Improve testing infra for fetchTree/fetchGit

[DavHau]

Is your feature request related to a problem? Please describe.
None of the code paths that could trigger actual networking seem to be tested as of now, limiting the test coverage to only file:// based actions.
I would like to add tests for potential bugs like this.

Describe the solution you'd like
Git already offers the git-http-backend CGI script. Therefore, all that's needed is a cgi capable http server. Python could be used, as it has all of that already built in, but having python as a dependency might be undesired due to bootstrapping complexity. Maybe there is a simpler cpp based http server that can be used.

Describe alternatives you've considered
Not testing network related code paths -> Instead rely on humans to find bugs during code review -> slow and unreliable.

Additional context

Priorities

Add 👍 to issues you find important.

[roberth]

I'd be happy to upstream into nixpkgs these modules that help set up a gitea. https://github.com/hercules-ci/hercules-ci-effects/tree/master/effects/testsupport

gitea is fairly quick to launch (unlike, say, gitlab) VM+gitea take about 10 s.

OTOH these a lot of value in keeping the test setup minimal.

might be undesired due to bootstrapping complexity.

Bootstrapping should be done without tests.

1. bootstrap without tests
2. have a Nix
3. bootstrap with tests, using Nix to get the test deps

Also this could be a checks test instead of an installCheckPhase test.

[DavHau]

Yes, it would be nice if you could upstream these modules. I played around a bit wit the python http server, but it seems like advanced path rewriting capabilities are required in order to use thegit-http-backend. Just using gitea might be simpler.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-12-01-nix-team-meeting-minutes-108/36402/1