEvent:
+        def wait(self, delay):
+            waits.append(delay)
+            now[0] += delay
+
+    def send(datagram):
+        sends.append(datagram)
+        session._last_send_ts = now[0]
+
+    monkeypatch.setattr(dtls_session.time, "monotonic", lambda: now[0])
+    session._stop = StopEvent()
+    session._min_req_interval = 0.2
+    session._last_send_ts = 0.0
+    session._send_dgram = send
+
+    for index in range(11):
+        session.subscribe(["resource", "vs", str(index)])
+
+    assert len(sends) == 11
+    assert waits == [pytest.approx(0.2)] * 10
+
+
+def test_existing_caller_pacing_before_subscribe_does_not_wait_twice(
+    monkeypatch,
+):
+    session = _session()
+    now = [100.05]
+    waits = []
+
+    class StopEvent:
+        def wait(self, delay):
+            waits.append(delay)
+            now[0] += delay
+
+    monkeypatch.setattr(dtls_session.time, "monotonic", lambda: now[0])
+    session._stop = StopEvent()
+    session._min_req_interval = 0.2
+    session._last_send_ts = 100.0
+    session._send_dgram = Mock()
+
+    session.pace()
+    session.subscribe(["mode", "vs", "0"])
+
+    assert waits == [pytest.approx(0.15)]
+    session._send_dgram.assert_called_once()
+
+
+def test_subscribe_rechecks_liveness_after_pacing_before_registering():
+    session = _session()
+    session._send_dgram = Mock()
+
+    def close_during_pacing():
+        session.conn = None
+
+    session.pace = close_during_pacing
+
+    with pytest.raises(SessionClosedError):
+        session.subscribe(["mode", "vs", "0"])
+
+    assert session._observe_tokens == {}
+    session._send_dgram.assert_not_called()
+
+
+def test_ack_ping_and_observe_deregister_are_not_paced():
+    session = _session()
+    session.pace = Mock(side_effect=AssertionError("control send was paced"))
+
+    class Connection:
+        def __init__(self):
+            self.sent = []
+
+        def send(self, datagram):
+            self.sent.append(datagram)
+
+        def bio_read(self, _size):
+            return b""
+
+    connection = Connection()
+    session.conn = connection
+
+    session.ping()
+    session._send_observe_dereg(b"\x40", ["mode", "vs", "0"])
+    session._dispatch_coap(
+        build_coap(TYPE_CON, 0x45, 0x1234, b"unknown", [], b"state")
+    )
+
+    session.pace.assert_not_called()
+    assert len(connection.sent) == 3
+    assert parse_coap(connection.sent[-1])[:4] == (TYPE_ACK, 0, 0x1234, b"")

tests/test_session_connect_deadline.py

@@ -0,0 +1,348 @@
+"""Deterministic tests for bounded DTLS handshake timing."""
