From b691a8e52cf418408a541af6adc05c51849c8f59 Mon Sep 17 00:00:00 2001
From: Kadir Can Ozden <101993364+bysiber@users.noreply.github.com>
Date: Fri, 20 Feb 2026 15:00:07 +0300
Subject: [PATCH] Add missing autoreset in Packer.pack_ext_type

pack_ext_type writes to the internal buffer but never checks
self._autoreset, unlike every other public pack method. This means
it always returns None and the packed data leaks into the output of
the next pack() call, corrupting the serialized stream.

Add the same autoreset pattern used by pack(), pack_map_pairs(),
pack_array_header(), and pack_map_header().
---
 msgpack/fallback.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/msgpack/fallback.py b/msgpack/fallback.py
index b02e47cf..6b54ffbf 100644
--- a/msgpack/fallback.py
+++ b/msgpack/fallback.py
@@ -861,6 +861,10 @@ def pack_ext_type(self, typecode, data):
             self._buffer.write(b"\xc9" + struct.pack(">I", L))
         self._buffer.write(struct.pack("B", typecode))
         self._buffer.write(data)
+        if self._autoreset:
+            ret = self._buffer.getvalue()
+            self._buffer = BytesIO()
+            return ret
 
     def _pack_array_header(self, n):
         if n <= 0x0F: