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gh-136234: Fix _SelectorSocketTransport.writelines to be robust to connection loss #136743

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gh-136234: Fix _SelectorSocketTransport.writelines to be robust to connection loss #136743

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20bd1cc
Fix _SelectorSocketTransport.writelines to be robust to connection loss
bmerry Jul 17, 2025
74ea990
Inline _write_after_conn_lost
bmerry Jul 21, 2025
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7 changes: 7 additions & 0 deletions Lib/asyncio/selector_events.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1174,6 +1174,13 @@ def writelines(self, list_of_data):
raise RuntimeError('unable to writelines; sendfile is in progress')
if not list_of_data:
return

if self._conn_lost:
if self._conn_lost >= constants.LOG_THRESHOLD_FOR_CONNLOST_WRITES:
logger.warning('socket.send() raised exception.')
self._conn_lost += 1
return

self._buffer.extend([memoryview(data) for data in list_of_data])
self._write_ready()
# If the entire buffer couldn't be written, register a write handler
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16 changes: 16 additions & 0 deletions Lib/test/test_asyncio/test_selector_events.py
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Expand Up @@ -854,6 +854,22 @@ def test_writelines_pauses_protocol(self):
self.assertTrue(self.sock.send.called)
self.assertTrue(self.loop.writers)

def test_writelines_after_connection_lost(self):
# GH-136234
transport = self.socket_transport()
self.sock.send = mock.Mock()
self.sock.send.side_effect = ConnectionResetError
transport.write(b'data1') # Will fail immediately, causing connection lost

transport.writelines([b'data2'])
self.assertFalse(transport._buffer)
self.assertFalse(self.loop.writers)

test_utils.run_briefly(self.loop) # Allow _call_connection_lost to run
transport.writelines([b'data2'])
self.assertFalse(transport._buffer)
self.assertFalse(self.loop.writers)

@unittest.skipUnless(selector_events._HAS_SENDMSG, 'no sendmsg')
def test_write_sendmsg_full(self):
data = memoryview(b'data')
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2025-07-17-16-12-23.gh-issue-136234.VmTxtj.rst
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@@ -0,0 +1,2 @@
Fix :meth:`asyncio.WriteTransport.writelines` to be robust to connection
failure, by using the same behavior as :meth:`~asyncio.WriteTransport.write`.
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