The #63309 hang class — gateway stuck at 'Connecting to Telegram
(attempt 1/8)' with no retry, no timeout, for minutes — can only occur
when the event loop thread itself is blocked in a synchronous call:
_await_with_thread_deadline's timer fires off-loop, but its expiry
hand-off (call_soon_threadsafe) still needs the loop to run, and the
gateway's outer wait_for is a pure loop timer. When the loop is pinned,
every layer goes silent simultaneously and the process wedges with no
evidence of where.
Two changes:
1. Loop-blocked watchdog in _await_with_thread_deadline: a second
daemon timer fires one grace period (5s) after the deadline; if the
loop still hasn't processed the expiry, it logs a WARNING from the
timer thread and faulthandler-dumps all thread stacks to stderr —
converting the silent hang into a trace that names the exact
blocking frame. A threading.Event set by the expiry callback (and on
normal exit) keeps completed awaits from ever being misreported.
2. discover_fallback_ips: the system-resolver leg runs
socket.getaddrinfo in a worker thread with no timeout, and
asyncio.gather waited on it unboundedly — a wedged OS resolver
stalled discovery for minutes between the two startup log lines. Its
result only feeds a log message, so it no longer gates discovery:
DoH legs (already client-bounded) are gathered alone and the system
leg is awaited with a _DOH_TIMEOUT cap, best-effort.
Refs #63309
Tests: 3 watchdog regressions (blocked-loop dump fires; responsive-loop
timeout does not; completed await does not) + 2 hung-resolver
regressions (DoH results returned promptly; worst-case seed fallback
stays bounded).
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