https://github.com/python/cpython/commit/43a4511b6e62437048366893499768f9130dad3b
commit: 43a4511b6e62437048366893499768f9130dad3b
branch: main
author: Armaan Sandhu <[email protected]>
committer: kumaraditya303 <[email protected]>
date: 2026-10-29T14:15:15Z
summary:
gh-159503: Avoid copying the unsent remainder of a bytes write in asyncio
selector transports (#159504)
files:
A Misc/NEWS.d/next/Library/2026-10-16-01-56-15.gh-issue-159503.WgYiow.rst
M Doc/library/asyncio-protocol.rst
M Lib/asyncio/selector_events.py
M Lib/test/test_asyncio/test_selector_events.py
diff --git a/Doc/library/asyncio-protocol.rst b/Doc/library/asyncio-protocol.rst
index 58f77feb311984..42c2d509d5a729 100644
--- a/Doc/library/asyncio-protocol.rst
+++ b/Doc/library/asyncio-protocol.rst
@@ -333,6 +333,9 @@ Write-only Transports
This method does not block; it buffers the data and arranges for it
to be sent out asynchronously.
+ The transport keeps a reference to *data* rather than copying. *data*
+ must not be mutated while being sent.
+
.. method:: WriteTransport.writelines(list_of_data)
Write a list (or any iterable) of data bytes to the transport.
diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index 57ba2f13246c94..15a3754262d078 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -1099,7 +1099,9 @@ def write(self, data):
# Not all was written; register write handler.
self._add_writer(self._sock_fd, self._write_ready)
- # Add it to the buffer.
+ # Add it to the buffer as a view, so partial sends don't copy it.
+ if not isinstance(data, memoryview):
+ data = memoryview(data)
self._buffer.append(data)
self._buffer_size += len(data)
self._maybe_pause_protocol()
diff --git a/Lib/test/test_asyncio/test_selector_events.py
b/Lib/test/test_asyncio/test_selector_events.py
index 74322105a7c757..e080225bd976d9 100644
--- a/Lib/test/test_asyncio/test_selector_events.py
+++ b/Lib/test/test_asyncio/test_selector_events.py
@@ -855,14 +855,23 @@ def test_write_partial_none(self):
self.assertEqual(list_to_buffer([b'data']), transport._buffer)
def test_write_tryagain(self):
- self.sock.send.side_effect = BlockingIOError
-
data = b'data'
- transport = self.socket_transport()
- transport.write(data)
-
- self.loop.assert_writer(7, transport._write_ready)
- self.assertEqual(list_to_buffer([b'data']), transport._buffer)
+ for buf in data, bytearray(data), memoryview(data):
+ with self.subTest(type=type(buf).__name__):
+ self.sock.send.side_effect = BlockingIOError
+
+ transport = self.socket_transport()
+ transport.write(buf)
+
+ self.loop.assert_writer(7, transport._write_ready)
+ self.assertEqual(list_to_buffer([b'data']), transport._buffer)
+ # gh-159503: the data is buffered as a view, not a copy
+ self.assertIsInstance(transport._buffer[0], memoryview)
+ self.assertIs(transport._buffer[0].obj, memoryview(buf).obj)
+ if isinstance(buf, bytearray):
+ # The buffered view pins the bytearray against resizing.
+ with self.assertRaises(BufferError):
+ buf.append(0)
def test_write_sendmsg_no_data(self):
self.sock.sendmsg = mock.Mock()
diff --git
a/Misc/NEWS.d/next/Library/2026-10-16-01-56-15.gh-issue-159503.WgYiow.rst
b/Misc/NEWS.d/next/Library/2026-10-16-01-56-15.gh-issue-159503.WgYiow.rst
new file mode 100644
index 00000000000000..158968d0c2a1ce
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-16-01-56-15.gh-issue-159503.WgYiow.rst
@@ -0,0 +1,3 @@
+Fix quadratic copying in :mod:`asyncio` selector socket transports on partial
+sends. A :class:`bytearray` passed to :meth:`~asyncio.WriteTransport.write`
+now stays locked against resizing until it has been sent.
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