From 060eef27c39e78acdbd0fadcd5f49b2d1a26ab73 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Tue, 7 Oct 2025 19:57:57 +0200
Subject: [PATCH] [3.13] gh-136234: Fix `SelectorSocketTransport.writelines` to
 be robust to connection loss (GH-136743) (GH-138702) (#139710)

[3.14] gh-136234: Fix `SelectorSocketTransport.writelines` to be robust to connection loss (GH-136743) (GH-138702)
(cherry picked from commit 5cd6cfe4cf6770b9424bb23f8c7c661f7a3db7b0)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
---
 Lib/asyncio/selector_events.py                   |  7 +++++++
 Lib/test/test_asyncio/test_selector_events.py    | 16 ++++++++++++++++
 ...025-07-17-16-12-23.gh-issue-136234.VmTxtj.rst |  2 ++
 3 files changed, 25 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2025-07-17-16-12-23.gh-issue-136234.VmTxtj.rst

diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index df5b1e49240f..8701467d4132 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -1170,6 +1170,13 @@ class _SelectorSocketTransport(_SelectorTransport):
             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
diff --git a/Lib/test/test_asyncio/test_selector_events.py b/Lib/test/test_asyncio/test_selector_events.py
index 1e7761ee38d7..bfb347e4a8cf 100644
--- a/Lib/test/test_asyncio/test_selector_events.py
+++ b/Lib/test/test_asyncio/test_selector_events.py
@@ -817,6 +817,22 @@ class SelectorSocketTransportTests(test_utils.TestCase):
         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')
diff --git a/Misc/NEWS.d/next/Library/2025-07-17-16-12-23.gh-issue-136234.VmTxtj.rst b/Misc/NEWS.d/next/Library/2025-07-17-16-12-23.gh-issue-136234.VmTxtj.rst
new file mode 100644
index 000000000000..044a601c9170
--- /dev/null
+++ b/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`.
-- 
2.47.3