SaveSync: dedupe bootstrap recompute job across API restarts

Pass a deterministic job_id and unique=True to low_prio_queue.enqueue
so a restart while a previous bootstrap recompute is still queued or
running no-ops the second enqueue. Without this, every API restart with
a NULL-hash row left would push another duplicate job onto the
low-priority queue; RQ would happily run both back-to-back, redoing the
same scans and content-hash reads against the filesystem.

RQ raises rq.exceptions.DuplicateJobError when unique=True hits an
existing job ID. Swallow it with a log line and let other enqueue
failures fall through to the generic exception path so they still get
logged with a traceback.

backend/startup.py

@@ -4,6 +4,7 @@ import asyncio
 
 import sentry_sdk
 from opentelemetry import trace
+from rq.exceptions import DuplicateJobError
 
 from config import (
     ENABLE_SCHEDULED_CONVERT_IMAGES_TO_WEBP,
@@ -47,6 +48,8 @@ from utils.context import initialize_context
 
 tracer = trace.get_tracer(__name__)
 
+RECOMPUTE_SAVE_HASHES_JOB_ID = "recompute_save_content_hashes:bootstrap"
+
 
 def _enqueue_recompute_save_hashes_if_needed() -> None:
     """Backfill content_hash for saves uploaded before the path-resolution
@@ -71,6 +74,8 @@ def _enqueue_recompute_save_hashes_if_needed() -> None:
     try:
         low_prio_queue.enqueue(
             recompute_save_content_hashes_task.run,
+            job_id=RECOMPUTE_SAVE_HASHES_JOB_ID,
+            unique=True,
             job_timeout=TASK_TIMEOUT,
             meta={
                 "task_name": recompute_save_content_hashes_task.title,
@@ -81,6 +86,11 @@ def _enqueue_recompute_save_hashes_if_needed() -> None:
             f"Enqueued recompute_save_content_hashes ({missing} saves with NULL content_hash); "
             "running on low-priority worker"
         )
+    except DuplicateJobError:
+        log.info(
+            "recompute_save_content_hashes already queued or running from a "
+            "previous restart; skipping enqueue"
+        )
     except Exception:
         log.exception(
             "Failed to enqueue recompute_save_content_hashes; admins can run it manually"

backend/tests/test_startup.py

@@ -1,6 +1,7 @@
 """Tests for startup-time auto-enqueue of the recompute task."""
 
 import startup
+from rq.exceptions import DuplicateJobError
 
 
 def test_enqueue_recompute_skips_when_no_missing_hashes(mocker):
@@ -38,6 +39,25 @@ def test_enqueue_recompute_fires_when_missing_hashes_present(mocker):
     # Job timeout must be passed; otherwise long-running recomputes get killed
     # by RQ's default short timeout on very large libraries.
     assert kwargs["job_timeout"] == startup.TASK_TIMEOUT
+    # Deterministic job_id + unique=True prevent duplicate enqueues across
+    # API process restarts while a previous recompute is still running.
+    assert kwargs["job_id"] == startup.RECOMPUTE_SAVE_HASHES_JOB_ID
+    assert kwargs["unique"] is True
+
+
+def test_enqueue_recompute_silently_skips_duplicate(mocker):
+    """A DuplicateJobError from RQ (job already queued/running) is logged and
+    swallowed, not propagated."""
+    mocker.patch.object(
+        startup.db_save_handler, "count_saves_missing_content_hash", return_value=10
+    )
+    mocker.patch.object(
+        startup.low_prio_queue,
+        "enqueue",
+        side_effect=DuplicateJobError("already enqueued"),
+    )
+
+    startup._enqueue_recompute_save_hashes_if_needed()
 
 
 def test_enqueue_recompute_swallows_count_error(mocker):