gh-160513: Call connection_lost() when start_tls() handshake is lost (GH-160514)

SSLProtocol.connection_lost() did not notify the application protocol
while a handshake was in progress, even when start_tls() had handed
over a protocol that had already seen connection_made().  With streams
this left StreamWriter.wait_closed() waiting forever.

ANT-2026-A6ZIAFVT
ANT-2026-4SKWM05E

Lib/asyncio/sslproto.py

@@ -406,13 +406,15 @@ class SSLProtocol(protocols.BufferedProtocol):
         if self._app_transport is not None:
             self._app_transport._closed = True
 
-        if self._state != SSLProtocolState.DO_HANDSHAKE:
-            if (
-                self._app_state == AppProtocolState.STATE_CON_MADE or
-                self._app_state == AppProtocolState.STATE_EOF
-            ):
-                self._app_state = AppProtocolState.STATE_CON_LOST
-                self._loop.call_soon(self._app_protocol.connection_lost, exc)
+        # The app protocol gets connection_lost() only if it got
+        # connection_made().  After start_tls() that is already the case
+        # while the handshake is in progress.
+        if (
+            self._app_state == AppProtocolState.STATE_CON_MADE or
+            self._app_state == AppProtocolState.STATE_EOF
+        ):
+            self._app_state = AppProtocolState.STATE_CON_LOST
+            self._loop.call_soon(self._app_protocol.connection_lost, exc)
         self._set_state(SSLProtocolState.UNWRAPPED)
         self._transport = None
         self._app_transport = None

Lib/test/test_asyncio/test_sslproto.py

@@ -2,6 +2,7 @@
 
 import logging
 import socket
+import struct
 import unittest
 import weakref
 from test import support
@@ -153,6 +154,29 @@ class SslProtoHandshakeTests(test_utils.TestCase):
         test_utils.run_briefly(self.loop)
         self.assertIsInstance(waiter.exception(), ConnectionAbortedError)
 
+    def test_connection_lost_during_handshake(self):
+        # The app protocol gets connection_lost() if and only if it got
+        # connection_made().  With start_tls() (call_connection_made=False)
+        # it is already connected when the handshake starts.
+        for call_connection_made in (True, False):
+            with self.subTest(call_connection_made=call_connection_made):
+                app_proto = mock.Mock(spec=asyncio.Protocol)
+                ssl_proto = sslproto.SSLProtocol(
+                    self.loop, app_proto, test_utils.dummy_ssl_context(),
+                    waiter=None, call_connection_made=call_connection_made)
+                self.connection_made(
+                    ssl_proto,
+                    do_handshake=mock.Mock(side_effect=ssl.SSLWantReadError)
+                )
+                exc = ConnectionResetError()
+                ssl_proto.connection_lost(exc)
+                test_utils.run_briefly(self.loop)
+                app_proto.connection_made.assert_not_called()
+                if call_connection_made:
+                    app_proto.connection_lost.assert_not_called()
+                else:
+                    app_proto.connection_lost.assert_called_once_with(exc)
+
     def test_connection_lost_when_busy(self):
         # gh-118950: SSLProtocol.connection_lost not being called when OSError
         # is thrown on asyncio.write.
@@ -570,9 +594,10 @@ class BaseStartTLS(func_tests.FunctionalTestCaseMixin):
                 sock.close()
 
         class ClientProto(asyncio.Protocol):
-            def __init__(self, on_data, on_eof):
+            def __init__(self, on_data, on_eof, on_con_lost):
                 self.on_data = on_data
                 self.on_eof = on_eof
+                self.on_con_lost = on_con_lost
                 self.con_made_cnt = 0
 
             def connection_made(proto, tr):
@@ -586,14 +611,18 @@ class BaseStartTLS(func_tests.FunctionalTestCaseMixin):
             def eof_received(self):
                 self.on_eof.set_result(True)
 
+            def connection_lost(self, exc):
+                self.on_con_lost.set_result(exc)
+
         async def client(addr):
             await asyncio.sleep(0.5)
 
             on_data = self.loop.create_future()
             on_eof = self.loop.create_future()
+            on_con_lost = self.loop.create_future()
 
             tr, proto = await self.loop.create_connection(
-                lambda: ClientProto(on_data, on_eof), *addr)
+                lambda: ClientProto(on_data, on_eof, on_con_lost), *addr)
 
             tr.write(HELLO_MSG)
 
@@ -604,11 +633,59 @@ class BaseStartTLS(func_tests.FunctionalTestCaseMixin):
                     self.loop.start_tls(tr, proto, client_context),
                     0.5)
 
+            self.assertIsNone(
+                await asyncio.wait_for(on_con_lost, support.LOOPBACK_TIMEOUT))
+
         with self.tcp_server(serve, timeout=self.TIMEOUT) as srv:
             self.loop.run_until_complete(
                 asyncio.wait_for(client(srv.addr),
                                  timeout=support.SHORT_TIMEOUT))
 
