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

@@ -958,19 +958,22 @@ def test_write_sendmsg_full(self):
 
     @unittest.skipUnless(selector_events._HAS_SENDMSG, 'no sendmsg')
     def test_write_sendmsg_partial(self):
-
-        data = memoryview(b'data')
-        self.sock.sendmsg = mock.Mock()
-        # Sent partial data
-        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.assertTrue(self.sock.sendmsg.called)
-        self.assertTrue(self.loop.writers)
-        self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
+        data = b'data'
+        for buf in data, memoryview(data):
+            with self.subTest(type=type(buf).__name__):
+                self.sock.sendmsg = mock.Mock()
+                # Sent partial data
+                self.sock.sendmsg.return_value = 2
+
+                transport = self.socket_transport(sendmsg=True)
+                transport._buffer.append(buf)
+                self.loop._add_writer(7, transport._write_ready)
+                transport._write_ready()
+                self.assertTrue(self.sock.sendmsg.called)
+                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)
 
     @unittest.skipUnless(selector_events._HAS_SENDMSG, 'no sendmsg')
     def test_write_sendmsg_half_buffer(self):
@@ -1071,15 +1074,19 @@ def test_write_ready_no_data(self):
         self.assertRaises(AssertionError, transport._write_ready)
 
     def test_write_ready_partial(self):
-        data = memoryview(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.loop.assert_writer(7, transport._write_ready)
-        self.assertEqual(list_to_buffer([b'ta']), transport._buffer)
+        data = b'data'
+        for buf in data, memoryview(data):
+            with self.subTest(type=type(buf).__name__):
+                self.sock.send.return_value = 2
+
+                transport = self.socket_transport()
+                transport._buffer.append(buf)
+                self.loop._add_writer(7, transport._write_ready)
+                transport._write_ready()
+                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)
 
     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

@@ -0,0 +1,4 @@
+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.