See there:

https://github.com/borgbackup/borg/pull/4247

I did the first changeset after seeing some strange exception popping up which it was handling another exception - which I assumed was related to memoryview.release not being called in the original code.

So it was clear to me, that we should use the CM there. So I added that (first changeset) and that made the code always fail (see first travis-ci link).

Then removed the CM again and replaced it with functionally equivalent try/finally (second changeset) - that worked.

So, the question is whether there is some issue in CPython's memoryview contextmanager code that make it fail in such a strange way.