scheduler: Plug FD leaks on destroy

[cosmo0920]

Previously, we didn't care to dispose FD on destroy.
This causes FD leaks on termination and relaunching frequently with using lib mode.
So, we need to plug FD leaks there.

Closes #11974.

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Testing
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Backporting

• Backport to latest stable release.

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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

  • Enhanced scheduler initialization and teardown for improved event channel state management.
  • Ensures proper cleanup of event channel resources during scheduler shutdown.

• Tests

  • Added comprehensive internal unit tests validating event channel cleanup behavior across scheduler lifecycle.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 00334a27-fdbb-4fe1-adf5-1705098c65ac

📥 Commits

Reviewing files that changed from the base of the PR and between b950d65 and 29c102f.

📒 Files selected for processing (3)

• src/flb_scheduler.c
• tests/internal/CMakeLists.txt
• tests/internal/scheduler.c

✅ Files skipped from review due to trivial changes (1)

• tests/internal/CMakeLists.txt

🚧 Files skipped from review as they are similar to previous changes (1)

• tests/internal/scheduler.c

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📝 Walkthrough

Walkthrough

flb_sched_create() now initializes ch_events[] to -1 and zeroes sched->event before channel creation, and passes &sched->event to mk_event_channel_create(). flb_sched_destroy() gains a guarded teardown that calls mk_event_channel_destroy() only when ch_events[0] != -1, then resets both entries. A new unit test verifies the pipe FDs are closed after destruction.

Changes

Scheduler event channel FD leak fix

Layer / File(s)	Summary
Guarded create and destroy logic in flb_sched src/flb_scheduler.c	flb_sched_create() sets ch_events[0] and ch_events[1] to -1 and zeroes sched->event as initial state, and passes &sched->event to mk_event_channel_create(). flb_sched_destroy() conditionally calls mk_event_channel_destroy() only when ch_events[0] != -1, then resets both entries to -1, preventing double-close on repeated init/destroy cycles.
Unit test for event channel cleanup tests/internal/scheduler.c, tests/internal/CMakeLists.txt	New test test_scheduler_event_channel_cleanup creates a config, event loop, and two schedulers, captures their ch_events pipe FDs, destroys the scheduler and event loop, then asserts flb_pipe_w/flb_pipe_r on each captured FD returns -1. Registered as event_channel_cleanup in TEST_LIST and added to the CMake UNIT_TESTS_FILES list.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐇 A pipe left open, a leak in the ground,
Now guarded by -1, safe and sound.
On destroy we check before we close,
No dangling FDs—that's how it goes!
Each scheduler cleaned, each handle freed,
The rabbit hops on, with no FDs to bleed. 🌿

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title 'scheduler: Plug FD leaks on destroy' accurately describes the main change: fixing file descriptor leaks in the scheduler destroy operation.
Linked Issues check	✅ Passed	The PR implements proper cleanup of ch_events file descriptors in flb_sched_destroy(), directly addressing the FD leak issue #11974 by releasing resources that were previously uncleaned.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing FD leaks in the scheduler: proper event channel destruction and tests verifying the cleanup, with no unrelated modifications.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch comos0920-plug-fd-leaks-on-lib-mode

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