Fix compaction abort rescheduling before queue pick

[xingbowang]

Summary

• AbortAllCompactions can race with an already-scheduled automatic compaction before the background worker calls PickCompactionFromQueue(). MaybeScheduleFlushOrCompaction() has already consumed one unscheduled_compactions_ credit when it scheduled the worker, but the CF is still present in compaction_queue_ with queued_for_compaction=true. If the worker returns kCompactionAborted before popping the CF, ResumeAllCompactions() can see unscheduled_compactions_ == 0 and fail to schedule the still-queued work, leaving compaction permanently stalled until DB restart.

• Restore the unscheduled compaction credit in the non-prepicked abort path, matching the existing BG-work-stopped handling for the same scheduled-before-pick state. Prepicked/manual compactions are not adjusted because they do not represent an unpopped automatic compaction_queue_ entry whose scheduling credit was consumed.

• Add AbortScheduledAutomaticCompactionBeforePick to deterministically reproduce the lost-credit race with a sync point at BackgroundCallCompaction:0. The test verifies that ResumeAllCompactions() schedules the still-queued automatic compaction by checking COMPACT_WRITE_BYTES advances and L0 file count drops.

Test Plan

• Without the implementation fix, DBCompactionAbortTest.AbortScheduledAutomaticCompactionBeforePick fails because COMPACT_WRITE_BYTES remains unchanged after ResumeAllCompactions().

[github-actions]

✅ clang-tidy: No findings on changed lines

Completed in 108.4s.

[github-actions]

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✅ Claude Code Review

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Summary

This PR correctly fixes a real race condition where AbortAllCompactions() can permanently stall compaction by losing an unscheduled_compactions_ credit when abort fires before PickCompactionFromQueue(). The fix is minimal, correctly scoped, thread-safe, and matches the existing BG-work-stopped pattern. No high-severity issues found.

High-severity findings (0):

No high-severity findings.

Full review (click to expand)

Findings

🔴 HIGH

None.

🟡 MEDIUM

M1. Inconsistent !is_prepicked guard in BG-work-stopped path -- db_impl_compaction_flush.cc:4080

• Issue: The existing BG-work-stopped path (line 4080) increments unscheduled_compactions_++ unconditionally (no !is_prepicked guard), while the new abort path correctly guards with !is_prepicked. For prepicked/manual compactions, no unscheduled_compactions_ credit was consumed by MaybeScheduleFlushOrCompaction(), so the unconditional increment in the BG-work-stopped path could create a spurious credit.
• Root cause: The BG-work-stopped path predates the abort fix and was written without considering the prepicked distinction. Its comment says "Since we didn't pop a cfd from the compaction queue" -- but prepicked compactions never had a queue entry to begin with.
• Suggested fix: Add the same !is_prepicked guard to the BG-work-stopped path for consistency:

  } else {
      status = error_handler_.GetBGError();
      if (!is_prepicked) {
          unscheduled_compactions_++;
      }
  }

  This is a pre-existing issue, not introduced by this PR. Could be addressed as a follow-up.

M2. Test coverage gaps for edge cases -- db_compaction_abort_test.cc

• Issue: The new test covers the primary scenario (single CF, single background thread, automatic compaction) but does not cover:
  • Multiple CFs in compaction_queue_ when abort fires (multiple credits lost and restored)
  • max_background_compactions > 1 (multiple workers aborting simultaneously)
  • Verifying that manual/prepicked compactions do NOT incorrectly increment unscheduled_compactions_ during abort
• Root cause: Test scope limited to the minimal reproduction case.
• Suggested fix: Consider adding a multi-CF or multi-thread variant in a follow-up. The current test is sufficient to validate the core fix.

🟢 LOW / NIT

L1. Consider centralizing credit restoration -- db_impl_compaction_flush.cc

• Issue: Credit restoration (unscheduled_compactions_++) now appears in four separate locations (lines 4080, 4070, 4161, 4174), each with slightly different guard conditions. This fragmentation makes the credit system harder to reason about.
• Suggested fix: Consider extracting a helper method in a follow-up:

  void RestoreUnscheduledCompactionCredit(bool is_prepicked) {
      if (!is_prepicked) { unscheduled_compactions_++; }
  }

Cross-Component Analysis

Context	Affected?	Verified?
WritePreparedTxnDB	No	Same scheduling path
FIFO/Universal compaction	Yes, works correctly	Style-agnostic scheduling
Multiple CFs	Yes, works correctly	Per-worker credit accounting
Multiple BG threads	Yes, works correctly	Each worker restores own credit
Nested abort/resume	Yes, works correctly	Counter semantics preserved
Manual compaction	No (guarded by !is_prepicked)	Verified via caller audit
Forwarded-to-bottom compaction	No (guarded by !is_prepicked)	Verified: original pick consumes credit, forwarded worker is prepicked
ReadOnly DB	No	No compaction scheduling

Key invariant verified: MaybeScheduleFlushOrCompaction() at line 3995 (in BackgroundCallCompaction cleanup) cannot consume the restored credit because compaction_aborted_ is still > 0 at that point. AbortAllCompactions() waits for bg_compaction_scheduled_ == 0 (line 3188), which happens AFTER line 3986's decrement, so ResumeAllCompactions() cannot be called until after the cleanup completes.

Positive Observations

• The fix precisely mirrors the existing BG-work-stopped pattern (line 4080), demonstrating good codebase awareness.
• The !is_prepicked guard is more correct than the existing BG-work-stopped path, showing attention to the credit accounting model.
• The test uses SyncPointAbortHelper consistently with other tests in the file.
• The comment on the new code clearly explains the invariant being maintained.
• The sync point "BackgroundCallCompaction:0" precisely targets the window between worker scheduling and queue pick, making the test deterministic.

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[meta-codesync]

@xingbowang has imported this pull request. If you are a Meta employee, you can view this in D106684155.