feat(fifolock): acquire and release Logic optimization

[CLFutureX]

1 Reduce redundant enqueue-dequeue operations when in non-blocking mode or when the current lock resource has not been acquired.
2 Add assert checks when releasing resources.
3 Clarify the definition of _waiters: it should only store objects that are in blocking wait.

[CLFutureX]

@simonrosenberg Hey, PTAL, Thanks

[blacksmith-sh]

[Automatic Post]: I have assigned @raymyers as a reviewer based on git blame information. Thanks in advance for the help!

[simonrosenberg]

@OpenHands /codereview-roasted
Read the PR description and read the diff to understand what this PR is doing.
Make sure it does improve the FIFOLock logic and doesn't break anything and post your code review here

[openhands-ai]

I'm on it! simonrosenberg can track my progress at all-hands.dev

[simonrosenberg]

🟢 Good Taste - Worth Merging

This is a solid refactoring that improves both performance and code clarity. The changes demonstrate good engineering principles by eliminating special cases and clarifying data structure semantics.

---

Overall Assessment

Verdict: ✅ Approved - Core logic is excellent, with minor improvement suggestions

The PR successfully achieves its three stated goals:

1. ✅ Reduces redundant enqueue-dequeue operations via fast-path and early non-blocking exit
2. ✅ Adds defensive assertion when releasing resources
3. ✅ Clarifies that _waiters only contains threads actively blocking (not the current owner)

All tests pass, including the critical FIFO fairness tests and stress tests. The refactoring maintains correctness while improving performance.

---

Key Insight

This PR eliminates a subtle special case: In the old design, the current lock owner remained in the _waiters queue until calling release(). The new design has cleaner semantics: threads enter _waiters when they start waiting and are removed immediately upon acquiring the lock. This makes the state machine simpler to reason about and eliminates the "owner in queue" special case.

---

Data Structure Analysis

Before

# Owner was kept in _waiters until release()
acquire() → enqueue → wait → acquire → stay in queue
release() → popleft() to remove self → notify next

After

# Owner is removed from _waiters upon acquisition
acquire() → enqueue → wait → acquire + popleft() → out of queue
release() → just notify next (no popleft needed)

The new semantics are cleaner: _waiters contains only threads that are currently waiting, never the current owner.

---

[IMPROVEMENT OPPORTUNITIES]

[fifo_lock.py, Line 36] Documentation: Update comment to reflect new semantics

The comment says "FIFO queue of waiting threads" which is correct, but given this PR explicitly clarifies the semantics, consider being more explicit:

self._waiters: deque[threading.Condition] = deque()
# FIFO queue of threads currently blocking on acquire()
# Threads are removed from queue when they acquire the lock

[fifo_lock.py, Lines 103-105] Assertion Clarity: Make assertion message more specific

The current assertion checks an invariant that should be impossible to violate if the logic is correct (since we already verified ownership). Consider clarifying what scenario this guards against:

assert self._count >= 1, (
    f"Release called with invalid count {self._count}. "
    "This indicates a race condition or logic error."
)

[fifo_lock.py, Lines 62-66] Readability: Add comment explaining fast-path optimization

While the logic is correct, a brief comment would help future readers understand why this fast path exists:

# Fast path: If lock is free and no one is waiting, acquire immediately
# This avoids creating a Condition and queue operations for the common case
if self._owner is None and not self._waiters:
    self._owner = ident
    self._count = 1
    return True

---

Complexity Assessment

The new code adds multiple paths through acquire(), but each path is simpler:

• Fast path (lines 62-66): 5 lines, no queue operations, no Condition creation
• Non-blocking path (lines 67-69): 3 lines, early exit without queue pollution
• Blocking path (lines 71+): Similar complexity to before, but clearer semantics

Overall complexity: Marginally better due to optimized common paths and eliminated special case.

---

Correctness Verification

✅ All paths properly synchronized: Fast path, non-blocking path, and blocking path all operate inside with self._mutex:

✅ FIFO ordering preserved: The queue discipline remains unchanged - threads are still notified in order

✅ Reentrancy works: The reentrant case (line 58-60) is unchanged and still works correctly

✅ No race conditions introduced: The removal of popleft() from release() is correct because threads now remove themselves in acquire() at line 78

✅ Timeout handling intact: Line 86 still removes from queue on timeout, maintaining correctness

---

Performance Analysis

Improvements for common scenarios:

1. Uncontended acquisition: Saves ~3 operations (Condition creation, enqueue, dequeue)
2. Non-blocking attempts: Saves ~4 operations (Condition creation, enqueue, queue search, dequeue)
3. Contended case: Same performance, but clearer semantics

These optimizations are meaningful for high-frequency lock operations.

---

Testing Coverage

Verified that all existing tests pass:

• ✅ Basic functionality (acquire, release, reentrancy)
• ✅ Context manager support
• ✅ Non-blocking behavior
• ✅ Timeout behavior
• ✅ FIFO fairness (critical for this lock type)
• ✅ Error handling (wrong thread, unowned lock)
• ✅ Stress test with 20 threads

The test suite is comprehensive and confirms the refactoring maintains correctness.

---

Pragmatic Assessment

Is this solving a real problem? Yes - reducing lock acquisition overhead for uncontended and non-blocking cases is a practical optimization.

Is the complexity justified? Yes - the code is actually simpler once you understand the cleaner semantics. The multiple paths are each straightforward.

Any backward compatibility concerns? None - this is an internal implementation change with identical external behavior.

---

Final Recommendation

LGTM 🚀

This PR demonstrates good engineering taste:

1. Simplifies the state machine by eliminating the "owner in queue" special case
2. Optimizes common paths without sacrificing correctness
3. Adds defensive programming (assertion) without over-engineering
4. Maintains all existing test coverage

The minor documentation improvements I suggested are optional - the code is correct as-is and ready to merge.

Great work on this refactoring!

[openhands-ai]

Code Review Complete ✅

I've completed a comprehensive code review of PR #1251 "feat(fifolock): acquire and release Logic optimization" and posted my findings directly to the PR.

Summary of Review

Overall Verdict: 🟢 Good Taste - LGTM - Approved for Merging

The PR successfully improves the FIFOLock implementation with:

✅ What the PR Does Well:

1. Eliminates redundant operations: Fast-path optimization avoids Condition creation and queue operations when lock is free
2. Cleaner semantics: _waiters now only contains threads actually blocking, not the current owner
3. Performance improvements: Saves 3-4 operations for uncontended and non-blocking cases
4. Maintains correctness: All 7 tests pass including critical FIFO fairness and stress tests

🔍 Key Architectural Insight:

The PR eliminates a subtle special case - in the old design, the current lock owner stayed in the _waiters queue until release(). The new design removes threads from the queue immediately upon acquisition, resulting in cleaner state machine semantics.

💡 Minor Improvement Suggestions (Optional):

I suggested three minor documentation/clarity improvements:

• Line 36: More explicit comment about _waiters semantics
• Lines 103-105: More descriptive assertion message
• Lines 62-66: Comment explaining fast-path optimization

These are optional style suggestions - the code is correct and ready to merge as-is.

Testing Verification:

• ✅ All 7 existing tests pass
• ✅ No breaking changes
• ✅ FIFO ordering preserved
• ✅ Reentrancy works correctly
• ✅ Thread safety maintained

The PR demonstrates good engineering taste and is ready for merge! 🚀

You can view the full detailed review here: #1251 (comment)

_{View full conversation}

[simonrosenberg]

thanks @CLFutureX !