Fix race in libbeat pipeline shutdown to ensure all events are gone

[blakerouse]

Proposed commit message

Acquire and release the client mutex in Close before calling signalClose to ensure any in-progress Publish call has finished incrementing the pending event counter. Without this, signalClose could observe zero pending events and complete shutdown immediately while a Publish was still in-flight between the isOpen check and AddEvent.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

None

Related issues

• Closes Race in libbeat pipeline client can shutdown without waiting for event acks #49390

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 4a399ed6-09d8-4422-95c2-bd4cdce445df

📥 Commits

Reviewing files that changed from the base of the PR and between 15c34c0 and 7f6e050.

📒 Files selected for processing (1)

• libbeat/publisher/pipeline/client.go

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📝 Walkthrough

Walkthrough

This PR fixes a race in the libbeat pipeline client's shutdown. client.Close now performs a brief mutex lock/unlock after onClosing to serialize with concurrent Publish calls so in-flight publishes register their pending events before Close signals and waits for ACKs. A regression test (TestCloseWaitsForInFlightPublish) verifies Close blocks until the in-flight event is ACKed. A changelog fragment documents the bug-fix.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	PR successfully addresses all objectives from issue #49390: synchronizes Close with Publish via mutex acquisition, prevents race between isOpen check and counter increment, ensures all in-flight events are waited for before shutdown.
Out of Scope Changes check	✅ Passed	All changes are scoped to fixing the pipeline client shutdown race: mutex synchronization in client.go, regression test for the specific race condition, and changelog fragment documenting the fix.

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[infra-vault-gh-plugin-prod]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[github-actions]

@Mergifyio backport 9.4 9.3 8.19

[mergify]

backport 9.4 9.3 8.19

✅ Backports have been created

Details

• #50677 [9.4](backport #50625) Fix race in libbeat pipeline shutdown to ensure all events are gone has been created for branch 9.4
• #50678 [9.3](backport #50625) Fix race in libbeat pipeline shutdown to ensure all events are gone has been created for branch 9.3
• #50679 [8.19](backport #50625) Fix race in libbeat pipeline shutdown to ensure all events are gone has been created for branch 8.19