test: testing sshfs with local ssh server

[lobis]

This PR adds an ssh server only to the ubuntu build of the CI to test accessing files over ssh via fsspec.

After testing it I realised that sshfs does not implement some interface required by uproot for this to work (cat_file) so it raises a NotImplementedError. I'm not sure if this is just not implemented and can be implemented or if there is a limitation why this has not been implemented (you cannot request a chunk of a file, you need to request it all?).

We could handle each fsspec case separately having a fallback in case some interface is not implemented, but I think it would complicate things too much.

After writing this PR I think it makes sense to raise a more detailed exception if fsspec does not implemented the required interface, so the user can post an issue in the corresponding fsspec repo. fsspec probably has a way to list the available methods (or it should!)

[jpivarski]

I hadn't known that fsspec provided an ssh backend, but it could be really useful. (I wonder about the usefulness of the S3 backend, given that S3 is a wrapper around HTTP.)

Does this test run in our CI? It's written in a way that it could run if sshd is installed, but that doesn't mean that it will run.

Okay, it doesn't actually run (Windows with Python 3.10):

SKIPPED [1] tests\test_0692_fsspec.py:87: ssh access to localhost failed with ssh access to localhost failed with ssh: connect to host localhost port 22: Connection refused

or in Ubuntu with Python 3.11:

SKIPPED [1] tests/test_0692_fsspec.py:98: sshfs needs to implement some interface

At least it is possible for this test to fail, if the attempt raised any exception other than NotImplementedError. (It would be strange for a test to not be able to fail.)

If you've tested this on your computer, you have sshd, and it passes, that's good enough for me. Even if you haven't, this is fine to merge as-is. If there's a problem with it that we discover later, then we'll deal with it then.

[lobis]

If you've tested this on your computer, you have sshd, and it passes, that's good enough for me. Even if you haven't, this is fine to merge as-is. If there's a problem with it that we discover later, then we'll deal with it then.

No, I actually never tested this. After running the test I saw that sshfs does not implement cat_file interface (required to request a chunk of the file), so it's not possible to use uproot to read ssh files with fsspec at this time.

[jpivarski]

So it will always fail with NotImplementedError until they (or we) add that feature. That's okay, it's a placeholder, and it will alert us to when they do add it (if it fails).