_session_task_is_stale() failed to detect a stale session lock when the owner
task completed and cleaned _session_tasks (del in _process_message_background's
finally) but _active_sessions was NOT released because _release_session_guard
skipped on a guard mismatch (a concurrent reset/new command or drain handoff
swapped _active_sessions[key] to a different guard). With no owner task left to
inspect, _session_task_is_stale reported 'not stale', the orphaned guard was
never healed, and the session deadlocked permanently — later messages received
but never dispatched.
Reorder the finally cleanup to release-then-conditional-delete: release the
guard first, then drop the _session_tasks entry ONLY if the guard was actually
released (session_key no longer in _active_sessions). On a guard mismatch the
done-task entry survives, so the on-entry self-heal (_session_task_is_stale ->
_heal_stale_session_lock) detects the stale lock and clears it on the next
inbound message.
Extracted the cleanup into a callable _cleanup_finished_session_task() helper so
the regression test drives the REAL production code path rather than a copy of
its logic (the original test inlined the fixed logic and passed regardless of
the production order — mutation-verified the rewritten tests now fail on the
buggy del-first order). Added a positive-path test (guard matches -> release +
delete) so both branches are pinned.
Co-authored-by: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>
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