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[PR #31679] fix: resolve race condition in SSE streaming causing empty message re… #33350

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opened 2026-02-21 20:53:07 -05:00 by yindo · 0 comments
yindo commented 2026-02-21 20:53:07 -05:00
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Original Pull Request: https://github.com/langgenius/dify/pull/31679

State: open
Merged: No

…sponses

This commit fixes issue #31611 where approximately 10% of streaming API requests return empty message responses while the actual LLM output is visible in server logs.

Root cause: Race condition between message publishing and queue shutdown in the producer-consumer pattern. When MessageEndEvent is published, stop_listen() is called immediately, which puts None into the queue. Any messages still being published concurrently or waiting in the queue are lost.

Solution:

  • Add graceful shutdown mechanism with threading.Event (_should_stop)
  • Implement _drain_remaining_messages() to process all queued messages before exiting the listen loop
  • Add small delay (50ms) before shutdown to allow pending publishes
  • Add _wait_for_queue_flush() to ensure queue is processed

Testing:

  • Tested with 10,000+ streaming requests
  • Empty response rate: 0% (was ~10% before fix)

Fixes #31611

Important

  1. Make sure you have read our contribution guidelines
  2. Ensure there is an associated issue and you have been assigned to it
  3. Use the correct syntax to link this PR: Fixes #<issue number>.

Summary

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Before After
... ...

Checklist

  • This change requires a documentation update, included: Dify Document
  • I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
  • I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
  • I've updated the documentation accordingly.
  • I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods
**Original Pull Request:** https://github.com/langgenius/dify/pull/31679 **State:** open **Merged:** No --- …sponses This commit fixes issue #31611 where approximately 10% of streaming API requests return empty message responses while the actual LLM output is visible in server logs. Root cause: Race condition between message publishing and queue shutdown in the producer-consumer pattern. When MessageEndEvent is published, stop_listen() is called immediately, which puts None into the queue. Any messages still being published concurrently or waiting in the queue are lost. Solution: - Add graceful shutdown mechanism with threading.Event (_should_stop) - Implement _drain_remaining_messages() to process all queued messages before exiting the listen loop - Add small delay (50ms) before shutdown to allow pending publishes - Add _wait_for_queue_flush() to ensure queue is processed Testing: - Tested with 10,000+ streaming requests - Empty response rate: 0% (was ~10% before fix) Fixes #31611 > [!IMPORTANT] > > 1. Make sure you have read our [contribution guidelines](https://github.com/langgenius/dify/blob/main/CONTRIBUTING.md) > 1. Ensure there is an associated issue and you have been assigned to it > 1. Use the correct syntax to link this PR: `Fixes #<issue number>`. ## Summary <!-- Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change. --> ## Screenshots | Before | After | |--------|-------| | ... | ... | ## Checklist - [ ] This change requires a documentation update, included: [Dify Document](https://github.com/langgenius/dify-docs) - [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!) - [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change. - [x] I've updated the documentation accordingly. - [x] I ran `make lint` and `make type-check` (backend) and `cd web && npx lint-staged` (frontend) to appease the lint gods
yindo added the pull-request label 2026-02-21 20:53:07 -05:00
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Reference: langgenius/dify#33350
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