gh-136234: Fix SelectorSocketTransport.writelines to be robust to connection loss (#136743)

Lib/asyncio/selector_events.py

@@ -1174,6 +1174,13 @@ def writelines(self, list_of_data):
             raise RuntimeError('unable to writelines; sendfile is in progress')
         if not list_of_data:
             return
+
+        if self._conn_lost:
+            if self._conn_lost >= constants.LOG_THRESHOLD_FOR_CONNLOST_WRITES:
+                logger.warning('socket.send() raised exception.')
+            self._conn_lost += 1
+            return
+
         self._buffer.extend([memoryview(data) for data in list_of_data])
         self._write_ready()
         # If the entire buffer couldn't be written, register a write handler

Lib/test/test_asyncio/test_selector_events.py

@@ -854,6 +854,22 @@ def test_writelines_pauses_protocol(self):
         self.assertTrue(self.sock.send.called)
         self.assertTrue(self.loop.writers)
 
+    def test_writelines_after_connection_lost(self):
+        # GH-136234
+        transport = self.socket_transport()
+        self.sock.send = mock.Mock()
+        self.sock.send.side_effect = ConnectionResetError
+        transport.write(b'data1')  # Will fail immediately, causing connection lost
+
+        transport.writelines([b'data2'])
+        self.assertFalse(transport._buffer)
+        self.assertFalse(self.loop.writers)
+
+        test_utils.run_briefly(self.loop)  # Allow _call_connection_lost to run
+        transport.writelines([b'data2'])
+        self.assertFalse(transport._buffer)
+        self.assertFalse(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/Library/2025-07-17-16-12-23.gh-issue-136234.VmTxtj.rst

@@ -0,0 +1,2 @@
+Fix :meth:`asyncio.WriteTransport.writelines` to be robust to connection
+failure, by using the same behavior as :meth:`~asyncio.WriteTransport.write`.