fix: a restart no longer strands downloads — the download lane closes its predecessor's runs as interrupted and frees the platform locks at boot (#4433)

A walk that outlives the 90s stop grace is SIGKILLed: its event never
finalizes and its platform lock is held for the 27-min TTL. The 30-min
sweep then errored every stranded event and bumped consecutive_failures,
backing sources off (and blocking backfills) as if the platform failed.

On worker_ready, the process consuming 'download' ends pre-boot
pending/running events as skipped with error_type 'interrupted', leaves
the source untouched so the next tick resumes it, and releases the
serialized platforms' locks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/celery_signals.py

@@ -23,7 +23,13 @@ import logging
 from datetime import UTC, datetime
 
 from celery.exceptions import SoftTimeLimitExceeded
-from celery.signals import task_failure, task_postrun, task_prerun, task_retry
+from celery.signals import (
+    task_failure,
+    task_postrun,
+    task_prerun,
+    task_retry,
+    worker_ready,
+)
 
 from .models import TaskRun
 from .tasks._sync_engine import sync_session_factory
@@ -213,3 +219,43 @@ def _on_retry(sender=None, request=None, reason=None, einfo=None, **_):
         error_message=str(reason) if reason else None,
         retry_count=getattr(request, "retries", 0),
     )
+
+
+def _consumed_queues(consumer) -> set[str]:
+    """The queue names this worker process consumes (its `-Q`), or empty when
+    they can't be read — which makes the boot hook below a no-op, not a guess."""
+    try:
+        return {q.name for q in consumer.task_consumer.queues}
+    except Exception:  # noqa: BLE001 — shape varies across celery versions
+        return set()
+
+
+@worker_ready.connect
+def _on_worker_ready(sender=None, **_):
+    """The download lane clears what its previous process left behind (#4433).
+
+    A restart SIGKILLs any walk that outlives the stop grace, so its event is
+    never finalized and its platform lock is never released. Both used to wait
+    out timers — a 30-min stall sweep that then blamed the source, and a 27-min
+    lock TTL that stalled every other source on the platform. Only the process
+    consuming `download` does this; the other lanes booting beside it must not.
+    Best-effort: a failure here is logged and the worker still starts.
+    """
+    if "download" not in _consumed_queues(sender):
+        return
+    booted_at = datetime.now(UTC)
+    try:
+        from .services.platform_lock import release_all_platform_locks
+        from .tasks.maintenance import interrupt_orphaned_download_events
+
+        with sync_session_factory()() as session:
+            closed = interrupt_orphaned_download_events(session, booted_at=booted_at)
+            session.commit()
+        released = release_all_platform_locks()
+        if closed or released:
+            log.info(
+                "download lane boot: closed %d orphaned download event(s) as "
+                "interrupted, released %d platform lock(s)", closed, released,
+            )
+    except Exception:  # noqa: BLE001 — never block the worker from starting
+        log.exception("download lane boot recovery failed")