+
+from __future__ import annotations
+
+import socket
+
+import pytest
+from OpenSSL import SSL
+
+from smartthings_local.errors import SessionTimeoutError
+from smartthings_local.protocol import dtls_session
+from smartthings_local.protocol.dtls_session import DtlsCoapSession
+from smartthings_local.protocol.endpoint import ResolvedUdpEndpoint
+
+
+class _Clock:
+    def __init__(self):
+        self.now = 100.0
+
+    def monotonic(self):
+        return self.now
+
+    def advance(self, seconds):
+        self.now += seconds
+
+
+class _Auth:
+    def __init__(self, clock=None, configure_delay=0.0):
+        self.clock = clock
+        self.configure_delay = configure_delay
+
+    def configure_context(self, _context):
+        if self.clock is not None:
+            self.clock.advance(self.configure_delay)
+
+
+class _Connection:
+    def __init__(self, outcomes=None, outputs=None, timer=None):
+        self.outcomes = list(outcomes or ())
+        self.outputs = list(outputs or ())
+        self.timer = timer
+        self.bio_writes = []
+        self.timeout_calls = 0
+
+    def set_connect_state(self):
+        return None
+
+    def set_ciphertext_mtu(self, _mtu):
+        return None
+
+    def do_handshake(self):
+        outcome = self.outcomes.pop(0) if self.outcomes else "want-read"
+        if outcome == "want-read":
+            raise SSL.WantReadError()
+        if isinstance(outcome, Exception):
+            raise outcome
+
+    def bio_read(self, _size):
+        if self.outputs:
+            return self.outputs.pop(0)
+        raise SSL.WantReadError()
+
+    def bio_write(self, data):
+        self.bio_writes.append(data)
+
+    def DTLSv1_get_timeout(self):
+        return self.timer
+
+    def DTLSv1_handle_timeout(self):
+        self.timeout_calls += 1
+
+
+class _Socket:
+    def __init__(self, clock, inbound=()):
+        self.clock = clock
+        self.inbound = list(inbound)
+        self.timeouts = []
+        self.sent = []
+        self.closed = False
+
+    def settimeout(self, timeout):
+        self.timeouts.append(timeout)
+
+    def send(self, data):
+        self.sent.append(data)
+        return len(data)
+
+    def recv(self, _size):
+        if self.inbound:
+            result = self.inbound.pop(0)
+            if isinstance(result, Exception):
+                self.clock.advance(self.timeouts[-1])
+                raise result
+            return result
+        self.clock.advance(self.timeouts[-1])
+        raise TimeoutError()
+
+    def close(self):
+        self.closed = True
+
+
+def _session(auth=None):
+    return DtlsCoapSession(
+        "device.example",
+        5684,
+        auth=auth or _Auth(),
+    )
+
+
+def _install_handshake(
+    monkeypatch,
+    clock,
+    *,
+    outcomes=(),
+    outputs=(),
+    inbound=(),
+    timer=None,
+):
+    connection = _Connection(outcomes, outputs, timer)
+    sock = _Socket(clock, inbound)
+    endpoint = ResolvedUdpEndpoint(
+        socket.AF_INET,
+        ("192.0.2.10", 5684),
+    )
+    open_calls = []
+
+    def open_socket(*args, **kwargs):
+        open_calls.append((args, kwargs))
+        sock.settimeout(kwargs["timeout"])
+        return sock, endpoint
+
+    monkeypatch.setattr(dtls_session.SSL, "Context", lambda *_args: object())
+    monkeypatch.setattr(
+        dtls_session.SSL,
+        "Connection",
+        lambda *_args: connection,
+    )
+    monkeypatch.setattr(
+        dtls_session,
+        "open_connected_udp_socket",
+        open_socket,
+    )
+    monkeypatch.setattr(dtls_session.time, "monotonic", clock.monotonic)
+    monkeypatch.setattr(dtls_session.time, "sleep", clock.advance)
+    monkeypatch.setattr(
+        dtls_session.time,
+        "time",
+        lambda: pytest.fail("wall clock must not control handshake deadlines"),
+    )
+    return connection, sock, endpoint, open_calls
+
+
+@pytest.mark.parametrize("timeout", (True, "1", object()))
