Implement queue locking mechanism in StreamingResponse to prevent race conditions during activity processing

[cleemullins]

Fixes #188

Fixed Concurrency Errors in streaming_response.py

I've identified and fixed several concurrency issues in the streaming response implementation:

Issues Found and Fixed

1. Added asyncio.Lock for queue synchronization

• Added self._queue_lock in __init__ to protect shared state access
• This lock prevents race conditions when multiple async contexts access the queue simultaneously

2. Fixed race condition in _chunk_queued flag

• Problem: The flag was reset at the beginning of create_activity(), which could cause race conditions
• Solution: Moved self._chunk_queued = False to execute after activity creation in all code paths
• This ensures the flag is properly reset before returning, preventing lost updates

3. Protected _queue_sync task management

• Problem: Checking and starting the drain task without synchronization could lead to multiple drain tasks running
• Solution:
  • Changed _queue_activity() to use a lock when checking and starting the drain task
  • Added a check for done() status to handle completed tasks correctly
  • This prevents multiple drain tasks from running simultaneously

4. Thread-safe queue operations

• Problem: self._queue.pop(0) could be accessed by multiple async contexts
• Solution:
  • Protected queue access with the lock in _drain_queue()
  • Added a double-check pattern to safely handle empty queue scenarios
  • Ensures the _queue_sync cleanup happens under lock protection

Key Changes Made

Location	Change
Line 54	Added self._queue_lock = asyncio.Lock()
Lines 273-318	Refactored _queue_next_chunk() to reset _chunk_queued after activity creation
Lines 320-332	Refactored _queue_activity() to use lock-protected task startup
Lines 334-362	Protected queue access in _drain_queue() with lock

Impact

These changes eliminate race conditions that could occur when:

• Multiple async contexts try to queue activities simultaneously
• The drain task is checked and started from different contexts
• Queue items are being added while being consumed
• The _chunk_queued flag is being read/written from different contexts

The implementation is now thread-safe for async operations and prevents potential data corruption or lost updates.

[Copilot]

Pull Request Overview

This PR implements a queue locking mechanism to address race conditions in the StreamingResponse class's activity processing. The changes introduce an asyncio.Lock to protect shared state during concurrent access and refactor queue management logic to ensure thread-safe operations.

Key Changes:

• Added asyncio.Lock to synchronize access to the activity queue and related state
• Refactored _chunk_queued flag reset logic to occur after activity creation rather than at the beginning
• Modified queue drain task management to use lock-protected startup and completion checks

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[Copilot]

Creating a task to start another task introduces unnecessary complexity and potential timing issues. The inner async function start_drain_if_needed() creates a task that may not complete before _queue_activity() returns, potentially allowing race conditions if _queue_activity() is called again immediately. Since _queue_activity() is not async, consider either making it async and awaiting the lock directly, or using a synchronous lock approach with asyncio.create_task(self._drain_queue()) protected by a different synchronization mechanism.

[Copilot]

The _chunk_queued flag is being modified without lock protection. Since this flag is accessed from multiple async contexts (as mentioned in the PR description), resetting it here and at line 309 should be protected by self._queue_lock to prevent race conditions where multiple threads might read stale values or encounter lost updates.

[Copilot]

Setting _chunk_queued = True occurs outside any lock protection. This creates a race condition where multiple callers of _queue_next_chunk() could simultaneously set the flag and queue activities. The flag assignment should be protected by self._queue_lock to ensure atomic read-modify-write operations.

[Copilot]

The queue append operation is not protected by the lock, but the queue access in _drain_queue() is protected. This creates a race condition where items could be added to self._queue while _drain_queue() is checking if the queue is empty or popping items. Move this append operation inside the lock in start_drain_if_needed() or add separate lock protection here.

[Copilot]

[nitpick] Accessing len(self._queue) for logging occurs outside the lock protection. While this is just for debugging, it could produce misleading log output if the queue size changes between the log call and the actual processing. Consider moving this inside the first lock acquisition or removing the count from the log message.

Suggested change

	logger.debug(f"Draining queue with {len(self._queue)} activities.")
	while self._queue:
	factory = self._queue.pop(0)
	# Use lock to safely access the queue
	async with self._queue_lock:
	while self._queue:
	# Use lock to safely access the queue
	async with self._queue_lock:
	logger.debug(f"Draining queue with {len(self._queue)} activities.")

[cleemullins]

After fighting with the PR for a day, I'm no longer convinced this is the right direction. We should hold this until the next release, and re-evaluate.

[Copilot]

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[cleemullins]

This PR shouldn't be merged. Closing. Will re-do to fix the open issue in the next release.