From 7b92b6bc4ccfe78072efda776b988e284cbfff98 Mon Sep 17 00:00:00 2001
From: aryansk <aryansk@users.noreply.github.com>
Date: Sun, 23 Aug 2026 18:43:54 +0530
Subject: [PATCH 1/2] asyncio: avoid sharing exception object between
 StreamReader and close waiter

StreamReaderProtocol.connection_lost() set the same exception object
on both the StreamReader's waiter and the Stream's _closed waiter.
Since gh-90082, Future stores the traceback at set_exception() time
and restores it with with_traceback() on every result() call, which
mutates the exception in place. Sharing one object between two
futures caused the second await (typically writer.wait_closed() in an
except block) to rewrite the traceback of the in-flight exception
being handled, erasing the real failure site (readexactly) and
replacing it with wait_closed frames.

Fix by copying the exception for the _closed waiter so each future
owns an independent object. Copy falls back to reconstructing via
type(exc)(*exc.args) when copy.copy fails.

Fixes #156278
---
 Lib/asyncio/streams.py | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/Lib/asyncio/streams.py b/Lib/asyncio/streams.py
index 954e132617ce42b..9988589d4f942a8 100644
--- a/Lib/asyncio/streams.py
+++ b/Lib/asyncio/streams.py
@@ -3,6 +3,7 @@
     'open_connection', 'start_server')
 
 import collections
+import copy
 import socket
 import sys
 import warnings
@@ -280,7 +281,29 @@ def connection_lost(self, exc):
             if exc is None:
                 self._closed.set_result(None)
             else:
-                self._closed.set_exception(exc)
+                # Avoid sharing the same exception object between the
+                # reader future and the close waiter.  Future.result()
+                # restores the traceback with `with_traceback()`, which
+                # mutates the exception in place; sharing one object
+                # between two futures rewrites the traceback of the
+                # in-flight exception being handled (gh-156278).
+                try:
+                    exc_copy = copy.copy(exc)
+                except Exception:
+                    try:
+                        exc_copy = type(exc)(*exc.args)
+                        # Preserve context attributes where possible.
+                        if hasattr(exc, "__cause__"):
+                            exc_copy.__cause__ = exc.__cause__
+                        if hasattr(exc, "__context__"):
+                            exc_copy.__context__ = exc.__context__
+                        if hasattr(exc, "__suppress_context__"):
+                            exc_copy.__suppress_context__ = exc.__suppress_context__
+                        if exc.__traceback__ is not None:
+                            exc_copy = exc_copy.with_traceback(exc.__traceback__)
+                    except Exception:
+                        exc_copy = exc
+                self._closed.set_exception(exc_copy)
         super().connection_lost(exc)
         self._stream_reader_wr = None
         self._task = None

From dd72b5d319632fb820fb95e5d5ecfdb4ffba17de Mon Sep 17 00:00:00 2001
From: aryansk <aryansk@users.noreply.github.com>
Date: Sun, 23 Aug 2026 19:03:58 +0530
Subject: [PATCH 2/2] Add NEWS entry for gh-156278

---
 .../Library/2026-08-23-13-30-00.gh-issue-156278.Z9l5a1.rst | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-08-23-13-30-00.gh-issue-156278.Z9l5a1.rst

diff --git a/Misc/NEWS.d/next/Library/2026-08-23-13-30-00.gh-issue-156278.Z9l5a1.rst b/Misc/NEWS.d/next/Library/2026-08-23-13-30-00.gh-issue-156278.Z9l5a1.rst
new file mode 100644
index 000000000000000..974644fd286549f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-08-23-13-30-00.gh-issue-156278.Z9l5a1.rst
@@ -0,0 +1,7 @@
+Fix :func:`asyncio.StreamWriter.wait_closed` traceback rewriting.
+
+``StreamReaderProtocol.connection_lost`` no longer shares the same
+exception object between the reader and the close waiter, which
+previously caused ``await writer.wait_closed()`` in an ``except``
+block to rewrite the ``__traceback__`` of the in-flight exception
+being handled.