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Add missing autoreset in Packer.pack_ext_type #663

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Add missing autoreset in Packer.pack_ext_type #663

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4 changes: 4 additions & 0 deletions msgpack/fallback.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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Copilot AI Apr 21, 2026

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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.

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@methane methane Apr 21, 2026

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@copilot apply changes based on this feedback

Comment on lines +864 to +867
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Copilot AI Apr 21, 2026

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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.

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def _pack_array_header(self, n):
if n <= 0x0F:
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