gh-127655: Ensure _SelectorSocketTransport.writelines pauses the protocol if needed

Lib/asyncio/selector_events.py

@@ -1175,6 +1175,7 @@ def writelines(self, list_of_data):
         # If the entire buffer couldn't be written, register a write handler
         if self._buffer:
             self._loop._add_writer(self._sock_fd, self._write_ready)
+            self._maybe_pause_protocol()
 
     def can_write_eof(self):
         return True

Lib/test/test_asyncio/test_selector_events.py

@@ -805,6 +805,18 @@ def test_writelines_send_partial(self):
         self.assertTrue(self.sock.send.called)
         self.assertTrue(self.loop.writers)
 
+    def test_writelines_pauses_protocol(self):
+        data = memoryview(b'data')
+        self.sock.send.return_value = 2
+        self.sock.send.fileno.return_value = 7
+
+        transport = self.socket_transport()
+        transport._high_water = 1
+        transport.writelines([data])
+        self.assertTrue(self.protocol.pause_writing.called)
+        self.assertTrue(self.sock.send.called)
+        self.assertTrue(self.loop.writers)
+
     @unittest.skipUnless(selector_events._HAS_SENDMSG, 'no sendmsg')
     def test_write_sendmsg_full(self):
         data = memoryview(b'data')

Misc/NEWS.d/next/Security/2024-12-05-21-35-19.gh-issue-127655.xpPoOf.rst

@@ -0,0 +1 @@
+Fixed the :mod:`asyncio` selector transport not pausing writes for the protocol when the buffer reaches the high water mark when using :meth:`asyncio.StreamWriter.writelines`.