gh-109534: fix reference leak when SSL handshake fails (#114074)

Author: ordinary-jamie

Lib/asyncio/selector_events.py

@@ -235,6 +235,10 @@ async def _accept_connection2(
                 await waiter
             except BaseException:
                 transport.close()
+                # gh-109534: When an exception is raised by the SSLProtocol object the
+                # exception set in this future can keep the protocol object alive and
+                # cause a reference cycle.
+                waiter = None
                 raise
                 # It's now up to the protocol to handle the connection.
 

Lib/asyncio/sslproto.py

@@ -579,6 +579,7 @@ def _on_handshake_complete(self, handshake_exc):
 
             peercert = sslobj.getpeercert()
         except Exception as exc:
+            handshake_exc = None
             self._set_state(SSLProtocolState.UNWRAPPED)
             if isinstance(exc, ssl.CertificateError):
                 msg = 'SSL handshake failed on verifying the certificate'

Misc/NEWS.d/next/Library/2024-01-15-18-42-44.gh-issue-109534.wYaLMZ.rst

@@ -0,0 +1,3 @@
+Fix a reference leak in
+:class:`asyncio.selector_events.BaseSelectorEventLoop` when SSL handshakes
+fail. Patch contributed by Jamie Phan.