[3.12] gh-105716: Support Background Threads in Subinterpreters Consistently (gh-109921) (gh-110707)

The existence of background threads running on a subinterpreter was preventing interpreters from getting properly destroyed, as well as impacting the ability to run the interpreter again. It also affected how we wait for non-daemon threads to finish.

We add PyInterpreterState.threads.main, with some internal C-API functions.

(cherry-picked from commit 1dd9dee45d)

Lib/threading.py

@@ -37,6 +37,7 @@ _daemon_threads_allowed = _thread.daemon_threads_allowed
 _allocate_lock = _thread.allocate_lock
 _set_sentinel = _thread._set_sentinel
 get_ident = _thread.get_ident
+_is_main_interpreter = _thread._is_main_interpreter
 try:
     get_native_id = _thread.get_native_id
     _HAVE_THREAD_NATIVE_ID = True
@@ -1566,7 +1567,7 @@ def _shutdown():
     # the main thread's tstate_lock - that won't happen until the interpreter
     # is nearly dead.  So we release it here.  Note that just calling _stop()
     # isn't enough:  other threads may already be waiting on _tstate_lock.
-    if _main_thread._is_stopped:
+    if _main_thread._is_stopped and _is_main_interpreter():
         # _shutdown() was already called
         return
 
@@ -1619,6 +1620,7 @@ def main_thread():
     In normal conditions, the main thread is the thread from which the
     Python interpreter was started.
     """
+    # XXX Figure this out for subinterpreters.  (See gh-75698.)
     return _main_thread
 
 # get thread-local implementation, either from the thread