pythongh-101225: Fix hang when passing Pipe instances to child in mul…

…tiprocessing

This PR uses the same backlog value when creating
a `.connection.Client` in `.resource_sharer` as is used in
`.manager`.

On macOS the default backlog (1) is small enough to cause the
socket accept queue to fill up when starting a number of children.

Lib/multiprocessing/resource_sharer.py

@@ -123,7 +123,7 @@ def _start(self):
         from .connection import Listener
         assert self._listener is None, "Already have Listener"
         util.debug('starting listener and thread for sending handles')
-        self._listener = Listener(authkey=process.current_process().authkey)
+        self._listener = Listener(authkey=process.current_process().authkey, backlog=16)
         self._address = self._listener.address
         t = threading.Thread(target=self._serve)
         t.daemon = True

Misc/NEWS.d/next/Library/2023-12-29-17-46-06.gh-issue-101225.QaEyxF.rst

@@ -0,0 +1,2 @@
+Fix a hang in :mod:`multiprocessing` when passing ``Pipe`` instances to
+workers on macOS.