+def test_connect_timeout_type_is_explicit(timeout):
+    with pytest.raises(TypeError, match="number or None"):
+        _session().connect(timeout=timeout)
+
+
+@pytest.mark.parametrize(
+    "timeout",
+    (
+        0,
+        -1,
+        float("nan"),
+        float("inf"),
+        float("-inf"),
+        10**1000,
+    ),
+)
+def test_connect_timeout_must_be_positive_and_finite(timeout):
+    with pytest.raises(ValueError, match="positive finite"):
+        _session().connect(timeout=timeout)
+
+
+def test_connect_timeout_caps_every_blocking_poll(monkeypatch):
+    clock = _Clock()
+    _connection, sock, _endpoint, open_calls = _install_handshake(
+        monkeypatch,
+        clock,
+    )
+    session = _session()
+
+    with pytest.raises(SessionTimeoutError):
+        session.connect(timeout=4.75)
+
+    assert clock.now == pytest.approx(104.75)
+    assert sock.closed
+    assert open_calls == [
+        (
+            ("device.example", 5684),
+            {
+                "family": socket.AF_UNSPEC,
+                "local_port": None,
+                "timeout": 0.5,
+            },
+        )
+    ]
+    assert max(sock.timeouts) <= 0.5
+    assert sock.timeouts[-1] == pytest.approx(0.25)
+
+
+def test_short_timeout_is_not_rounded_up_to_poll_interval(monkeypatch):
+    clock = _Clock()
+    _connection, sock, _endpoint, open_calls = _install_handshake(
+        monkeypatch,
+        clock,
+    )
+
+    with pytest.raises(SessionTimeoutError):
+        _session().connect(timeout=0.125)
+
+    assert clock.now == pytest.approx(100.125)
+    assert open_calls[0][1]["timeout"] == pytest.approx(0.125)
+    assert sock.timeouts == pytest.approx([0.125, 0.125])
+
+
+def test_default_timeout_uses_session_constant(monkeypatch):
+    clock = _Clock()
+    _connection, sock, _endpoint, _open_calls = _install_handshake(
+        monkeypatch,
+        clock,
+    )
+    session = _session()
+    session.HANDSHAKE_TIMEOUT_S = 0.2
+
+    with pytest.raises(SessionTimeoutError):
+        session.connect()
+
+    assert clock.now == pytest.approx(100.2)
+    assert sock.closed
+
+
+def test_context_setup_consumes_the_same_deadline(monkeypatch):
+    clock = _Clock()
+    socket_opened = False
+
+    def open_socket(*_args, **_kwargs):
+        nonlocal socket_opened
+        socket_opened = True
+        raise AssertionError("expired setup must not open a socket")
+
+    monkeypatch.setattr(dtls_session.SSL, "Context", lambda *_args: object())
+    monkeypatch.setattr(dtls_session.SSL, "Connection", lambda *_args: _Connection())
+    monkeypatch.setattr(dtls_session, "open_connected_udp_socket", open_socket)
+    monkeypatch.setattr(dtls_session.time, "monotonic", clock.monotonic)
+    session = _session(_Auth(clock, configure_delay=0.2))
+
+    with pytest.raises(SessionTimeoutError):
+        session.connect(timeout=0.1)
+
+    assert not socket_opened
+
+
+def test_socket_setup_consumes_the_same_deadline(monkeypatch):
+    clock = _Clock()
+    connection = _Connection()
+    connection.do_handshake = lambda: pytest.fail(
+        "expired socket setup must not start a handshake"
+    )
+    sock = _Socket(clock)
+    endpoint = ResolvedUdpEndpoint(
+        socket.AF_INET,
+        ("192.0.2.10", 5684),
+    )
+
+    def open_socket(*_args, **kwargs):
+        sock.settimeout(kwargs["timeout"])
+        clock.advance(0.2)
+        return sock, endpoint
+
+    monkeypatch.setattr(dtls_session.SSL, "Context", lambda *_args: object())
+    monkeypatch.setattr(
+        dtls_session.SSL,
+        "Connection",
+        lambda *_args: connection,
+    )
+    monkeypatch.setattr(dtls_session, "open_connected_udp_socket", open_socket)
+    monkeypatch.setattr(dtls_session.time, "monotonic", clock.monotonic)
+
+    with pytest.raises(SessionTimeoutError):
+        _session().connect(timeout=0.1)
+
+    assert sock.closed
+
+
+def test_successful_handshake_preserves_connected_session_state(monkeypatch):
