Three robustness/hardening fixes surfaced by /sh-security-review on the
agent-team listener/coordinator surface. None alter AUTHZ-01 allowlist
behavior or the first-answer-wins compare-and-set semantics.
1. Respawn close() leak (CWE-772). The watchdog _supervise_slack_listener
respawned the inbound Slack listener without tearing down the dead one,
leaking a Socket Mode WebSocket / SDK thread set per flap. Now the dead
listener is closed before respawn (new _close_dead_listener, idempotent),
AND _run_listener has a finally that always closes the listener so a
crashed serve() releases its socket. SlackListener.close() is idempotent,
so the belt-and-braces close stays a safe no-op.
2. Broadened exception guard in handle_event (CWE-248). The submit block
only caught ValueError; the accept path (submit_answer ->
_question_turn/_question_thread) can raise KeyError on a concurrently
mutated row, and the CAS can raise sqlite3.Error. An uncaught exception
would escape into the Bolt dispatch. Added a separate `except Exception`
that logs at WARNING (not silent, not debug) and returns None. The
existing ValueError-as-debug behavior is unchanged; authorization still
runs first, so the trust boundary is not widened.
3. channel_ref partial-unique index (defense-in-depth). Added
uq_pending_questions_open_channel_ref — a PARTIAL UNIQUE index on
(channel_ref) WHERE channel_ref IS NOT NULL AND status='open' — so two
OPEN rows can never share a non-null channel_ref (a thread_ts can never
map to two open questions). Installed in init_db AND unconditionally in
migrate (idempotent IF NOT EXISTS) so existing v1 DBs gain it. NULLs and
closed rows are excluded; mirrored verbatim into schema.sql.
Tests: +8 (was 960, now 968). New: schema partial-unique reject/null/closed/
migrate cases; handle_event KeyError + sqlite3.Error swallow cases;
coordinator close-before-respawn + run_listener-closes-on-crash. Fixed the
operator-cli test fixture to use a per-question channel_ref (it previously
inserted multiple open rows sharing one ref, which the new index correctly
rejects).
