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gh-155941: Close the transport when a plain client_connected_cb raises #155942

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gh-155941: Close the transport when a plain client_connected_cb raises #155942

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12 changes: 11 additions & 1 deletion Lib/asyncio/streams.py
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Original file line number Diff line number Diff line change
Expand Up @@ -239,7 +239,17 @@ def connection_made(self, transport):
self._over_ssl = transport.get_extra_info('sslcontext') is not None
if self._client_connected_cb is not None:
writer = StreamWriter(transport, self, reader, self._loop)
res = self._client_connected_cb(reader, writer)
try:
res = self._client_connected_cb(reader, writer)
except Exception as exc:
self._loop.call_exception_handler({
'message': 'Unhandled exception in client_connected_cb',
'exception': exc,
'transport': transport,
})
transport.close()
self._strong_reader = None
return
if coroutines.iscoroutine(res):
def callback(task):
if task.cancelled():
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32 changes: 32 additions & 0 deletions Lib/test/test_asyncio/test_streams.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1267,6 +1267,38 @@ async def handle_echo(reader, writer):
messages = self._basetest_unhandled_exceptions(handle_echo)
self.assertEqual(messages, [])

def test_unhandled_exception_sync_callback(self):
# An exception raised by a plain-function client_connected_cb is
# reported like the coroutine case and the transport is closed.
port = socket_helper.find_unused_port()

messages = []
self.loop.set_exception_handler(lambda loop, ctx: messages.append(ctx))

async def client():
rd, wr = await asyncio.open_connection('localhost', port)
async with asyncio.timeout(60):
data = await rd.read()
self.assertEqual(data, b'') # the server closed the connection
wr.close()
await wr.wait_closed()

async def main():
def handle_echo(reader, writer):
raise Exception('test')

server = await asyncio.start_server(
handle_echo, 'localhost', port)
await server.start_serving()
await client()
server.close()
await server.wait_closed()

self.loop.run_until_complete(main())

self.assertEqual(messages[0]['message'],
'Unhandled exception in client_connected_cb')

def test_open_connection_happy_eyeball_refcycles(self):
port = socket_helper.find_unused_port()
async def main():
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-17-21-00-00.gh-issue-155941.strmCb.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Fix :func:`asyncio.start_server` when a plain-function *client_connected_cb*
raises: the error is now reported like the coroutine case and the transport
is closed, instead of leaving the connection open forever (which also made
:meth:`asyncio.Server.wait_closed` hang).
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