test: set Celery config through a celery_conf fixture, not monkeypatch

pytest 9.2.0 (released 2026-10-27, changelog #10644) changed how
monkeypatch.setattr restores an attribute the object does not hold in
its own __dict__: undo now deletes it instead of assigning the old value
back. Celery's conf serves every key through __getattr__, so undo called
delattr, the delete raised AttributeError, and pytest skipped the rest
of that test's undos. That made 53 teardown errors across six files
(cleanup API, task-run signals, tasks admin, backup, library audit, the
soft-limit salvage test), and could leak patches into later tests.

CI installs pytest unpinned, so the first run after the release picked it
up: the weekly base refresh of main, 2026-11-01, and every run since. The
gate held, so nothing was published.

tests/conftest.py gains `celery_conf(**values)`: it records each key's
value through conf[key], writes with conf.update(), and restores the
saved values after the test. The six files use it.

Task #6864.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/conftest.py

@@ -85,6 +85,29 @@ def db_sync(sync_engine):
             session.rollback()
 
 
+@pytest.fixture
+def celery_conf():
+    """Set Celery config keys for one test: `celery_conf(task_always_eager=True)`.
+    The values they had before the test come back after it.
+
+    Use this, not `monkeypatch.setattr(celery.conf, ...)`. Celery serves config
+    keys through `__getattr__`, so monkeypatch sees no attribute to restore and
+    deletes the key on undo instead; the delete raises, and every later undo of
+    that test is skipped, leaking its patches into the tests after it.
+    """
+    from backend.app.celery_app import celery
+
+    saved: dict[str, object] = {}
+
+    def _set(**values: object) -> None:
+        for key in values:
+            saved.setdefault(key, celery.conf[key])
+        celery.conf.update(values)
+
+    yield _set
+    celery.conf.update(saved)
+
+
 @pytest_asyncio.fixture
 async def app():
     return create_app()

tests/test_api_cleanup.py

@@ -11,15 +11,14 @@ import pytest
 from PIL import Image
 from sqlalchemy import func, select
 
-from backend.app.celery_app import celery
 from backend.app.models import ImageRecord, LibraryAuditRun
 
 pytestmark = pytest.mark.integration
 
 
 @pytest.fixture(autouse=True)
-def disable_celery_eager(monkeypatch):
-    monkeypatch.setattr(celery.conf, "task_always_eager", False)
+def disable_celery_eager(celery_conf):
+    celery_conf(task_always_eager=False)
 
 
 def _sha256_min_dim_token(min_w: int, min_h: int) -> str:

tests/test_download_source_task.py

@@ -167,7 +167,7 @@ async def test_finalize_soft_limited_is_noop_without_running_event(db_sync):
 @pytest.mark.integration
 @pytest.mark.asyncio
 async def test_download_source_catches_soft_limit_and_salvages_event(
-    db_sync, monkeypatch,
+    db_sync, monkeypatch, celery_conf,
 ):
     """End-to-end wiring: when the inner run raises SoftTimeLimitExceeded,
     download_source's handler must flip the in-flight event to error
@@ -178,8 +178,7 @@ async def test_download_source_catches_soft_limit_and_salvages_event(
     import backend.app.tasks.download as dl
     from backend.app.models import DownloadEvent
 
-    monkeypatch.setattr(celery.conf, "task_always_eager", True)
-    monkeypatch.setattr(celery.conf, "task_eager_propagates", False)
+    celery_conf(task_always_eager=True, task_eager_propagates=False)
     # This test exercises the soft-limit salvage path, not the per-platform
     # serialization. Neutralize the Redis lock so it can't defer/recurse under
     # eager mode (and so it doesn't contend with test_platform_lock).

tests/test_task_run_signals.py

@@ -20,10 +20,9 @@ pytestmark = pytest.mark.integration
 
 
 @pytest.fixture(autouse=True)
-def _eager_celery(monkeypatch):
+def _eager_celery(celery_conf):
     """task_always_eager so signals fire in-process during the test."""
-    monkeypatch.setattr(celery.conf, "task_always_eager", True)
-    monkeypatch.setattr(celery.conf, "task_eager_propagates", False)
+    celery_conf(task_always_eager=True, task_eager_propagates=False)
 
