msgpack.fallback.Packer.pack is not thread safe

[yaccz]

msgpack.fallback.Packer.pack is not thread safe while the cython implementation seems to be by acquiring GIL. This may be a difference at least worth mentioning somewhere.

[jfolz]

The GIL does not make code thread-safe. It merely ensures that reference-counting is done correctly. AFAIK neither implementation is thread-safe.

[methane]

Like many classes in Python stdlib, it's is not thread safe unless it's documented as "thread safe"
explicitly. Even if Cython implementation is thread safe (I don't confirmed it yet), it's implementation detail.
You must use Cython implementation in single thread, or use mutex too.