From 1cdd242a25d18886b7e036d061665d5a3ef220c6 Mon Sep 17 00:00:00 2001
From: kn1g78 <huan9d0w@gmail.com>
Date: Sat, 27 Jun 2026 23:20:01 +0800
Subject: [PATCH 1/2] asyncio: update StreamReader transport after
 StreamWriter.start_tls

Keep StreamReader in sync with the upgraded transport after StreamWriter.start_tls(), and add a regression test for the transport replacement.
---
 Lib/asyncio/streams.py                |  7 +++++++
 Lib/test/test_asyncio/test_streams.py | 28 +++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/Lib/asyncio/streams.py b/Lib/asyncio/streams.py
index a28c11e928f806..d4867424098b69 100644
--- a/Lib/asyncio/streams.py
+++ b/Lib/asyncio/streams.py
@@ -216,6 +216,9 @@ def _stream_reader(self):
     def _replace_transport(self, transport):
         self._transport = transport
         self._over_ssl = transport.get_extra_info('sslcontext') is not None
+        reader = self._stream_reader
+        if reader is not None:
+            reader._replace_transport(transport)
 
     def connection_made(self, transport):
         if self._reject_connection:
@@ -475,6 +478,10 @@ def set_transport(self, transport):
         assert self._transport is None, 'Transport already set'
         self._transport = transport
 
+    def _replace_transport(self, transport):
+        assert self._transport is not None, 'Transport not set'
+        self._transport = transport
+
     def _maybe_resume_transport(self):
         if self._paused and len(self._buffer) <= self._limit:
             self._paused = False
diff --git a/Lib/test/test_asyncio/test_streams.py b/Lib/test/test_asyncio/test_streams.py
index fb774895a7e52a..911087a128f971 100644
--- a/Lib/test/test_asyncio/test_streams.py
+++ b/Lib/test/test_asyncio/test_streams.py
@@ -861,6 +861,34 @@ async def run_test():
         self.loop.run_until_complete(run_test())
         self.assertEqual(messages, [])
 
+    def test_streamwriter_start_tls_updates_reader_transport(self):
+        reader = asyncio.StreamReader(loop=self.loop)
+        protocol = asyncio.StreamReaderProtocol(reader, loop=self.loop)
+        old_transport = mock.Mock()
+        old_transport.get_extra_info.return_value = None
+        old_transport.is_closing.return_value = False
+        protocol.connection_made(old_transport)
+
+        writer = asyncio.StreamWriter(old_transport, protocol, reader, self.loop)
+
+        ssl_context = mock.sentinel.ssl_context
+        new_transport = mock.Mock()
+        new_transport.get_extra_info.return_value = ssl_context
+        self.loop.start_tls = mock.AsyncMock(return_value=new_transport)
+
+        self.loop.run_until_complete(writer.start_tls(ssl_context))
+
+        self.loop.start_tls.assert_awaited_once_with(
+            old_transport, protocol, ssl_context,
+            server_side=False, server_hostname=None,
+            ssl_handshake_timeout=None,
+            ssl_shutdown_timeout=None,
+        )
+        self.assertIs(writer.transport, new_transport)
+        self.assertIs(protocol._transport, new_transport)
+        self.assertIs(reader._transport, new_transport)
+        self.assertTrue(protocol._over_ssl)
+
     def test_streamreader_constructor_without_loop(self):
         with self.assertRaisesRegex(RuntimeError, 'no current event loop'):
             asyncio.StreamReader()

From 6d2274358b2857c5414b61220cef67eb4d82d2f7 Mon Sep 17 00:00:00 2001
From: kn1g78 <huan9d0w@gmail.com>
Date: Sun, 28 Jun 2026 18:51:53 +0800
Subject: [PATCH 2/2] gh-152431: add news entry for asyncio start_tls fix

---
 .../next/Library/2026-06-28-00-00-00.gh-issue-152431.Ja3K9m.rst | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-06-28-00-00-00.gh-issue-152431.Ja3K9m.rst

diff --git a/Misc/NEWS.d/next/Library/2026-06-28-00-00-00.gh-issue-152431.Ja3K9m.rst b/Misc/NEWS.d/next/Library/2026-06-28-00-00-00.gh-issue-152431.Ja3K9m.rst
new file mode 100644
index 00000000000000..bda2dfd7cd9d89
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-28-00-00-00.gh-issue-152431.Ja3K9m.rst
@@ -0,0 +1,2 @@
+Fix ``asyncio.StreamWriter.start_tls()`` to keep the linked
+``StreamReader`` transport in sync with the upgraded transport.