Bug report

Bug description:

PyDict_AddWatcher, PyDict_ClearWatcher, PyDict_Watch, PyDict_Unwatch, and _PyDict_SendEvent are not safe to call concurrently in free-threaded builds. The shared callback array and per-dict _ma_watcher_tag can race when watchers are registered, removed, or triggered from multiple threads.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-145235: Make dict watcher API thread-safe for free-threaded builds #145233
• [3.15] gh-145235: Make dict watcher API thread-safe for free-threaded builds (gh-145233) #149690
• [3.14] gh-145235: Make dict watcher API thread-safe for free-threaded builds (gh-145233) #149691

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Note: Running test.test_free_threading.test_dict_watcher (added in #145233) emits tsan warnings in the free-threaded tsan build.