Celery disable_sync_subtasks setting not being respected

The Fix

pip install celery==4.4.0rc5

Based on closed celery/celery issue #5736 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -769,6 +769,7 @@ def join(self, timeout=None, propagate=True, interval=0.5,
                 timeout=remaining, propagate=propagate,
                 interval=interval, no_ack=no_ack, on_interval=on_interval,
+                disable_sync_subtasks=disable_sync_subtasks,
             )
             if callback:

repro.py

software -> celery:4.3.0 (rhubarb) kombu:4.6.4 py:3.5.6
            billiard:3.6.1.0 py-amqp:2.5.1
platform -> system:Linux arch:64bit, ELF
            kernel version:4.19.71-1-lts imp:CPython
loader   -> celery.loaders.default.Loader
settings -> transport:amqp results:disabled

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install celery==4.4.0rc5\nWhen NOT to use: Do not use this fix if you require synchronous execution of subtasks.\n\n

Why This Fix Works in Production

Trigger: R = retval = fun(*args, **kwargs)
Mechanism: The disable_sync_subtasks setting was not being applied correctly in the ResultSet implementation
Why the fix works: Fixes the issue where the disable_sync_subtasks setting was not respected in ResultSet. (first fixed release: 4.4.0rc5).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.5 in real deployments (not just unit tests).
The disable_sync_subtasks setting was not being applied correctly in the ResultSet implementation
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #5736
Fix PR: https://github.com/celery/celery/pull/5737
First fixed release: 4.4.0rc5
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.36

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“ # Checklist  - [x] I hav”

Issue thread · issue description · source

Failure Signature (Search String)

R = retval = fun(*args, **kwargs)

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "/home/gsfish/.pyenv/versions/scan_detect/lib/python3.5/site-packages/celery/app/trace.py", line 385, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/home/gsfish/.pyenv/versions/scan_detect/lib/python3.5/site-packages/celery/app/trace.py", line 648, in __protected_call__
    return self.run(*args, **kwargs)
  File "/home/gsfish/work/netease/project/scan_detect/tasks.py", line 106, in test
    return result_set.get(disable_sync_subtasks=False)
  File "/home/gsfish/.pyenv/versions/scan_detect/lib/python3.5/site-packages/celery/result.py", line 697, in get
    on_interval=on_interval,
  File "/home/gsfish/.pyenv/versions/scan_detect/lib/python3.5/site-packages/celery/result.py", line 765, in join
    interval=interval, no_ack=no_ack, on_interval=on_interval,
  File "/home/gsfish/.pyenv/versions/scan_detect/lib/python3.5/site-packages/celery/result.py", line 205, in get
    assert_will_not_block()
  File "/home/gsfish/.pyenv/versions/scan_detect/lib/python3.5/site-packages/celery/result.py", line 41, in assert_will_not_block
    raise RuntimeError(E_WOULDBLOCK)
RuntimeError: Never call result.get() within a task!
See http://docs.celeryq.org/en/latest/userguide/tasks.html#task-synchronous-subtasks

Minimal Reproduction

repro.py

software -> celery:4.3.0 (rhubarb) kombu:4.6.4 py:3.5.6
            billiard:3.6.1.0 py-amqp:2.5.1
platform -> system:Linux arch:64bit, ELF
            kernel version:4.19.71-1-lts imp:CPython
loader   -> celery.loaders.default.Loader
settings -> transport:amqp results:disabled

Environment

Python: 3.5

What Broke

Subtasks were executed synchronously despite the disable_sync_subtasks setting being enabled.

Why It Broke

The disable_sync_subtasks setting was not being applied correctly in the ResultSet implementation

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install celery==4.4.0rc5

When NOT to use: Do not use this fix if you require synchronous execution of subtasks.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/celery/celery/pull/5737

First fixed release: 4.4.0rc5

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not use this fix if you require synchronous execution of subtasks.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.

Version Compatibility Table

Version	Status
4.4.0rc5	Fixed

Related Issues

No related fixes found.

Cluster: celery:configuration-error Celery hub Celery best practices All hubs All clusters

Related clusters: Data consistency Duplicates Duplicate processing

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Celery disable_sync_subtasks setting not being respected while using ResultSet (Fix)