> static_buffers is not a static variable. It is auto local variable.
> So I think other thread don't hijack it.

Oh yes, quite right. I should have looked closer at the code first before commenting. I think this can be closed as not-a-bug, unless +tzickel has example code that gives the wrong output?

> perhaps add an if to check if the backing object is really mutable ? (Py_buffer.readonly)

It's not just the buffer data being mutable that's an issue, it's the owning object. It's possible for an object to expose a read-only buffer, but also allow the buffer (including its size or address) to be mutated through its own API.

> Also, semi related, (dunno where to discuss it), would a good .join() optimization be to add an optional length parameter, like .join(iterable, length=10)

You could always open a separate bug for it, but I can't see it catching on given that one needs to modify one's code for it.