+    clock = _Clock()
+    connection, sock, endpoint, _open_calls = _install_handshake(
+        monkeypatch,
+        clock,
+        outcomes=("want-read", "success"),
+        inbound=(b"synthetic server flight",),
+    )
+    session = _session()
+
+    session.connect(timeout=1.0)
+
+    assert connection.bio_writes == [b"synthetic server flight"]
+    assert session.conn is connection
+    assert session.sock is sock
+    assert session.endpoint is endpoint
+    assert session.dest == endpoint.sockaddr
+    assert not sock.closed
+
+
+def test_connect_services_openssl_retransmit_timer(monkeypatch):
+    clock = _Clock()
+    outbound = b"\x16\xfe\xfd" + b"\x00" * 8 + b"\x00\x01x"
+    connection, sock, _endpoint, _open_calls = _install_handshake(
+        monkeypatch,
+        clock,
+        outcomes=("want-read", "want-read", "success"),
+        outputs=(outbound, outbound),
+        inbound=(TimeoutError(), b"synthetic server flight"),
+        timer=0.0,
+    )
+
+    _session().connect(timeout=2.0)
+
+    assert connection.timeout_calls == 1
+    assert sock.sent == [outbound, outbound]
+    assert connection.bio_writes == [b"synthetic server flight"]
+
+
+@pytest.mark.parametrize("success_delay", (0.1, 0.2))
+def test_handshake_success_at_or_after_deadline_is_retained(
+    monkeypatch,
+    success_delay,
+):
+    clock = _Clock()
+    connection, sock, endpoint, _open_calls = _install_handshake(
+        monkeypatch,
+        clock,
+    )
+
+    def late_success():
+        clock.advance(success_delay)
+
+    connection.do_handshake = late_success
+
+    session = _session()
+    session.connect(timeout=0.1)
+
+    assert session.conn is connection
+    assert session.sock is sock
+    assert session.endpoint is endpoint
+    assert session.dest == endpoint.sockaddr
+    assert not sock.closed

tests/test_session_interruption.py

@@ -0,0 +1,316 @@
+"""Deterministic tests for connection-attempt cancellation."""
+
+from __future__ import annotations
+
+import select
+import socket
+import threading
+import time
+import traceback
+
+import pytest
+from OpenSSL import SSL
+
+from smartthings_local.errors import SessionClosedError, SessionError
+from smartthings_local.protocol import dtls_session
+from smartthings_local.protocol.dtls_session import (
+    ConnectCancellation,
+    DtlsCoapSession,
+)
+from smartthings_local.protocol.endpoint import ResolvedUdpEndpoint
+
+
+class _Auth:
+    def __init__(self, on_configure=None):
+        self.on_configure = on_configure
+
+    def configure_context(self, _context):
+        if self.on_configure is not None:
+            self.on_configure()
+
+
+class _Connection:
+    def __init__(self, *, started=None, on_success=None, succeed=False):
+        self.started = started
+        self.on_success = on_success
+        self.succeed = succeed
+        self.bio_writes = []
+        self.handshake_calls = 0
+
+    def set_connect_state(self):
+        return None
+
+    def set_ciphertext_mtu(self, _mtu):
+        return None
+
+    def do_handshake(self):
+        self.handshake_calls += 1
+        if self.started is not None:
+            self.started.set()
+        if self.succeed:
+            if self.on_success is not None:
+                self.on_success()
+            return
+        raise SSL.WantReadError()
+
+    def bio_read(self, _size):
+        raise SSL.WantReadError()
+
+    def bio_write(self, datagram):
+        self.bio_writes.append(datagram)
+        self.succeed = True
+
+    def DTLSv1_get_timeout(self):
+        return None
+
+    def shutdown(self):
+        return None
+
+
+def _session(auth=None):
+    return DtlsCoapSession(
+        "device.example",
+        5684,
+        auth=auth or _Auth(),
+    )
+
+
+def _install_connection(monkeypatch, connection, data_socket, *, on_open=None):
+    endpoint = ResolvedUdpEndpoint(
+        socket.AF_INET,
+        ("192.0.2.10", 5684),
+    )
+
+    def open_socket(*_args, **_kwargs):
