Prevent concurrent interactive flows listening on same port when running on Windows

msal/oauth2cli/authcode.py

@@ -7,6 +7,7 @@
 """
 import logging
 import socket
+import sys
 from string import Template
 import threading
 import time
@@ -103,7 +104,17 @@ def log_message(self, format, *args):
         logger.debug(format, *args)  # To override the default log-to-stderr behavior
 
 
-class _AuthCodeHttpServer(HTTPServer):
+class _AuthCodeHttpServer(HTTPServer, object):
+    def __init__(self, server_address, *args, **kwargs):
+        _, port = server_address
+        if port and (sys.platform == "win32" or is_wsl()):
+            # The default allow_reuse_address is True. It works fine on non-Windows.
+            # On Windows, it undesirably allows multiple servers listening on same port,
+            # yet the second server would not receive any incoming request.
+            # So, we need to turn it off.
+            self.allow_reuse_address = False
+        super(_AuthCodeHttpServer, self).__init__(server_address, *args, **kwargs)
+
     def handle_timeout(self):
         # It will be triggered when no request comes in self.timeout seconds.
         # See https://docs.python.org/3/library/socketserver.html#socketserver.BaseServer.handle_timeout

tests/test_authcode.py

@@ -0,0 +1,26 @@
+import unittest
+import socket
+import sys
+
+from msal.oauth2cli.authcode import AuthCodeReceiver
+
+
+class TestAuthCodeReceiver(unittest.TestCase):
+    def test_setup_at_a_given_port_and_teardown(self):
+        port = 12345  # Assuming this port is available
+        with AuthCodeReceiver(port=port) as receiver:
+            self.assertEqual(port, receiver.get_port())
+
+    def test_setup_at_a_ephemeral_port_and_teardown(self):
+        port = 0
+        with AuthCodeReceiver(port=port) as receiver:
+            self.assertNotEqual(port, receiver.get_port())
+
+    def test_no_two_concurrent_receivers_can_listen_on_same_port(self):
+        port = 12345  # Assuming this port is available
+        with AuthCodeReceiver(port=port) as receiver:
+            expected_error = OSError if sys.version_info[0] > 2 else socket.error
+            with self.assertRaises(expected_error):
+                with AuthCodeReceiver(port=port) as receiver2:
+                    pass
+