gh-101225: Increase the socket backlog when creating a multiprocessing.connection.Listener (#113567)

Increase the backlog for multiprocessing.connection.Listener` objects created
 by `multiprocessing.manager` and `multiprocessing.resource_sharer` to
 significantly reduce the risk of getting a connection refused error when creating
 a `multiprocessing.connection.Connection` to them.

Lib/multiprocessing/managers.py

@@ -156,7 +156,7 @@ class Server(object):
         Listener, Client = listener_client[serializer]
 
         # do authentication later
-        self.listener = Listener(address=address, backlog=16)
+        self.listener = Listener(address=address, backlog=128)
         self.address = self.listener.address
 
         self.id_to_obj = {'0': (None, ())}

Lib/multiprocessing/resource_sharer.py

@@ -123,7 +123,7 @@ class _ResourceSharer(object):
         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=128)
         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 +1,4 @@
+Increase the backlog for :class:`multiprocessing.connection.Listener` objects created
+by :mod:`multiprocessing.manager` and :mod:`multiprocessing.resource_sharer` to
+significantly reduce the risk of getting a connection refused error when creating
+a :class:`multiprocessing.connection.Connection` to them.