Storages: use-after-free race between SegmentReadTaskScheduler and SegmentReaderPoolManager shutdown (#10917)

[ti-chi-bot]

This is an automated cherry-pick of #10917

What problem does this PR solve?

Issue Number: close #10903

Problem Summary:

During shutdown, SegmentReaderPoolManager::stop() destroys all reader_pools and their WorkQueue mutexes, but SegmentReadTaskScheduler's schedLoop background thread is still running. When schedLoop wakes from its 2ms sleep, it calls scheduleOneRound() → SegmentReaderPoolManager::addTask() → accesses already-destroyed reader_pools, causing SIGSEGV (use-after-free).

What is changed and how it works?

Add a public `stop()` method to `SegmentReadTaskScheduler` that sets the stop flag and joins the background thread. Call it **before** `SegmentReaderPoolManager::stop()` in both production (`Server.cpp`) and test (`gtests_dbms_main.cpp`) shutdown paths.

- `SegmentReadTaskScheduler.h`: expose `stop()` as public method; rename `std::atomic<bool> stop` → `stop_flag` to avoid name collision
- `SegmentReadTaskScheduler.cpp`: implement `stop()`, destructor delegates to it
- `Server.cpp`: call `SegmentReadTaskScheduler::instance().stop()` before `SegmentReaderPoolManager::instance().stop()`
- `gtests_dbms_main.cpp`: same reorder

Also fix two minor typos along the way:
- `reapPeningPools` → `reapPendingPools` in class comment
- `max_free_threds` → `max_free_threads` in test main

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Manual test: ./gtests_dbms --gtest_filter="DeltaMergeStoreTest.ReadLegacyStringDataCFTiny" — test passes and process exits cleanly without SIGSEGV.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

Summary by CodeRabbit

Release Notes

• Bug Fixes
  • Improved shutdown stability by ensuring the segment read task scheduler stops before dependent reader pools are torn down, reducing risk of invalid task enqueuing during shutdown.
• Tests
  • Corrected a thread-pool initialization parameter name in the test harness to ensure consistent setup and teardown behavior.

[coderabbitai]

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[JaySon-Huang]

/test pull-unit-next-gen

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JaySon-Huang, yongman

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• OWNERS [JaySon-Huang,yongman]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-06-24 14:00:48.179171032 +0000 UTC m=+128931.842396545: ☑️ agreed by JaySon-Huang.
• 2026-06-25 03:44:20.626100078 +0000 UTC m=+178344.289325581: ☑️ agreed by yongman.

[JaySon-Huang]

/test pull-unit-next-gen