+        if on_open is not None:
+            on_open()
+        return data_socket, endpoint
+
+    monkeypatch.setattr(dtls_session.SSL, "Context", lambda *_args: object())
+    monkeypatch.setattr(
+        dtls_session.SSL,
+        "Connection",
+        lambda *_args: connection,
+    )
+    monkeypatch.setattr(
+        dtls_session,
+        "open_connected_udp_socket",
+        open_socket,
+    )
+    return endpoint
+
+
+def _run_connect(session, cancel):
+    outcome = {}
+
+    def worker():
+        try:
+            session.connect(timeout=2.0, cancel=cancel)
+        except Exception as error:  # noqa: BLE001 - captured for assertion
+            outcome["error"] = error
+        else:
+            outcome["connected"] = True
+
+    thread = threading.Thread(target=worker)
+    thread.start()
+    return thread, outcome
+
+
+@pytest.mark.parametrize("cancel", (True, threading.Event(), object(), "signal"))
+def test_connect_cancel_type_is_explicit(cancel):
+    with pytest.raises(TypeError, match="ConnectCancellation or None"):
+        _session().connect(cancel=cancel)
+
+
+def test_pre_cancelled_connect_stops_before_context_setup(monkeypatch):
+    cancel = ConnectCancellation()
+    cancel.set()
+    monkeypatch.setattr(
+        dtls_session.SSL,
+        "Context",
+        lambda *_args: pytest.fail("cancelled connect configured TLS"),
+    )
+
+    with pytest.raises(SessionClosedError):
+        _session().connect(cancel=cancel)
+
+
+def test_cancel_during_context_setup_stops_before_socket_setup(monkeypatch):
+    cancel = ConnectCancellation()
+    monkeypatch.setattr(dtls_session.SSL, "Context", lambda *_args: object())
+    monkeypatch.setattr(
+        dtls_session,
+        "open_connected_udp_socket",
+        lambda *_args, **_kwargs: pytest.fail(
+            "cancelled connect opened a socket"
+        ),
+    )
+
+    with pytest.raises(SessionClosedError):
+        _session(_Auth(cancel.set)).connect(cancel=cancel)
+
+
+def test_cancel_during_socket_setup_closes_before_handshake(monkeypatch):
+    cancel = ConnectCancellation()
+    connection = _Connection(succeed=True)
+    data_socket, peer = socket.socketpair()
+    _install_connection(
+        monkeypatch,
+        connection,
+        data_socket,
+        on_open=cancel.set,
+    )
+
+    try:
+        with pytest.raises(SessionClosedError):
+            _session().connect(cancel=cancel)
+        assert data_socket.fileno() == -1
+        assert connection.handshake_calls == 0
+    finally:
+        peer.close()
+
+
+def test_socket_signal_wakes_every_subscribed_waiter():
+    cancel = ConnectCancellation()
+    first = cancel._subscribe()
+    second = cancel._subscribe()
+    try:
+        cancel.set()
+        readable, _, _ = select.select(
+            (first[0], second[0]),
+            (),
+            (),
+            0,
+        )
+        assert set(readable) == {first[0], second[0]}
+        assert cancel._unsubscribe(*first)
+        assert cancel._unsubscribe(*second)
+    finally:
+        for reader, writer in (first, second):
+            reader.close()
+            writer.close()
+
+
+def test_cancel_wakes_blocked_connect_without_poll_latency(monkeypatch):
+    cancel = ConnectCancellation()
+    started = threading.Event()
+    connection = _Connection(started=started)
+    data_socket, peer = socket.socketpair()
+    _install_connection(monkeypatch, connection, data_socket)
+    session = _session()
+    thread, outcome = _run_connect(session, cancel)
+
+    try:
+        assert started.wait(1.0)
+        before = time.monotonic()
+        cancel.set()
+        thread.join(1.0)
+        elapsed = time.monotonic() - before
+
+        assert not thread.is_alive()
+        assert elapsed < 0.25
+        assert isinstance(outcome.get("error"), SessionClosedError)
+        assert data_socket.fileno() == -1
+        assert session.sock is None
+        assert session.conn is None
