* gh-151518: Avoid STW starvation of attaching threads Free-threaded stop-the-world pauses can otherwise starve a thread trying to reattach after it was suspended while detached. A tight manual gc.collect() loop can release and immediately request the next stop-the-world pause, repeatedly parking the detached thread before it can attach and make progress. Add a distinct _Py_THREAD_SUSPENDED_DETACHED state for tstates parked from DETACHED. tstate_wait_attach() marks an attach waiter only after observing that detached-origin suspended state, and park_detached_threads() skips only those active waiters on later stop-the-world passes. The ordinary successful tstate_try_attach() path remains the baseline CAS-only path. Teach the related stop-the-world paths about both suspended states, including start_the_world() and tstate_delete_common(). Keep the new wait flag after the existing hot free-threaded _PyThreadStateImpl fields so their offsets do not move. Add a free-threaded GC regression test that runs a subprocess with a tight gc.collect() worker and verifies the main thread can reattach after sleeping and stop the worker. * gh-151518: Track attach waiters in the thread state Represent active attach waiters with suspended-waiting and detached-waiting states. Preserve waiter registration when the world resumes, and keep passive detached threads immediately parkable. Restore the thread-state padding and retain the single-CAS uncontended attach path. * gh-151518: Simplify the GC fairness regression Use explicit warmup imports and a joined non-daemon collector. Run the child through script_helper with a faulthandler watchdog so a stalled attachment still fails with a traceback. Describe the bug as a fairness issue in the NEWS entry. * gh-151518: Exercise STW fairness directly * gh-151518: Preserve attach waiters when resuming BRC suspensions Share the waiter-aware resume transition between stop-the-world pauses and biased reference count merging so a concurrently registering waiter keeps its opportunity to attach. Restore the support import needed by the new upstream GC regression and align the thread-state constants. * gh-151518: Strengthen the STW attach fairness regression Hold each test pause for 10 ms and repeat pauses in C to reduce the opportunities for a waiting thread to run between pause requests. Clarify that parking rechecks the thread state before sleeping. * gh-151518: Restrict STW test pauses to free-threaded builds STW is a no-op in GIL builds. Avoid waiting in the helper there, since WASI cannot perform the blocking futex operation. * gh-151518: Wake only registered attach waiters Avoid a parking-lot lookup when resuming a suspended thread that has not tried to reattach. Use the old state from the successful CAS so concurrent waiter registration still receives a wakeup.

Commits

Commits on Sep 25, 2026

gh-151518: Avoid STW starvation of attaching threads (#152826)

gh-151464: exclude '<>' token from tokenize output (#154854)

gh-157649: Merge test_cppext into test_cext (#158176)

gh-158001: Fix test_ctypes.test_version_var() on s390x (#158193)