bpo-43952: Fix multiprocessing Listener authkey bug (GH-25845)

Listener.accept() no longer hangs when authkey is an empty bytes object.

Lib/multiprocessing/connection.py

@@ -476,8 +476,9 @@ def accept(self):
         '''
         if self._listener is None:
             raise OSError('listener is closed')
+
         c = self._listener.accept()
-        if self._authkey:
+        if self._authkey is not None:
             deliver_challenge(c, self._authkey)
             answer_challenge(c, self._authkey)
         return c

Lib/test/_test_multiprocessing.py

@@ -3504,6 +3504,25 @@ def test_context(self):
         if self.TYPE == 'processes':
             self.assertRaises(OSError, l.accept)
 
+    def test_empty_authkey(self):
+        # bpo-43952: allow empty bytes as authkey
+        def handler(*args):
+            raise RuntimeError('Connection took too long...')
+
+        def run(addr, authkey):
+            client = self.connection.Client(addr, authkey=authkey)
+            client.send(1729)
+
+        key = b""
+
+        with self.connection.Listener(authkey=key) as listener:
+            threading.Thread(target=run, args=(listener.address, key)).start()
+            with listener.accept() as d:
+                self.assertEqual(d.recv(), 1729)
+
+        if self.TYPE == 'processes':
+            self.assertRaises(OSError, listener.accept)
+
     @unittest.skipUnless(util.abstract_sockets_supported,
                          "test needs abstract socket support")
     def test_abstract_socket(self):

Misc/NEWS.d/next/Library/2021-05-03-11-04-12.bpo-43952.Me7fJe.rst

@@ -0,0 +1,2 @@
+Fix :meth:`multiprocessing.connection.Listener.accept()` to accept empty bytes
+as authkey. Not accepting empty bytes as key causes it to hang indefinitely.