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/services/platform_lock.py

@@ -60,3 +60,22 @@ def platform_lock(platform: str, *, ttl_seconds: int):
     except redis.RedisError as exc:  # pragma: no cover - broker outage
         log.warning("platform_lock unavailable for %s: %s", platform, exc)
         return None
+
+
+def release_all_platform_locks() -> int:
+    """Drop every serialized platform's lock. Returns how many were held.
+
+    Only for the download lane's boot (#4433). A worker that restarts mid-walk
+    is SIGKILLed past its stop grace, so its `finally` never releases the lock,
+    and the TTL keeps every other source on that platform bouncing for up to
+    27 minutes after the new worker is ready. At boot no walk of ours can be
+    running, so a held lock names a dead one. Assumes one download consumer —
+    the only shape FC deploys; a second replica booting would free a live
+    walk's lock (not corrupt it: the walk runs on, a second walk may overlap it).
+    """
+    try:
+        client = _redis()
+        return int(client.delete(*(f"{_LOCK_PREFIX}{p}" for p in SERIALIZED_PLATFORMS)))
+    except redis.RedisError as exc:  # pragma: no cover - broker outage
+        log.warning("could not release platform locks at boot: %s", exc)
+        return 0