Address review: slice memoryviews directly, test bytearray, trim NEWS

Author: Ar-maan05

Doc/library/asyncio-protocol.rst

@@ -333,6 +333,8 @@ Write-only Transports
    This method does not block; it buffers the data and arranges for it
    to be sent out asynchronously.
 
+   Do not modify *data* until it has been sent.
+
 .. method:: WriteTransport.writelines(list_of_data)
 
    Write a list (or any iterable) of data bytes to the transport.

Lib/asyncio/selector_events.py

@@ -1143,7 +1143,10 @@ def _adjust_leftover_buffer(self, nbytes: int) -> None:
             if b_len <= nbytes:
                 nbytes -= b_len
             else:
-                buffer.appendleft(memoryview(b)[nbytes:])
+                # Keep a view of the remainder, not a copy.
+                if not isinstance(b, memoryview):
+                    b = memoryview(b)
+                buffer.appendleft(b[nbytes:])
                 break
 
     def _write_send(self):
@@ -1154,8 +1157,10 @@ def _write_send(self):
             buffer = self._buffer.popleft()
             n = self._sock.send(buffer)
             if n != len(buffer):
-                # Not all data was written
-                self._buffer.appendleft(memoryview(buffer)[n:])
+                # Not all data was written; keep a view, not a copy.
+                if not isinstance(buffer, memoryview):
+                    buffer = memoryview(buffer)
+                self._buffer.appendleft(buffer[n:])
             self._buffer_size -= n
         except (BlockingIOError, InterruptedError):
             self._buffer.appendleft(buffer)

Lib/test/test_asyncio/test_selector_events.py

@@ -959,7 +959,7 @@ def test_write_sendmsg_full(self):
     @unittest.skipUnless(selector_events._HAS_SENDMSG, 'no sendmsg')
     def test_write_sendmsg_partial(self):
         data = b'data'
-        for buf in data, memoryview(data):
+        for buf in data, bytearray(data), memoryview(data):
             with self.subTest(type=type(buf).__name__):
                 self.sock.sendmsg = mock.Mock()
                 # Sent partial data
@@ -973,7 +973,15 @@ def test_write_sendmsg_partial(self):
                 self.assertTrue(self.loop.writers)
                 self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
                 # gh-159503: the remainder is a view, not a copy
-                self.assertIs(transport._buffer[0].obj, data)
+                rest = transport._buffer[0]
+                self.assertIsInstance(rest, memoryview)
+                self.assertIs(
+                    rest.obj,
+                    buf.obj if isinstance(buf, memoryview) else buf)
+                if isinstance(buf, bytearray):
+                    # The unsent view pins the bytearray against resizing.
+                    with self.assertRaises(BufferError):
+                        buf.append(0)
 
     @unittest.skipUnless(selector_events._HAS_SENDMSG, 'no sendmsg')
     def test_write_sendmsg_half_buffer(self):
@@ -1075,7 +1083,7 @@ def test_write_ready_no_data(self):
 
     def test_write_ready_partial(self):
         data = b'data'
-        for buf in data, memoryview(data):
+        for buf in data, bytearray(data), memoryview(data):
             with self.subTest(type=type(buf).__name__):
                 self.sock.send.return_value = 2
 
@@ -1086,7 +1094,15 @@ def test_write_ready_partial(self):
                 self.loop.assert_writer(7, transport._write_ready)
                 self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
                 # gh-159503: the remainder is a view, not a copy
-                self.assertIs(transport._buffer[0].obj, data)
+                rest = transport._buffer[0]
+                self.assertIsInstance(rest, memoryview)
+                self.assertIs(
+                    rest.obj,
+                    buf.obj if isinstance(buf, memoryview) else buf)
+                if isinstance(buf, bytearray):
+                    # The unsent view pins the bytearray against resizing.
+                    with self.assertRaises(BufferError):
+                        buf.append(0)
 
     def test_write_ready_partial_none(self):
         data = b'data'

Misc/NEWS.d/next/Library/2026-10-16-01-56-15.gh-issue-159503.WgYiow.rst

@@ -1,4 +1,3 @@
-Fix quadratic copying in :mod:`asyncio` selector socket transports when a
-large :class:`bytes` write is only partly sent while other data is buffered.
-The unsent remainder is now kept as a :class:`memoryview` instead of being
-copied on every partial send.
+Fix quadratic copying in :mod:`asyncio` selector socket transports on partial
+sends. A :class:`bytearray` passed to ``write()`` now stays locked against
+resizing until it has been sent.