+        assert not cancel._writers
+    finally:
+        cancel.set()
+        thread.join(1.0)
+        peer.close()
+
+
+def test_reported_handshake_success_wins_cancel_during_unsubscribe(
+    monkeypatch,
+):
+    class CancelDuringUnsubscribe(ConnectCancellation):
+        def _unsubscribe(self, reader, writer):
+            self.set()
+            return super()._unsubscribe(reader, writer)
+
+    cancel = CancelDuringUnsubscribe()
+    connection = _Connection(succeed=True)
+    data_socket, peer = socket.socketpair()
+    endpoint = _install_connection(monkeypatch, connection, data_socket)
+    session = _session()
+
+    try:
+        session.connect(cancel=cancel)
+        assert cancel.is_set()
+        assert session.sock is data_socket
+        assert session.conn is connection
+        assert session.endpoint is endpoint
+        assert session.dest == endpoint.sockaddr
+        assert not cancel._writers
+    finally:
+        session.close()
+        peer.close()
+
+
+def test_cancel_during_backend_failure_does_not_read_unset_completion(
+    monkeypatch,
+):
+    cancel = ConnectCancellation()
+    connection = _Connection()
+
+    def fail_after_cancel():
+        cancel.set()
+        raise SSL.Error("synthetic backend failure")
+
+    connection.do_handshake = fail_after_cancel
+    data_socket, peer = socket.socketpair()
+    _install_connection(monkeypatch, connection, data_socket)
+    session = _session()
+
+    try:
+        with pytest.raises(SessionClosedError):
+            session.connect(cancel=cancel)
+        assert data_socket.fileno() == -1
+        assert session.sock is None
+        assert session.conn is None
+        assert not cancel._writers
+    finally:
+        peer.close()
+
+
+def test_successful_connect_does_not_set_cancel_or_close_session(monkeypatch):
+    cancel = ConnectCancellation()
+    connection = _Connection()
+    data_socket, peer = socket.socketpair()
+    endpoint = _install_connection(monkeypatch, connection, data_socket)
+    peer.send(b"synthetic server flight")
+    session = _session()
+
+    try:
+        session.connect(cancel=cancel)
+        assert not cancel.is_set()
+        assert session.sock is data_socket
+        assert session.conn is connection
+        assert session.endpoint is endpoint
+        assert connection.bio_writes == [b"synthetic server flight"]
+        assert not cancel._writers
+    finally:
+        session.close()
+        peer.close()
+
+
+def test_cancellation_socket_failure_is_redacted_and_closes_udp(monkeypatch):
+    class FailingCancellation(ConnectCancellation):
+        def _subscribe(self):
+            raise OSError("credential-value at device.example")
+
+    connection = _Connection()
+    data_socket, peer = socket.socketpair()
+    _install_connection(monkeypatch, connection, data_socket)
+
+    try:
+        with pytest.raises(SessionError) as exc:
+            _session().connect(cancel=FailingCancellation())
+
+        formatted = "".join(traceback.format_exception(exc.value))
+        assert data_socket.fileno() == -1
+        assert exc.value.__context__ is None
+        assert "credential-value" not in formatted
+        assert "device.example" not in formatted
+    finally:
+        peer.close()

tests/test_worker_cleanup.py

@@ -3,7 +3,12 @@
 from __future__ import annotations
 
 import threading
+from types import SimpleNamespace
 
+import pytest
+
+from mqtt_demo import bridge as bridge_module
+from mqtt_demo.bridge import PushBridge
 from smartthings_local.ocf.keepalive import KeepaliveTask
 from smartthings_local.ocf.observe_refresh import ObserveRefreshTask
 from smartthings_local.ocf.poll_scheduler import PollScheduler, PollTier
@@ -73,3 +78,238 @@ def test_poll_scheduler_worker_stops_without_leaking_thread():
         [PollTier("idle", interval_s=3600.0, paths=())],
     )
     _assert_worker_stops(scheduler.run_forever, "test-poll-scheduler")
+
+
+class _SessionWorker:
