tests: scope failpoint lock counts to each test's own keys (fixes #525, mitigates #516)

[eduralph]

The txn_cleanup_* failpoint tests count locks with a whole-keyspace scan (count_locks / wait_for_locks_count over ..). txn_cleanup_locks_batch_size stubs cleanup via the before-cleanup-locks failpoint and leaves its locks in place — and since they're async-commit locks, they resolve in the background and drain at an unpredictable rate. A later serial test's unbounded scan then catches the still-draining residue and over-counts — the flaky 575 vs 512 / 513 vs 512 in #525.

Fix (test-only)

• Add count_locks_of / wait_for_locks_count_of that count only the caller's own keys, making the assertions immune to residual locks from other tests.
• Switch all txn_cleanup_* count assertions to the scoped variants.
• Have txn_cleanup_locks_batch_size resolve its own locks before returning, so it no longer seeds residue for the next serial test.
• Remove the now-unused whole-keyspace wrappers.

No client (src/) change.

Fixes #525.

Scope note: this does not address #516 (the txn_cleanup_2pc_locks 600s timeout). An earlier revision of this description suggested it might, by removing residue that amplifies cleanup_locks — that reasoning was wrong: txn_cleanup_2pc_locks runs first in the serial suite (it is consistently reported as ( 1/54 )), so no residue from the other cleanup tests can reach it. #516 is a separate, still-undiagnosed stall and is left untouched here.

Testing

Ran the suite repeatedly against a local multi-region cluster (MULTI_REGION=1, the aggressive-split config/tikv.toml) — stable across 10+ runs at ~5s each; the previously-observed count drift does not recur. make check clean.

Summary by CodeRabbit

• Tests
  • Improved transaction cleanup and lock-resolution test reliability by checking locks only for the keys involved in each test.
  • Reduced flaky assertions caused by asynchronous lock draining between tests.
  • Added explicit cleanup handling to ensure locks are fully released before test completion.

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📥 Commits

Reviewing files that changed from the base of the PR and between 5cc7b3f and dde652f.

📒 Files selected for processing (1)

• tests/failpoint_tests.rs

📝 Walkthrough

Walkthrough

Failpoint transaction tests now scope lock counting and polling to their generated keys. Cleanup scenarios also perform explicit cleanup where needed and verify key-specific committed or rolled-back results.

Changes

Failpoint lock assertions

Layer / File(s)	Summary
Add key-scoped lock helpers tests/failpoint_tests.rs	Removes whole-keyspace lock helpers and adds helpers to count locks for supplied keys and wait for the scoped count to reach an expected value.
Update transaction cleanup tests tests/failpoint_tests.rs	Replaces whole-keyspace lock checks with key-scoped assertions, adds explicit cleanup in one scenario, and verifies committed or rolled-back state for the test keys.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• tikv/client-rust#547: Adds failpoint rollback-after-prewrite tests that also assert pessimistic transaction locks are cleared.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: scoping failpoint lock counts to each test’s own keys.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[eduralph]

@pingyu - ping for review