gh-159503: Avoid copying the unsent remainder of a bytes write in asyncio selector transports

Lib/asyncio/selector_events.py

@@ -1143,7 +1143,7 @@ def _adjust_leftover_buffer(self, nbytes: int) -> None:
             if b_len <= nbytes:
                 nbytes -= b_len
             else:
-                buffer.appendleft(b[nbytes:])
+                buffer.appendleft(memoryview(b)[nbytes:])
                 break
 
     def _write_send(self):
@@ -1155,7 +1155,7 @@ def _write_send(self):
             n = self._sock.send(buffer)
             if n != len(buffer):
                 # Not all data was written
-                self._buffer.appendleft(buffer[n:])
+                self._buffer.appendleft(memoryview(buffer)[n:])
             self._buffer_size -= n
         except (BlockingIOError, InterruptedError):
             self._buffer.appendleft(buffer)

Lib/test/test_asyncio/test_selector_events.py

@@ -972,6 +972,22 @@ def test_write_sendmsg_partial(self):
         self.assertTrue(self.loop.writers)
         self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
 
+    @unittest.skipUnless(selector_events._HAS_SENDMSG, 'no sendmsg')
+    def test_write_sendmsg_partial_bytes_no_copy(self):
+        # gh-159503: the unsent remainder of a bytes object must not be
+        # copied, otherwise a large write costs O(n**2).
+        data = b'data'
+        self.sock.sendmsg = mock.Mock()
+        self.sock.sendmsg.return_value = 2
+
+        transport = self.socket_transport(sendmsg=True)
+        transport._buffer.append(data)
+        self.loop._add_writer(7, transport._write_ready)
+        transport._write_ready()
+        self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
+        self.assertIsInstance(transport._buffer[0], memoryview)
+        self.assertIs(transport._buffer[0].obj, data)
+
     @unittest.skipUnless(selector_events._HAS_SENDMSG, 'no sendmsg')
     def test_write_sendmsg_half_buffer(self):
         data = [memoryview(b'data1'), memoryview(b'data2')]
@@ -1081,6 +1097,20 @@ def test_write_ready_partial(self):
         self.loop.assert_writer(7, transport._write_ready)
         self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
 
+    def test_write_ready_partial_bytes_no_copy(self):
+        # gh-159503: the unsent remainder of a bytes object must not be
+        # copied, otherwise a large write costs O(n**2).
+        data = b'data'
+        self.sock.send.return_value = 2
+
+        transport = self.socket_transport()
+        transport._buffer.append(data)
+        self.loop._add_writer(7, transport._write_ready)
+        transport._write_ready()
+        self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
+        self.assertIsInstance(transport._buffer[0], memoryview)
+        self.assertIs(transport._buffer[0].obj, data)
+
     def test_write_ready_partial_none(self):
         data = b'data'
         self.sock.send.return_value = 0

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

@@ -0,0 +1 @@
+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.