+    def __init__(self, *args, **kwargs):
+        self.stop = None
+        self.started = threading.Event()
+        self.exited = threading.Event()
+        self.on_reachable = kwargs.get("on_reachable")
+        self.on_unreachable = kwargs.get("on_unreachable")
+        self.last_success_ts = 0.0
+
+    def run_forever(self, stop):
+        self.stop = stop
+        self.started.set()
+        stop.wait()
+        self.exited.set()
+
+
+class _JoinedSession:
+    def __init__(self, workers, start_index, error=None):
+        self.workers = workers
+        self.start_index = start_index
+        self.error = error
+
+    def join(self):
+        assert all(
+            worker.started.wait(_THREAD_DEADLINE_S)
+            for worker in self.workers[self.start_index:]
+        )
+        if self.error is not None:
+            raise self.error
+
+
+class _BlockingJoinedSession(_JoinedSession):
+    def __init__(self, workers):
+        super().__init__(workers, 0)
+        self.joined = threading.Event()
+        self.release = threading.Event()
+
+    def join(self):
+        super().join()
+        self.joined.set()
+        assert self.release.wait(_THREAD_DEADLINE_S)
+
+
+def _bridge():
+    bridge = object.__new__(PushBridge)
+    bridge.descriptor = SimpleNamespace(
+        observe_paths=(),
+        poll_tiers=[],
+        is_active=lambda _state: False,
+    )
+    bridge.shared = SimpleNamespace(PING_INTERVAL_S=3600.0)
+    bridge.app = SimpleNamespace(klass="test")
+    bridge.log = SimpleNamespace(info=lambda *args: None, warning=lambda *args: None)
+    bridge.cache = SimpleNamespace(links={})
+    bridge.stop = threading.Event()
+    bridge._session_stop_lock = threading.Lock()
+    bridge._session_stop = None
+    bridge.scheduler = None
+    bridge.keepalive = None
+    bridge.observe_refresh = None
+    bridge._seed_from_device0 = lambda _session: None
+    bridge._retag_logger_with_serial = lambda: None
+    bridge.maybe_publish_state = lambda **kwargs: None
+    bridge.set_availability = lambda _online: None
+    return bridge
+
+
+@pytest.mark.parametrize(
+    ("session_count", "join_error"),
+    [
+        pytest.param(2, None, id="reconnect"),
+        pytest.param(1, RuntimeError("reader failed"), id="reader-error"),
+    ],
+)
+def test_bridge_retires_session_workers_before_returning(
+    monkeypatch, session_count, join_error
+):
+    workers = []
+
+    def worker_factory(*args, **kwargs):
+        worker = _SessionWorker(*args, **kwargs)
+        workers.append(worker)
+        return worker
+
+    monkeypatch.setattr(bridge_module, "PollScheduler", worker_factory)
+    monkeypatch.setattr(bridge_module, "KeepaliveTask", worker_factory)
+    monkeypatch.setattr(bridge_module, "ObserveRefreshTask", worker_factory)
+
+    bridge = _bridge()
+
+    session_stops = []
+    for session_index in range(session_count):
+        start_index = len(workers)
+        session = _JoinedSession(
+            workers,
+            start_index,
+            error=join_error if session_index == session_count - 1 else None,
+        )
+        if session.error is None:
+            bridge._run_session_inner(session)
+        else:
+            with pytest.raises(RuntimeError, match="reader failed"):
+                bridge._run_session_inner(session)
+
+        session_workers = workers[start_index:]
+        assert len(session_workers) == 3
+        assert len({id(worker.stop) for worker in session_workers}) == 1
+        session_stops.append(session_workers[0].stop)
+
+    assert len(workers) == session_count * 3
+    assert len({id(stop) for stop in session_stops}) == session_count
+    assert all(stop is not bridge.stop for stop in session_stops)
+    assert all(stop.is_set() for stop in session_stops)
+    assert not bridge.stop.is_set()
+    assert all(worker.exited.is_set() for worker in workers)
+    keepalive_workers = tuple(
+        workers[index] for index in range(1, len(workers), 3)
