A poll-failure-triggered reconnect while in observe mode left OBSERVE
silently dead: the new session had zero subscriptions, mode stayed
'observe' so the hot/warm sub-poll fallback stayed disabled, and the
ObserveRefreshTask kept retrying against the closed session instead of
the live one. Downgrading to poll mode on reconnect hands recovery to
the existing poll-mode retry path, which re-subscribes on the new
session.
Also snapshot ObserveManager._notified before intersecting it against
the subscribed set, closing a rare set-mutated-during-iteration race
between the DTLS reader thread (on_notification) and the executor
thread computing the grace-period success fraction.
Marc Billow
7024b170fe