gh-124858: fix happy eyeballs refcyles (#124859)

Lib/asyncio/base_events.py

@@ -17,7 +17,6 @@ import collections
 import collections.abc
 import concurrent.futures
 import errno
-import functools
 import heapq
 import itertools
 import os
@@ -1140,11 +1139,18 @@ class BaseEventLoop(events.AbstractEventLoop):
                     except OSError:
                         continue
             else:  # using happy eyeballs
-                sock, _, _ = await staggered.staggered_race(
-                    (functools.partial(self._connect_sock,
-                                       exceptions, addrinfo, laddr_infos)
-                     for addrinfo in infos),
-                    happy_eyeballs_delay, loop=self)
+                sock = (await staggered.staggered_race(
+                    (
+                        # can't use functools.partial as it keeps a reference
+                        # to exceptions
+                        lambda addrinfo=addrinfo: self._connect_sock(
+                            exceptions, addrinfo, laddr_infos
+                        )
+                        for addrinfo in infos
+                    ),
+                    happy_eyeballs_delay,
+                    loop=self,
+                ))[0]  # can't use sock, _, _ as it keeks a reference to exceptions
 
             if sock is None:
                 exceptions = [exc for sub in exceptions for exc in sub]

Lib/asyncio/staggered.py

@@ -133,6 +133,7 @@ async def staggered_race(coro_fns, delay, *, loop=None):
                         raise d.exception()
         return winner_result, winner_index, exceptions
     finally:
+        del exceptions
         # Make sure no tasks are left running if we leave this function
         for t in running_tasks:
             t.cancel()

Lib/test/test_asyncio/test_streams.py

@@ -1200,6 +1200,24 @@ class StreamTests(test_utils.TestCase):
         messages = self._basetest_unhandled_exceptions(handle_echo)
         self.assertEqual(messages, [])
 
+    def test_open_connection_happy_eyeball_refcycles(self):
+        port = socket_helper.find_unused_port()
+        async def main():
+            exc = None
+            try:
+                await asyncio.open_connection(
+                    host="localhost",
+                    port=port,
+                    happy_eyeballs_delay=0.25,
+                )
+            except* OSError as excs:
+                # can't use assertRaises because that clears frames
+                exc = excs.exceptions[0]
+            self.assertIsNotNone(exc)
+            self.assertListEqual(gc.get_referrers(exc), [])
+
+        asyncio.run(main())
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Library/2024-10-01-17-12-20.gh-issue-124858.Zy0tvT.rst

@@ -0,0 +1 @@
+Fix reference cycles left in tracebacks in :func:`asyncio.open_connection` when used with ``happy_eyeballs_delay``