+    )
+    assert all(worker.on_reachable is None for worker in keepalive_workers)
+    assert all(worker.on_unreachable is None for worker in keepalive_workers)
+    assert bridge.scheduler is None
+    assert bridge.keepalive is None
+    assert bridge.observe_refresh is None
+    assert bridge._session_stop is None
+
+
+def test_bridge_retires_started_worker_when_later_thread_fails_to_start(
+    monkeypatch,
+):
+    workers = []
+
+    def worker_factory(*args, **kwargs):
+        worker = _SessionWorker(*args, **kwargs)
+        workers.append(worker)
+        return worker
+
+    monkeypatch.setattr(bridge_module, "PollScheduler", worker_factory)
+    monkeypatch.setattr(bridge_module, "KeepaliveTask", worker_factory)
+    monkeypatch.setattr(bridge_module, "ObserveRefreshTask", worker_factory)
+
+    bridge = _bridge()
+
+    original_start = threading.Thread.start
+    start_count = 0
+
+    def fail_second_start(thread):
+        nonlocal start_count
+        start_count += 1
+        if start_count == 2:
+            raise RuntimeError("synthetic thread start failure")
+        original_start(thread)
+
+    monkeypatch.setattr(threading.Thread, "start", fail_second_start)
+
+    with pytest.raises(RuntimeError, match="synthetic thread start failure"):
+        bridge._run_session_inner(SimpleNamespace(join=lambda: None))
+
+    assert len(workers) == 3
+    assert workers[0].started.wait(_THREAD_DEADLINE_S)
+    assert workers[0].exited.wait(_THREAD_DEADLINE_S)
+    assert workers[0].stop is not bridge.stop
+    assert workers[0].stop.is_set()
+    assert workers[1].stop is None
+    assert workers[2].stop is None
+    assert bridge.scheduler is None
+    assert bridge.keepalive is None
+    assert bridge.observe_refresh is None
+    assert bridge._session_stop is None
+
+
+def test_request_stop_wakes_current_session_workers(monkeypatch):
+    workers = []
+
+    def worker_factory(*args, **kwargs):
+        worker = _SessionWorker(*args, **kwargs)
+        workers.append(worker)
+        return worker
+
+    monkeypatch.setattr(bridge_module, "PollScheduler", worker_factory)
+    monkeypatch.setattr(bridge_module, "KeepaliveTask", worker_factory)
+    monkeypatch.setattr(bridge_module, "ObserveRefreshTask", worker_factory)
+
+    bridge = _bridge()
+    session = _BlockingJoinedSession(workers)
+    session_thread = threading.Thread(
+        target=bridge._run_session_inner,
+        args=(session,),
+        daemon=True,
+    )
+    session_thread.start()
+
+    try:
+        assert session.joined.wait(_THREAD_DEADLINE_S)
+        session_stop = bridge._session_stop
+        assert session_stop is not None
+        assert not session_stop.is_set()
+
+        bridge.request_stop()
+
+        assert bridge.stop.is_set()
+        assert session_stop.is_set()
+        assert all(
+            worker.exited.wait(_THREAD_DEADLINE_S) for worker in workers
+        )
+        assert session_thread.is_alive()
+    finally:
+        session.release.set()
+        session_thread.join(_THREAD_DEADLINE_S)
+
+    assert not session_thread.is_alive()
+    assert bridge._session_stop is None
+
+
+def test_session_workers_observe_stop_requested_before_handoff(monkeypatch):
+    workers = []
+
+    def worker_factory(*args, **kwargs):
+        worker = _SessionWorker(*args, **kwargs)
+        workers.append(worker)
+        return worker
+
+    monkeypatch.setattr(bridge_module, "PollScheduler", worker_factory)
+    monkeypatch.setattr(bridge_module, "KeepaliveTask", worker_factory)
+    monkeypatch.setattr(bridge_module, "ObserveRefreshTask", worker_factory)
+
+    bridge = _bridge()
+    bridge.request_stop()
+    bridge._run_session_inner(_JoinedSession(workers, 0))
+
+    assert len(workers) == 3
+    assert all(worker.stop.is_set() for worker in workers)
+    assert all(worker.exited.wait(_THREAD_DEADLINE_S) for worker in workers)
+    assert bridge._session_stop is None