[3.10] gh-88863: Clear ref cycles to resolve leak when asyncio.open_connection raises (GH-95739) (#99722)

Break reference cycles to resolve memory leak, by
removing local exception and future instances from the frame.
(cherry picked from commit 995f617)

Co-authored-by: Dong Uk, Kang <[email protected]>

Author: frostbyte134

Lib/asyncio/base_events.py

@@ -971,6 +971,8 @@ async def _connect_sock(self, exceptions, addr_info, local_addr_infos=None):
             if sock is not None:
                 sock.close()
             raise
+        finally:
+            exceptions = my_exceptions = None
 
     async def create_connection(
             self, protocol_factory, host=None, port=None,
@@ -1063,17 +1065,20 @@ async def create_connection(
 
             if sock is None:
                 exceptions = [exc for sub in exceptions for exc in sub]
-                if len(exceptions) == 1:
-                    raise exceptions[0]
-                else:
-                    # If they all have the same str(), raise one.
-                    model = str(exceptions[0])
-                    if all(str(exc) == model for exc in exceptions):
+                try:
+                    if len(exceptions) == 1:
                         raise exceptions[0]
-                    # Raise a combined exception so the user can see all
-                    # the various error messages.
-                    raise OSError('Multiple exceptions: {}'.format(
-                        ', '.join(str(exc) for exc in exceptions)))
+                    else:
+                        # If they all have the same str(), raise one.
+                        model = str(exceptions[0])
+                        if all(str(exc) == model for exc in exceptions):
+                            raise exceptions[0]
+                        # Raise a combined exception so the user can see all
+                        # the various error messages.
+                        raise OSError('Multiple exceptions: {}'.format(
+                            ', '.join(str(exc) for exc in exceptions)))
+                finally:
+                    exceptions = None
 
         else:
             if sock is None:
@@ -1862,6 +1867,8 @@ def _run_once(self):
 
         event_list = self._selector.select(timeout)
         self._process_events(event_list)
+        # Needed to break cycles when an exception occurs.
+        event_list = None
 
         # Handle 'later' callbacks that are ready.
         end_time = self.time() + self._clock_resolution

Lib/asyncio/selector_events.py

@@ -497,7 +497,11 @@ async def sock_connect(self, sock, address):
 
         fut = self.create_future()
         self._sock_connect(fut, sock, address)
-        return await fut
+        try:
+            return await fut
+        finally:
+            # Needed to break cycles when an exception occurs.
+            fut = None
 
     def _sock_connect(self, fut, sock, address):
         fd = sock.fileno()
@@ -519,6 +523,8 @@ def _sock_connect(self, fut, sock, address):
             fut.set_exception(exc)
         else:
             fut.set_result(None)
+        finally:
+            fut = None
 
     def _sock_write_done(self, fd, fut, handle=None):
         if handle is None or not handle.cancelled():
@@ -542,6 +548,8 @@ def _sock_connect_cb(self, fut, sock, address):
             fut.set_exception(exc)
         else:
             fut.set_result(None)
+        finally:
+            fut = None
 
     async def sock_accept(self, sock):
         """Accept a connection.

Lib/asyncio/windows_events.py

@@ -439,7 +439,11 @@ def select(self, timeout=None):
             self._poll(timeout)
         tmp = self._results
         self._results = []
-        return tmp
+        try:
+            return tmp
+        finally:
+            # Needed to break cycles when an exception occurs.
+            tmp = None
 
     def _result(self, value):
         fut = self._loop.create_future()
@@ -821,6 +825,8 @@ def _poll(self, timeout=None):
                 else:
                     f.set_result(value)
                     self._results.append(f)
+                finally:
+                    f = None
 
         # Remove unregistered futures
         for ov in self._unregistered:

Misc/NEWS.d/next/Library/2022-08-06-12-18-07.gh-issue-88863.NnqsuJ.rst

@@ -0,0 +1,3 @@
+To avoid apparent memory leaks when :func:`asyncio.open_connection` raises,
+break reference cycles generated by local exception and future instances
+(which has exception instance as its member var). Patch by Dong Uk, Kang.