When a thread exits with pending asyncio Tasks, PyThreadState_Clear() moves them
onto the shared interp->asyncio_tasks_head list, but each Task still records the
_Py_ThreadId() of the thread that created it. unregister_task() skips the
stop-the-world pause whenever that id matches the current thread. That also
happens for a later thread that reuses the exited thread's pthread_t, and for
destructors that run later in PyThreadState_Clear(). The resulting unlocked
llist_remove() calls on the shared list race with each other and can leave an
already-removed node (next == NULL) linked into it. asyncio.all_tasks(), which
asyncio.run() calls on shutdown, then crashes with SIGSEGV in
_asyncio_all_tasks().

Record the unique id (PyThreadState.id) of the creating thread state instead,
and only take the lock-free path while that thread state is not being cleared.

Also drop the assertion in add_tasks_llist() that every linked Task is pending.
A Task's state is set before unregister_task() unlinks it, and that removal can
be waiting for the stop-the-world pause during which the list is walked.
add_one_task() already filters out Tasks that are done.

…ncio-task-unregister-race