+    def _test_start_tls_streams_failed_handshake(self, serve, expected_exc,
+                                                 **kwargs):
+        # wait_closed() must not hang if the connection is lost while
+        # the handshake started by start_tls() is still in progress.
+        client_context = test_utils.simple_client_sslcontext()
+
+        async def client(addr):
+            reader, writer = await asyncio.open_connection(*addr)
+            with self.assertRaises(expected_exc):
+                await writer.start_tls(client_context, **kwargs)
+            writer.close()
+            with self.assertRaises(expected_exc):
+                await writer.wait_closed()
+
+        with self.tcp_server(serve, timeout=self.TIMEOUT) as srv:
+            self.loop.run_until_complete(
+                asyncio.wait_for(client(srv.addr),
+                                 timeout=support.SHORT_TIMEOUT))
+
+    def test_start_tls_streams_handshake_reset(self):
+        def serve(sock):
+            sock.settimeout(self.TIMEOUT)
+            sock.recv(1024 * 1024)  # ClientHello
+            # Send RST instead of FIN.
+            sock.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER,
+                            struct.pack('ii', 1, 0))
+            sock.close()
+
+        self._test_start_tls_streams_failed_handshake(
+            serve, ConnectionError)
+
+    def test_start_tls_streams_handshake_timeout(self):
+        def serve(sock):
+            sock.settimeout(self.TIMEOUT)
+            try:
+                while sock.recv(1024 * 1024):
+                    pass
+            except ConnectionError:
+                pass
+            finally:
+                sock.close()
+
+        self._test_start_tls_streams_failed_handshake(
+            serve, ConnectionAbortedError, ssl_handshake_timeout=0.1)
+
     @socket_helper.skip_if_tcp_blackhole
     def test_start_tls_server_1(self):
         HELLO_MSG = b'1' * self.PAYLOAD_SIZE

Lib/test/test_asyncio/test_streams.py

@@ -14,6 +14,7 @@ except ImportError:
 
 import asyncio
 from test.test_asyncio import utils as test_utils
+from test import support
 from test.support import socket_helper
 
 
@@ -861,6 +862,56 @@ class StreamTests(test_utils.TestCase):
         self.loop.run_until_complete(run_test())
         self.assertEqual(messages, [])
 
+    @unittest.skipIf(ssl is None, 'No ssl module')
+    def test_start_tls_connection_lost_during_handshake(self):
+        # The protocol must get connection_lost() when the connection is
+        # lost while the start_tls() handshake is in progress, so that
+        # wait_closed() completes instead of hanging forever.
+
+        async def server_handler(client_reader, client_writer):
+            line = await client_reader.readline()
+            self.assertEqual(line, b"STARTTLS\n")
+            client_writer.write(b"go ahead\n")
+            await client_writer.drain()
+            with self.assertRaises(ConnectionAbortedError):
+                await client_writer.start_tls(
+                    test_utils.simple_server_sslcontext(),
+                    ssl_handshake_timeout=0.1)
+            client_writer.close()
+            with self.assertRaises(ConnectionAbortedError):
+                await asyncio.wait_for(client_writer.wait_closed(),
+                                       support.SHORT_TIMEOUT)
+            handler_done.set()
+
+        async def client(addr):
+            reader, writer = await asyncio.open_connection(*addr)
+            writer.write(b"STARTTLS\n")
+            await writer.drain()
+            line = await reader.readline()
+            self.assertEqual(line, b"go ahead\n")
+            # Never send a ClientHello: the server's handshake times out.
+            data = await reader.read()
+            self.assertEqual(data, b"")
+            writer.close()
+            await writer.wait_closed()
+
+        async def run_test():
+            server = await asyncio.start_server(
+                server_handler, socket_helper.HOSTv4, 0)
+            server_addr = server.sockets[0].getsockname()
+
+            await client(server_addr)
+            await asyncio.wait_for(handler_done.wait(),
+                                   support.SHORT_TIMEOUT)
+            server.close()
+            await server.wait_closed()
+
+        handler_done = asyncio.Event()
+        messages = []
+        self.loop.set_exception_handler(lambda loop, ctx: messages.append(ctx))
+        self.loop.run_until_complete(run_test())
+        self.assertEqual(messages, [])
+
     def test_streamwriter_start_tls_updates_reader_transport(self):
         reader = asyncio.StreamReader(loop=self.loop)
         protocol = asyncio.StreamReaderProtocol(reader, loop=self.loop)

Misc/NEWS.d/next/Library/2026-10-22-08-06-39.gh-issue-160513.q7RkZd.rst

@@ -0,0 +1,5 @@
+Fix :meth:`asyncio.loop.start_tls` and :meth:`asyncio.StreamWriter.start_tls`
+not calling the protocol's :meth:`~asyncio.BaseProtocol.connection_lost`
+when the connection is lost, the handshake times out or the call is
+cancelled during the TLS handshake. :meth:`asyncio.StreamWriter.wait_closed`
+no longer waits forever after such a failure.