test(celery): arm faulthandler at fixture scope, not in run_beat

Previous attempt put faulthandler.dump_traceback_later inside run_beat,
but the py3.7 CI hang turns out to happen earlier — start_worker() never
returns, so run_beat is never reached and the dump is never armed.

Move the diagnostic into an autouse fixture in
tests/integrations/celery/integration_tests/conftest.py so it covers the
entire test body. Next CI hang should land a thread dump in the log
showing where start_worker is wedged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sl0thentr0py

tests/integrations/celery/integration_tests/__init__.py

@@ -1,4 +1,3 @@
-import faulthandler
 import os
 import signal
 import sys
@@ -66,12 +65,6 @@ def run_beat(celery_app, runtime_seconds=1, loglevel="warning", quiet=True):
     Run Celery Beat that immediately starts tasks.
     The Celery Beat instance is automatically terminated after `runtime_seconds`.
     """
-    # If anything below wedges this (forked) child process, dump all of its
-    # thread stacks to stderr and exit, so a future hang surfaces a diagnosis
-    # in the CI log instead of just stalling pytest-forked's waitpid in the
-    # parent. Generous budget: kill_beat itself can wait up to ~30s for the
-    # pidfile, plus runtime_seconds, plus celery beat shutdown.
-    faulthandler.dump_traceback_later(45, exit=True)
     logger.info("Starting Celery Beat...")
     pid_file = os.path.join(tempfile.mkdtemp(), f"celery-beat-{os.getpid()}.pid")
 
@@ -87,7 +80,4 @@ def run_beat(celery_app, runtime_seconds=1, loglevel="warning", quiet=True):
         quiet=quiet,
         pidfile=pid_file,
     )
-    try:
-        beat_instance.run()
-    finally:
-        faulthandler.cancel_dump_traceback_later()
+    beat_instance.run()

tests/integrations/celery/integration_tests/conftest.py

@@ -0,0 +1,16 @@
+import faulthandler
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _dump_child_stacks_on_hang():
+    """
+    Arm faulthandler inside the (pytest-forked) child process so that if a
+    test wedges, the child dumps all of its thread stacks to stderr and exits
+    instead of stalling pytest-forked's waitpid in the parent. Cancels on
+    successful teardown so passing tests behave normally.
+    """
+    faulthandler.dump_traceback_later(45, exit=True)
+    yield
+    faulthandler.cancel_dump_traceback_later()