gh-71936: Fix race condition in multiprocessing.Pool

Proxes of shared objects register a Finalizer in BaseProxy._incref(), and it
will call BaseProxy._decref() when it is GCed. This may cause a race condition
with Pool(maxtasksperchild=None) on Windows.

A connection will be closed and raised TypeError when a GC occurs between
_ConnectionBase._check_writable() and _ConnectionBase._send_bytes() in
_ConnectionBase.send() in the second or later task.

BaseProxy does not count references well.

Author: hattya

Lib/multiprocessing/managers.py

@@ -850,20 +850,21 @@ def _incref(self):
         dispatch(conn, None, 'incref', (self._id,))
         util.debug('INCREF %r', self._token.id)
 
-        self._idset.add(self._id)
+        refid = f'{self._id}.{id(self)}'
+        self._idset.add(refid)
 
         state = self._manager and self._manager._state
 
         self._close = util.Finalize(
             self, BaseProxy._decref,
-            args=(self._token, self._authkey, state,
+            args=(self._token, refid, self._authkey, state,
                   self._tls, self._idset, self._Client),
             exitpriority=10
             )
 
     @staticmethod
-    def _decref(token, authkey, state, tls, idset, _Client):
-        idset.discard(token.id)
+    def _decref(token, refid, authkey, state, tls, idset, _Client):
+        idset.discard(refid)
 
         # check whether manager is still alive
         if state is None or state.value == State.STARTED:

Misc/ACKS

@@ -706,6 +706,7 @@ Larry Hastings
 Tim Hatch
 Zac Hatfield-Dodds
 Shane Hathaway
+Akinori Hattori
 Michael Haubenwallner
 Janko Hauser
 Flavian Hautbois

Misc/NEWS.d/next/Library/2022-10-15-10-18-20.gh-issue-71936.MzJjc_.rst

@@ -0,0 +1 @@
+Fix a race condition in :class:`multiprocessing.Pool`.