Add missing autoreset in Packer.pack_ext_type

[bysiber]

Packer.pack_ext_type() writes to the internal buffer but never checks self._autoreset. Every other public pack method (pack, pack_map_pairs, pack_array_header, pack_map_header) has this pattern:

if self._autoreset:
    ret = self._buffer.getvalue()
    self._buffer = BytesIO()
    return ret

Without it, pack_ext_type() always returns None, and the packed ext data stays in the buffer. The next call to pack() then returns both the extension data and the new value concatenated together, corrupting the serialized stream.

packer = msgpack.Packer()
result = packer.pack_ext_type(5, b'\x01\x02\x03')
# result is None, expected bytes

packer.pack(99)
# Returns 7 bytes (ext header + ext data + int) instead of 1 byte

[Copilot]

Pull request overview

This PR addresses a long-standing behavior mismatch in Packer.pack_ext_type() where the pure-Python fallback implementation writes to the internal buffer but doesn’t honor self._autoreset, causing pack_ext_type() to return None and potentially corrupt subsequent packed output.

Changes:

• Add autoreset handling to msgpack/fallback.py:Packer.pack_ext_type() so it returns bytes and clears the internal buffer when self._autoreset is enabled.

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[Copilot]

Please add a regression test that asserts Packer().pack_ext_type(...) returns the packed bytes when autoreset is enabled, and that the internal buffer is cleared (e.g., a subsequent packer.pack(99) should not include the previously packed ext bytes). This would have caught the original bug and will prevent it from reappearing.

[methane]

@copilot apply changes based on this feedback

[Copilot]

This adds autoreset handling for the pure-Python fallback Packer only. The default C-extension implementation (msgpack/_packer.pyx:pack_ext_type) still doesn’t return bytes or reset its internal buffer when autoreset is enabled, so msgpack.Packer().pack_ext_type(...) will remain broken in typical installs unless the Cython path is updated too.