Summary

If the default callback obtains a memoryview through getbuffer() and then returns a sufficiently large value, Packer.pack() reallocates its internal buffer while the memoryview still references the old storage.
Accessing the memoryview afterward causes a segmentation fault, and an ASan build reports a heap-use-after-free.

Versions

msgpack 1.2.2, CPython 3.12.3, Ubuntu 24.04 x86_64, glibc 2.39.

Reproducer

import msgpack

packer = None
exported = None

class Unsupported:
    pass

def default(obj):
    global exported
    exported = packer.getbuffer()
    return b"A" * (2 * 1024 * 1024)

packer = msgpack.Packer(default=default, autoreset=False)
packer.pack([Unsupported()])
msgpack.unpackb(exported)

Running it produces:

Segmentation fault (core dumped)

Sanitizer result

An ASan build reports a heap-use-after-free in unpack_execute.
The allocation was freed by msgpack_pack_write after the callback exported a memoryview of the buffer.

ERROR: AddressSanitizer: heap-use-after-free
READ of size 1
    #0 unpack_execute msgpack/unpack_template.h:161
freed by:
    #1 msgpack_pack_write msgpack/pack.h:47

I found this while fuzzing Python C extension modules.
It looks like a bug to me, but I may be missing an intended constraint, so I would appreciate confirmation.