pythongh-135836: Fix IndexError in asyncio.create_connection with empty exceptions list (python#135845)

Co-authored-by: Kumar Aditya <[email protected]>

Author: 2 people

Lib/asyncio/base_events.py

@@ -1161,7 +1161,7 @@ async def create_connection(
                         raise ExceptionGroup("create_connection failed", exceptions)
                     if len(exceptions) == 1:
                         raise exceptions[0]
-                    else:
+                    elif exceptions:
                         # If they all have the same str(), raise one.
                         model = str(exceptions[0])
                         if all(str(exc) == model for exc in exceptions):
@@ -1170,6 +1170,9 @@ async def create_connection(
                         # the various error messages.
                         raise OSError('Multiple exceptions: {}'.format(
                             ', '.join(str(exc) for exc in exceptions)))
+                    else:
+                        # No exceptions were collected, raise a timeout error
+                        raise TimeoutError('create_connection failed')
                 finally:
                     exceptions = None
 

Lib/test/test_asyncio/test_base_events.py

@@ -1190,6 +1190,36 @@ def getaddrinfo(*args, **kw):
                 self.loop.run_until_complete(coro)
             self.assertTrue(sock.close.called)
 
+    @patch_socket
+    def test_create_connection_happy_eyeballs_empty_exceptions(self, m_socket):
+        # See gh-135836: Fix IndexError when Happy Eyeballs algorithm
+        # results in empty exceptions list
+
+        async def getaddrinfo(*args, **kw):
+            return [(socket.AF_INET, socket.SOCK_STREAM, 0, '', ('127.0.0.1', 80)),
+                    (socket.AF_INET6, socket.SOCK_STREAM, 0, '', ('::1', 80))]
+
+        def getaddrinfo_task(*args, **kwds):
+            return self.loop.create_task(getaddrinfo(*args, **kwds))
+
+        self.loop.getaddrinfo = getaddrinfo_task
+
+        # Mock staggered_race to return empty exceptions list
+        # This simulates the scenario where Happy Eyeballs algorithm
+        # cancels all attempts but doesn't properly collect exceptions
+        with mock.patch('asyncio.staggered.staggered_race') as mock_staggered:
+            # Return (None, []) - no winner, empty exceptions list
+            async def mock_race(coro_fns, delay, loop):
+                return None, []
+            mock_staggered.side_effect = mock_race
+
+            coro = self.loop.create_connection(
+                MyProto, 'example.com', 80, happy_eyeballs_delay=0.1)
+
+            # Should raise TimeoutError instead of IndexError
+            with self.assertRaisesRegex(TimeoutError, "create_connection failed"):
+                self.loop.run_until_complete(coro)
+
     def test_create_connection_host_port_sock(self):
         coro = self.loop.create_connection(
             MyProto, 'example.com', 80, sock=object())

Misc/NEWS.d/next/Library/2025-06-23-11-04-25.gh-issue-135836.-C-c4v.rst

@@ -0,0 +1 @@
+Fix :exc:`IndexError` in :meth:`asyncio.loop.create_connection` that could occur when the Happy Eyeballs algorithm resulted in an empty exceptions list during connection attempts.