GeezerVison

Summary

Pool.__init__ used a multiprocessing.SimpleQueue for its internal _change_notifier, a mechanism that wakes up the worker handler thread when the pool state changes (close, terminate, cache empty). SimpleQueue creates two Lock objects backed by POSIX named semaphores via sem_open(), which requires /dev/shm to be mounted as tmpfs.

The change notifier never crosses a process boundary. It is only accessed by threads within the parent process (the worker handler, result handler, and main thread). Named semaphores were never needed here.

This replaces the SimpleQueue with a _ChangeNotifier class backed by multiprocessing.connection.Pipe (which wraps os.pipe()). It provides the same interface (put, get, empty, _reader) without requiring sem_open().

Impact

Both multiprocessing.Pool and multiprocessing.pool.ThreadPool fail on platforms without /dev/shm, even though ThreadPool only uses threads and never needs inter-process synchronization. Affected platforms include AWS Lambda (no /dev/shm), Android and iOS (sem_open() unavailable), and Docker containers with restricted shared memory.

Changes

• Lib/multiprocessing/pool.py: Add _ChangeNotifier class, replace the one line in Pool.__init__ that created the SimpleQueue
• Lib/test/_test_multiprocessing.py: Three new tests — notifier type verification + end-to-end pool.map, notification drain under load, worker process recovery after SIGKILL
• Misc/NEWS.d/next/Library/: NEWS entry

Testing

Full test_multiprocessing_spawn suite: 447 tests passed, 0 failures. Reference leak check passed.

• Issue: Some multiprocessing APIs should be possible to import or use even when multiprocessing itself isn't available #134634