Revert "Don't grab GIL from C-Core during shutdown (#37762)"

This change appears to cause a deadlock.

This reverts commit 4b53c2bac3.

src/python/grpcio/grpc/_cython/_cygrpc/credentials.pyx.pxi

@@ -12,7 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import atexit
 
 def _spawn_callback_in_thread(cb_func, args):
   t = ForkManagedThread(target=cb_func, args=args)
@@ -66,43 +65,11 @@ cdef int _get_metadata(void *state,
   return 0  # Asynchronous return
 
 
-# Protects access to GIL from _destroy() and to g_shutting_down.
-cdef mutex g_shutdown_mu
-cdef int g_shutting_down = 0
-
-# This is called by C-core when the plugin is destroyed, which may race between
-# GIL destruction during process shutdown. Since GIL destruction happens after
-# Python's exit handlers, we mark that Python is shutting down from an exit
-# handler and don't grab GIL in this function afterwards using a C mutex.
-# Access to g_shutting_down and GIL must be mutex protected here.
-cdef void _destroy(void *state) nogil:
-  global g_shutdown_mu
-  global g_shutting_down
-  g_shutdown_mu.lock()
-  if g_shutting_down == 0:
-    with gil:
-      cpython.Py_DECREF(<object>state)
-  g_shutdown_mu.unlock()
+cdef void _destroy(void *state) except * with gil:
+  cpython.Py_DECREF(<object>state)
   grpc_shutdown()
 
 
-g_shutdown_handler_registered = False
-
-def _maybe_register_shutdown_handler():
-  global g_shutdown_handler_registered
-  if g_shutdown_handler_registered:
-    return
-  g_shutdown_handler_registered = True
-  atexit.register(_on_shutdown)
-
-def _on_shutdown():
-  global g_shutdown_mu
-  global g_shutting_down
-  g_shutdown_mu.lock()
-  g_shutting_down = 1
-  g_shutdown_mu.unlock()
-
-
 cdef class MetadataPluginCallCredentials(CallCredentials):
 
   def __cinit__(self, metadata_plugin, name):
@@ -116,7 +83,6 @@ cdef class MetadataPluginCallCredentials(CallCredentials):
     c_metadata_plugin.state = <void *>self._metadata_plugin
     c_metadata_plugin.type = self._name
     cpython.Py_INCREF(self._metadata_plugin)
-    _maybe_register_shutdown_handler()
     fork_handlers_and_grpc_init()
     # TODO(yihuazhang): Expose min_security_level via the Python API so that
     # applications can decide what minimum security level their plugins require.