 
 @pytest.mark.asyncio

tests/test_tasks_admin.py

@@ -15,9 +15,8 @@ pytestmark = pytest.mark.integration
 
 
 @pytest.fixture(autouse=True)
-def _eager_celery(monkeypatch):
-    monkeypatch.setattr(celery.conf, "task_always_eager", True)
-    monkeypatch.setattr(celery.conf, "task_eager_propagates", False)
+def _eager_celery(celery_conf):
+    celery_conf(task_always_eager=True, task_eager_propagates=False)
 
 
 @pytest.fixture(autouse=True)

tests/test_tasks_backup.py

@@ -10,16 +10,14 @@ from pathlib import Path
 import pytest
 from sqlalchemy import select
 
-from backend.app.celery_app import celery
 from backend.app.models import BackupRun, ImportSettings
 
 pytestmark = pytest.mark.integration
 
 
 @pytest.fixture(autouse=True)
-def _eager_celery(monkeypatch):
-    monkeypatch.setattr(celery.conf, "task_always_eager", True)
-    monkeypatch.setattr(celery.conf, "task_eager_propagates", False)
+def _eager_celery(celery_conf):
+    celery_conf(task_always_eager=True, task_eager_propagates=False)
 
 
 @pytest.fixture(autouse=True)

tests/test_tasks_library_audit.py

@@ -9,7 +9,6 @@ from PIL import Image
 from sqlalchemy import select
 
 import backend.app.tasks.library_audit  # noqa: F401 — celery registration
-from backend.app import celery_app
 from backend.app.models import ImageRecord, LibraryAuditRun
 
 pytestmark = pytest.mark.integration
@@ -37,7 +36,7 @@ def _mk_image(db_sync, tmp_path, *, mode, color, name):
 
 
 def test_scan_library_for_rule_populates_matched_ids_for_transparency(
-    db_sync, tmp_path, monkeypatch,
+    db_sync, tmp_path, celery_conf,
 ):
     transparent_rec, _ = _mk_image(
         db_sync, tmp_path, mode="RGBA", color=(0, 0, 0, 0), name="trans.png",
@@ -53,10 +52,10 @@ def test_scan_library_for_rule_populates_matched_ids_for_transparency(
     db_sync.commit()
     audit_id = audit.id
 
-    monkeypatch.setattr(celery_app.celery.conf, "task_always_eager", True)
+    celery_conf(task_always_eager=True)
     from backend.app.tasks.library_audit import scan_library_for_rule
     scan_library_for_rule.run(audit_id)
-    monkeypatch.setattr(celery_app.celery.conf, "task_always_eager", False)
+    celery_conf(task_always_eager=False)
 
     matched = db_sync.execute(
         select(LibraryAuditRun.matched_ids).where(LibraryAuditRun.id == audit_id)
@@ -124,7 +123,7 @@ def test_scan_resumes_and_accumulates_matched(db_sync, tmp_path, monkeypatch):
 
 
 def test_scan_library_for_rule_skips_missing_files_gracefully(
-    db_sync, tmp_path, monkeypatch,
+    db_sync, tmp_path, celery_conf,
 ):
     rec, path = _mk_image(
         db_sync, tmp_path, mode="RGBA", color=(0, 0, 0, 0), name="ghost.png",
@@ -138,10 +137,10 @@ def test_scan_library_for_rule_skips_missing_files_gracefully(
     db_sync.commit()
     audit_id = audit.id
 
-    monkeypatch.setattr(celery_app.celery.conf, "task_always_eager", True)
+    celery_conf(task_always_eager=True)
     from backend.app.tasks.library_audit import scan_library_for_rule
     scan_library_for_rule.run(audit_id)
-    monkeypatch.setattr(celery_app.celery.conf, "task_always_eager", False)
+    celery_conf(task_always_eager=False)
 
     status = db_sync.execute(
         select(LibraryAuditRun.status).where(LibraryAuditRun.id == audit_id)