engine: fix endless loop on reload

[CharlieR-o-o-t]

Fix endless loop on fluent-bit reload.

Fixes #10670, #10518, #9927, #9354

Short description:
mk_event_timeout_destroy() should be called only after flb_engine_shutdown(). Otherwise, the shutdown timer will be removed right after the chunk retry finishes. This may cause all inputs to stop, the engine to become blocked during reloads, and the shutdown logic to never be triggered because config->shutdown_fd was already released.

Testing
I have reported bug #10670 , it well documented with exact steps to reproduce.
Tested

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

Summary by CodeRabbit

• Bug Fixes
  • Improved shutdown process to ensure the shutdown timer remains active during the grace period and task flushing, leading to a more reliable and graceful engine shutdown.

[coderabbitai]

Walkthrough

The change in src/flb_engine.c moves the destruction of the shutdown timer event from the initial handling of the FLB_ENGINE_SHUTDOWN event to the final shutdown sequence. This ensures the timer remains active during the shutdown grace period and task flushing, and is only destroyed right before the engine exits.

Changes

Cohort / File(s)	Change Summary
Engine Shutdown Timer Handling src/flb_engine.c	Relocated the destruction of the shutdown timer event from the early shutdown event handler to the final shutdown sequence, ensuring the timer persists throughout the shutdown grace period and task flushing.

Sequence Diagram(s)

sequenceDiagram
    participant Engine
    participant ShutdownTimer

    Engine->>Engine: Detect FLB_ENGINE_SHUTDOWN event
    Engine->>ShutdownTimer: Keep timer active (no destruction)
    Engine->>Engine: Process pending tasks and chunks
    Engine->>ShutdownTimer: Destroy timer (after tasks/chunks processed)
    Engine->>Engine: Complete shutdown and exit

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Estimated code review effort

🎯 2 (Simple) | ⏱️ ~7 minutes

Assessment against linked issues

Objective	Addressed	Explanation
Prevent endless loop and ensure proper handling of SIGHUP/SIGTERM and log processing after reload (#10670)	✅
Ensure shutdown timer event is managed correctly during shutdown and reload (#10670)	✅

Assessment against linked issues: Out-of-scope changes

No out-of-scope changes found.

Poem

In the engine’s gentle hum at night,
A timer once vanished out of sight.
Now it lingers, calm and wise,
Guiding shutdown ‘til goodbyes.
No more loops that never end—
The rabbit winks, “All signals mend!”
🐇✨

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Reviewing files that changed from the base of the PR and between 2b9741c and cafcca4.

📒 Files selected for processing (1)

• src/flb_engine.c (1 hunks)

🔇 Additional comments (1)

src/flb_engine.c (1)

1104-1107: LGTM! Critical fix for shutdown timer lifecycle management.

This change properly fixes the endless loop issue by ensuring the shutdown timer event remains active throughout the entire shutdown grace period and task flushing phase. Moving the mk_event_timeout_destroy call to the final shutdown sequence (right before function return) prevents premature timer destruction that could cause the shutdown process to hang indefinitely.

The previous early destruction of the timer likely contributed to the endless loop bug described in issue #10670, where Fluent Bit would stop processing signals and fail to complete the reload/shutdown process properly.

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[edsiper]

@CharlieR-o-o-t thank you !

[CharlieR-o-o-t]

Hello @edsiper ,
Looks like this fix not included in new release (Fluent Bit 4.0.7).
Do you know if it will be part of v4.0.8?

[cosmo0920]

Ah, sorry. I've overlooked for this PR. I sent a backport PR in #10709.
It will be included in v4.0.8.

[zwscn2014]

Ah, sorry. I've overlooked for this PR. I sent a backport PR in #10709. It will be included in v4.0.8.

Thank you for including this PR in v4.0.8! Could you please let me know if there is an estimated release date for this version?

[CharlieR-o-o-t]

one more PR to fix: #10720

@cosmo0920 , could you take a look? that's important fix.