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gh-155888: Fix asyncio writelines() hanging on an empty last chunk #155889

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Merged
kumaraditya303 merged 1 commit into from
Aug 16, 2026
+20 −0
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gh-155888: Fix asyncio writelines() hanging on an empty last chunk #155889

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5 changes: 5 additions & 0 deletions Lib/asyncio/selector_events.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1198,8 +1198,13 @@ def writelines(self, list_of_data):
return

for data in list_of_data:
# gh-155888: an empty chunk can never be drained, so never buffer it
if not data:
continue
self._buffer.append(memoryview(data))
self._buffer_size += len(data)
if not self._buffer:
return
self._write_ready()
# If the entire buffer couldn't be written, register a write handler
if self._buffer:
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13 changes: 13 additions & 0 deletions Lib/test/test_asyncio/test_events.py
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Original file line number Diff line number Diff line change
Expand Up @@ -560,6 +560,19 @@ def writer(data):
r.close()
self.assertEqual(read, data)

def test_writelines_empty_chunk(self):
# gh-155888: an empty chunk can never be drained, so never buffer it
rsock, wsock = socket.socketpair()
self.addCleanup(rsock.close)

async def main():
reader, writer = await asyncio.open_connection(sock=wsock)
writer.writelines([b'data', b''])
writer.close()
await asyncio.wait_for(writer.wait_closed(), support.SHORT_TIMEOUT)

self.loop.run_until_complete(main())

@unittest.skipUnless(hasattr(signal, 'SIGKILL'), 'No SIGKILL')
def test_add_signal_handler(self):
caught = 0
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-16-11-38-17.gh-issue-155888.pO-nAp.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix :meth:`asyncio.WriteTransport.writelines` hanging the transport when the
last data chunk is empty.
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