Celery In celery >=4.2, group_result.waiting() produces

The Fix

pip install celery==4.4.0rc5

Based on closed celery/celery issue #4938 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -241,6 +241,7 @@ Simon Schmidt, 2017/05/19
 Samuel Dion-Girardeau, 2017/05/29
 Aydin Sen, 2017/06/14
+Vinod Chandru, 2017/07/11
 Preston Moore, 2017/06/18
 Nicolas Mota, 2017/08/10

repro.py

# tasks.py
from celery import Celery
from celery import group

app = Celery('tasks',
             backend='redis://',
             broker='pyamqp://guest@localhost//')


@app.task()
def A():
    result = group(pow2.s(i) for i in range(3)).apply_async()
    result.save()
    restore.apply_async(kwargs={"result_id": result.id})


@app.task(bind=True)
def restore(self, result_id):
    group_result = app.GroupResult.restore(result_id)
    if group_result.waiting():
        self.retry()


@app.task()
def pow2(i):
    return i ** 2


if __name__ == '__main__':
    A.apply_async()

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 your application relies on synchronous task execution.\n\n

Why This Fix Works in Production

Trigger: ready_task = parent.ready()
Mechanism: The weak reference to the bound method in promises prevents proper callback execution
Why the fix works: Fixes a RuntimeError in group_result.waiting() when launched within a task in Celery 4.2. (first fixed release: 4.4.0rc5).

Why This Breaks in Prod

Triggered by an upgrade/regression window: 16.138043 breaks; 4.4.0rc5 is the first fixed release.
Shows up under Python 3.6 in real deployments (not just unit tests).
The weak reference to the bound method in promises prevents proper callback execution
Surfaces as: ready_task = parent.ready()

Proof / Evidence

GitHub issue: #4938
Fix PR: https://github.com/celery/celery/pull/4131
First fixed release: 4.4.0rc5
Affected versions: 16.138043
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.24

Discussion

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

Jump to Sources Open on GitHub

“I think it could have been written as a chord **except** for the fact that if any of the tasks of the group fail, we…”

@ewjoachim · 2018-08-09 · repro detail · source

“@ewjoachim thanks for the detailed description, but can you please describe your use case?”

@georgepsarakis · 2018-08-09 · source

“Is there any update on this? We seem to have a similar failure, but for us it comes from calling result.ready() inside a task”

@dolfim · 2018-11-29 · source

“BUMP Any update? I have the same error on celery 4.3.0 or master branch when using .join() on group”

@lstolcman · 2019-06-19 · source

Failure Signature (Search String)

ready_task = parent.ready()

Error Message

Stack trace

error.txt

Error Message
-------------
ready_task = parent.ready()
  File "/usr/local/lib/python3.6/site-packages/celery/result.py", line 311, in ready
    return self.state in self.backend.READY_STATES
  File "/usr/local/lib/python3.6/site-packages/celery/result.py", line 471, in state
    return self._get_task_meta()['status']
  File "/usr/local/lib/python3.6/site-packages/celery/result.py", line 410, in _get_task_meta
    return self._maybe_set_cache(self.backend.get_task_meta(self.id))
  File "/usr/local/lib/python3.6/site-packages/celery/result.py", line 404, in _maybe_set_cache
    self.on_ready(self)
  File "/usr/local/lib/python3.6/site-packages/vine/promises.py", line 150, in __call__
    svpending(*ca, **ck)
  File "/usr/local/lib/python3.6/site-packages/vine/synchronization.py", line 67, in __call__
    self.p(*self.args, **self.kwargs)
  File "/usr/local/lib/python3.6/site-packages/vine/promises.py", line 150, in __call__
    svpending(*ca, **ck)
  File "/usr/local/lib/python3.6/site-packages/vine/promises.py", line 143, in __call__
    return self.throw()
  File "/usr/local/lib/python3.6/site-packages/vine/promises.py", line 140, in __call__
    retval = fun(*final_args, **final_kwargs)
  File "/usr/local/lib/python3.6/site-packages/celery/result.py", line 877, in _on_ready
    ResultSet._on_ready(self)
  File "/usr/local/lib/python3.6/site-packages/celery/result.py", line 519, in _on_ready
    self._ca
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
[2019-06-19 03:04:27,712: ERROR/ForkPoolWorker-8] Task process_run_task[fe246ffd-3a55-4bbd-88d8-c17862a80de8] raised unexpected: RuntimeError('Never call result.get() within a task!\nSee http://docs.celeryq.org/en/latest/userguide/tasks.html#task-synchronous-subtasks\n')
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/celery/app/trace.py", line 385, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/celery/app/trace.py", line 648, in __protected_call__
    return self.run(*args, **kwargs)
  File "/code/myapp/apps/core/tasks.py", line 65, in process_run_task
    result.join()
  File "/usr/local/lib/python3.7/site-packages/celery/result.py", line 748, in join
    assert_will_not_block()
  File "/usr/local/lib/python3.7/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

# tasks.py
from celery import Celery
from celery import group

app = Celery('tasks',
             backend='redis://',
             broker='pyamqp://guest@localhost//')


@app.task()
def A():
    result = group(pow2.s(i) for i in range(3)).apply_async()
    result.save()
    restore.apply_async(kwargs={"result_id": result.id})


@app.task(bind=True)
def restore(self, result_id):
    group_result = app.GroupResult.restore(result_id)
    if group_result.waiting():
        self.retry()


@app.task()
def pow2(i):
    return i ** 2


if __name__ == '__main__':
    A.apply_async()

Environment

Python: 3.6

Why It Broke

The weak reference to the bound method in promises prevents proper callback execution

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 your application relies on synchronous task execution.

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/4131

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 your application relies on synchronous task execution.

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

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
16.138043	Broken
4.4.0rc5	Fixed

Related Issues

No related fixes found.

Cluster: celery:auth Celery hub Celery best practices All hubs All clusters

Related clusters: Configuration error Data consistency Duplicates

Sources

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

Celery In celery >=4.2, group_result.waiting() produces RuntimeError when launched within a task (Fix)