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 1/3] 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:

From 1406cd49ccc275ec15b8e42130fa5ed46e5f81ef Mon Sep 17 00:00:00 2001
From: Inada Naoki <songofacandy@gmail.com>
Date: Mon, 1 Jun 2026 18:55:47 +0900
Subject: [PATCH 2/3] add autoreset support to Packer.pack_ext_type()

---
 msgpack/_packer.pyx | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/msgpack/_packer.pyx b/msgpack/_packer.pyx
index 94d1462c..75b3f7f5 100644
--- a/msgpack/_packer.pyx
+++ b/msgpack/_packer.pyx
@@ -291,6 +291,10 @@ cdef class Packer:
             raise ValueError("ext data too large")
         msgpack_pack_ext(&self.pk, typecode, len(data))
         msgpack_pack_raw_body(&self.pk, data, len(data))
+        if self.autoreset:
+            buf = PyBytes_FromStringAndSize(self.pk.buf, self.pk.length)
+            self.pk.length = 0
+            return buf
 
     @cython.critical_section
     def pack_array_header(self, long long size):

From 907a1c5e8d5c950817727717ff8c6759febf25ab Mon Sep 17 00:00:00 2001
From: Inada Naoki <songofacandy@gmail.com>
Date: Mon, 1 Jun 2026 19:10:02 +0900
Subject: [PATCH 3/3] add test

---
 pyproject.toml         |  7 +++++++
 test/test_extension.py | 11 ++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/pyproject.toml b/pyproject.toml
index c69d5a7c..4e4510b4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -19,6 +19,7 @@ classifiers = [
     "Programming Language :: Python :: Implementation :: CPython",
     "Programming Language :: Python :: Implementation :: PyPy",
 ]
+dependencies = []
 
 [project.urls]
 Homepage = "https://msgpack.org/"
@@ -43,3 +44,9 @@ lint.select = [
     "I", # isort
     #"UP", pyupgrade
 ]
+
+[dependency-groups]
+dev = [
+    "cython>=3.2.5",
+    "pytest>=9.0.3",
+]
diff --git a/test/test_extension.py b/test/test_extension.py
index aaf0fd92..61852f15 100644
--- a/test/test_extension.py
+++ b/test/test_extension.py
@@ -6,7 +6,7 @@
 
 def test_pack_ext_type():
     def p(s):
-        packer = msgpack.Packer()
+        packer = msgpack.Packer(autoreset=False)
         packer.pack_ext_type(0x42, s)
         return packer.bytes()
 
@@ -20,6 +20,15 @@ def p(s):
     assert p(b"A" * 0x00012345) == b"\xc9\x00\x01\x23\x45\x42" + b"A" * 0x00012345  # ext 32
 
 
+def test_pack_ext_type_autoreset():
+    packer = msgpack.Packer()
+
+    assert packer.pack_ext_type(0x42, b"A") == b"\xd4\x42A"
+    assert packer.bytes() == b""
+    assert packer.pack_ext_type(0x42, b"ABC") == b"\xc7\x03\x42ABC"
+    assert packer.bytes() == b""
+
+
 def test_unpack_ext_type():
     def check(b, expected):
         assert msgpack.unpackb(b) == expected