Fix deadlock in AsyncWrapper thread pool shutdown

[shunping]

During test execution or pipeline teardown, calling reset_state on AsyncWrapper intermittently hangs the pipeline indefinitely, leading to test runner timeouts (https://github.com/apache/beam/actions/runs/25590690572/job/75127752557?pr=38425).

The traceback of timeout is shown below:

________________________ AsyncTest_1.test_duplicates __________________________
[gw0] darwin -- Python 3.13.13 /Users/runner/work/beam/beam/sdks/python/target/.tox/py313-macos/bin/python

self = <apache_beam.transforms.async_dofn_test.AsyncTest_1 testMethod=test_duplicates>

    def setUp(self):
      super().setUp()
>     async_lib.AsyncWrapper.reset_state()

apache_beam/transforms/async_dofn_test.py:102: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
target/.tox/py313-macos/lib/python3.13/site-packages/apache_beam/transforms/async_dofn.py:169: in reset_state
    pool.acquire(AsyncWrapper.initialize_pool(1)).shutdown(
/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/concurrent/futures/thread.py:239: in shutdown
    t.join()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <Thread(ThreadPoolExecutor-180_0, stopped 14025928704)>, timeout = None

    def join(self, timeout=None):
        """Wait until the thread terminates.
    
        This blocks the calling thread until the thread whose join() method is
        called terminates -- either normally or through an unhandled exception
        or until the optional timeout occurs.
    
        When the timeout argument is present and not None, it should be a
        floating-point number specifying a timeout for the operation in seconds
        (or fractions thereof). As join() always returns None, you must call
        is_alive() after join() to decide whether a timeout happened -- if the
        thread is still alive, the join() call timed out.
    
        When the timeout argument is not present or None, the operation will
        block until the thread terminates.
    
        A thread can be join()ed many times.
    
        join() raises a RuntimeError if an attempt is made to join the current
        thread as that would cause a deadlock. It is also an error to join() a
        thread before it has been started and attempts to do so raises the same
        exception.
    
        """
        if not self._initialized:
            raise RuntimeError("Thread.__init__() not called")
        if not self._started.is_set():
            raise RuntimeError("cannot join thread before it is started")
        if self is current_thread():
            raise RuntimeError("cannot join current thread")
    
        # the behavior of a negative timeout isn't documented, but
    self._bootstrap_inner()
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/threading.py", line 1044, in _bootstrap_inner
    self.run()
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/threading.py", line 995, in run
    self._target(*self._args, **self._kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/managers.py", line 179, in serve_forever
    self.stop_event.wait(1)
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/threading.py", line 660, in wait
    signaled = self._cond.wait(timeout)
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/threading.py", line 363, in wait
    gotit = waiter.acquire(True, timeout)

The deadlock occurs in the following order:

• The main thread executing reset_state acquires AsyncWrapper._lock and calls pool.shutdown(wait=True). It remains blocked waiting for active worker threads to finish while holding the lock.
• When an active worker thread finishes processing an item, the underlying future completes and tries to invoke its done callbacks on that same worker thread. The registered callback decrement_items_in_buffer, however, attempts to acquire AsyncWrapper._lock to decrement the buffer count. This leads to a deadlock.

To fix it, we move the shutdown call outside of the lock scope, so while the main thread is waiting for worker threads to complete, it won't hold the lock.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an intermittent deadlock issue occurring during the teardown of AsyncWrapper thread pools. By decoupling the shutdown process from the internal state lock, the main thread can now safely wait for worker threads to complete without blocking the callbacks that require the same lock. Additionally, defensive checks and a robust reproduction test have been added to ensure stability.

Highlights

• Deadlock Resolution: Refactored the reset_state method in AsyncWrapper to perform thread pool shutdowns outside of the internal lock scope, preventing deadlocks during cleanup.
• Safety Improvements: Added a safety check in decrement_items_in_buffer to ensure the UUID exists in the buffer before attempting to decrement, avoiding potential key errors.
• Regression Testing: Introduced a new test case test_reset_state_hang_reproduction that uses a separate process to verify that reset_state no longer hangs the pipeline.

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[shunping]

r: @dustin12

[gemini-code-assist]

Code Review

This pull request addresses a deadlock issue in AsyncWrapper.reset_state by moving the pool shutdown process outside of the global lock and adds a regression test to detect such hangs. A review comment correctly identifies that the pool.acquire() call remains inside the lock, which could still cause a deadlock if the pool is at capacity, and provides a suggestion to move it outside the lock scope.

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