Borg: implement borg mount with pyfuse3

[ThomasWaldmann]

llfuse is (more or less) unmaintained, we need to support pyfuse3 in future (master branch only) to implement borg mount.

llfuse support must not get removed yet, as some platforms can not yet run pyfuse3 (e.g. macOS and FreeBSD). so we must have both implementations in parallel in the codebase.

as pyfuse3 started as a fork of llfuse, the api might be similar - so we could check if some borg code could be used for both of the borg mount implementations.

pyfuse3-based implementation must include automated tests, same or similar as we have now for the llfuse-based implementation.

bounty: https://www.bountysource.com/issues/93368903-implement-borg-mount-with-pyfuse3

[ThomasWaldmann]

I intend to put a bounty on this, but bountysource currently has technical issues...

[selurvedu]

I've got an impression that Bountysource got broken months ago.

[ThomasWaldmann]

Got a reply from BS support, but still not working.

[ThomasWaldmann]

Note: check #4948 - does it behave the same with pyfuse3 and can we find out why?

[ThomasWaldmann]

could transfer 50 USD from another (not wanted any more) bounty via my personal account, but still can't access organization funds.

anybody interested in this can help by increasing the bounty.

[Gu1nness]

I'm taking this one !

[ThomasWaldmann]

@Gu1nness I started implementing it when I assigned the issue to myself. Maybe should have better left a comment also.

Guess I will be finished soon. How far is your work? We could have another bounty (if BS works again) for some further work also.

[Gu1nness]

Ah I didn't see you assigned it to yourself!
For now I'm working on the imports and all the places where llfuse is used, but it took only ten minutes for me for what I did.
So I guess I'll leave it to you and will work on the next one :)

[ThomasWaldmann]

@Gu1nness if you like, review #5430.