Celery Synchronous subtask guard blocks chord() when

The Fix

pip install celery==4.4.0rc5

Based on closed celery/celery issue #4873 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -262,3 +262,4 @@ Chris Mitchell, 2018/02/27
 Federico Bond, 2018/06/20
 Tom Booth, 2018/07/06
+Axel haustant, 2018/08/14
diff --git a/celery/canvas.py b/celery/canvas.py
index d5b2b755eb1..c85e8d7c045 100644

repro.py

$ celery -A minimal_eager_chord report
software -> celery:4.2.0 (windowlicker) kombu:4.2.1 py:3.5.3
            billiard:3.5.0.3 py-amqp:2.2.2
platform -> system:Darwin arch:64bit imp:CPython
loader   -> celery.loaders.app.AppLoader
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 your application relies on blocking behavior for task execution.\n\n

Why This Fix Works in Production

Trigger: No error occurs
Mechanism: The assert_will_not_block() function is triggered incorrectly with CELERY_TASK_ALWAYS_EAGER=True
Why the fix works: Ensures chords can work with CELERY_TASK_ALWAYS_EAGER and fixes issue #4873. (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

The assert_will_not_block() function is triggered incorrectly with CELERY_TASK_ALWAYS_EAGER=True
Production symptom (often without a traceback): No error occurs

Proof / Evidence

GitHub issue: #4873
Fix PR: https://github.com/celery/celery/pull/4979
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: 0.62

Discussion

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

Jump to Sources Open on GitHub

“Same issue here: our tests are using CELERY_TASK_ALWAYS_EAGER=True and tests using chords() fail with the same stacktrace”

@noirbizarre · 2018-08-02 · source

Failure Signature (Search String)

No error occurs
~/.pyenv/versions/3.5.3/envs/capstone/src/celery/celery/result.py in assert_will_not_block()

Copy-friendly signature

signature.txt

Failure Signature
-----------------
No error occurs
~/.pyenv/versions/3.5.3/envs/capstone/src/celery/celery/result.py in assert_will_not_block()

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
No error occurs
~/.pyenv/versions/3.5.3/envs/capstone/src/celery/celery/result.py in assert_will_not_block()

Minimal Reproduction

repro.py

$ celery -A minimal_eager_chord report
software -> celery:4.2.0 (windowlicker) kombu:4.2.1 py:3.5.3
            billiard:3.5.0.3 py-amqp:2.2.2
platform -> system:Darwin arch:64bit imp:CPython
loader   -> celery.loaders.app.AppLoader
settings -> transport:amqp results:disabled

What Broke

Chords fail to execute, causing test failures and blocking behavior.

Why It Broke

The assert_will_not_block() function is triggered incorrectly with CELERY_TASK_ALWAYS_EAGER=True

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 blocking behavior for 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/4979

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 blocking behavior for 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
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 Synchronous subtask guard blocks chord() when CELERY_TASK_ALWAYS_EAGER=True (Fix)