Add support for ssh config include

[mrmeszaros]

Fixes #1892
Fixes #1609

Should also fix fabric/fabric#2215 once released

Some solutions felt a bit dirty to me, so feel free to give feedback.

[PeterJCLaw]

(Speaking as someone who's been following this feature for a while & still interested; prompted by the closure of #1892 which this presumably replaces)

Thanks for putting this together @mrmeszaros :)

Do you feel this PR is ready for review/testing etc. (absolutely no worries if not!) -- wondering if I could help by offering a review and some testing perhaps? (I realise there's a merge conflict, though it appears trivial to resolve)

[mrmeszaros]

@PeterJCLaw I am 90% confident this is ready, so feel free to review it.

I tried adding some unit tests, but it would be nice if You could try and break it, to see where are the weak points.

10% in me says, that the internal representation could be improved, but it would need some more think juice. If You have ideas, please feel free to share, I am open for discussions.

PS.: I used ssh -G $TEST_HOST for a reference how OpenSSH compiles the config files.
PPS.: I will (hopefully) resolve the conflict today and push it up here. (lol)

[csueiras]

I installed this branch in my local repository and when calling get_hostnames() in the config object, I got the error:

.venv/lib/python3.9/site-packages/paramiko/config.py", line 430, in get_hostnames
    hosts.update(entry["host"])
KeyError: 'host'

Which I can see was added as part of this PR here:

https://github.com/mrmeszaros/paramiko/blame/39d82a15da01885c4688dddee2d1ea18ec5b0a05/paramiko/config.py#L430

My failing config has a Match host <myhost> exec ... (I installed Invoke per the documentation for support for exec).

[mrmeszaros]

@csueiras I will look into it - thanks for trying this out <3

Could You please send a failing config?
Or maybe give some hints about the ordering of the directives / stanzas etc. - to help me reproduce the issue?

[mrmeszaros]

@csueiras I have done some investigating! I have some mixed news.

Firstly:

It seems to me, that this was not introduced by my code (this is the good/bad news part).
I have tested Your usecase on the main branch, and it throws the same KeyError.
The get_hostnames() method is poorly tested - it only occurs in one test-case (that uses the robey test config file):

https://github.com/paramiko/paramiko/blob/51eb55debf2ebfe56f38378005439a029a48225f/tests/test_config.py#L370C1-L372C1

I am not sure how it should work, and how it is currently used. I did not find a lot of documentation regarding how it should handle host directives inside a match.

Secondly

I think I can provide a hot-fix (this is the good news part).
However, I am unsure of it's purpose, so for that I would need more info.
So, feel free to give some help.

Up Next

I think we should find/create a separate Issue / PR and ping the maintainer.
Hopefully that one - being a hot-fix - should be merged more quickly.