* 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.
Trent Nelson
f156510960