commitIdFromGitRepo: fix stackoverflow if many branches are used.

[Mic92]

If many branches are created than builtins.match stack overflows because
of a bug in libstdc++: see NixOS/nix#2147

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[lheckemann]

It's with a heavy heart that I'm ❤️ ing this, since it's terrible and shouldn't be necessary. But at the same time I'm happy for a workaround.

(fixes #42379)

[edolstra]

IMHO a function like commitIdFromGitRepo shouldn't exist in the first place. Nix is not a general purpose language and we shouldn't be processing .git internal files from Nix expressions. It's better to use builtin functions like fetchGit or fetchTree that give you the revision without any magic.

[cole-h]

@edolstra Do you have an example of how one would use fetchGit or fetchTree to get the revision? I'd gladly start using those two over commitIdFromGitRepo if there is a way to communicate this information in Nix.

[lheckemann]

fetchGit will also import the whole thing into the store AFAIU, so I don't think that's desirable — particularly since this is used to generate NixOS version info.

[Mic92]

IMHO a function like commitIdFromGitRepo shouldn't exist in the first place. Nix is not a general purpose language and we shouldn't be processing .git internal files from Nix expressions. It's better to use builtin functions like fetchGit or fetchTree that give you the revision without any magic.

fetchGit would be a lot slower though since it copies everything to the nix store.

[Mic92]

I would leave fetchGit to another pr.

[lheckemann]

Backported to 20.03: e7c435c

[lheckemann]

I guess an @edolstra -approved solution might be making builtins.fetchGit lazier, so that it only imports the path into the store when the value of outPath is used, allowing access to rev and revCount without importing it into the store?

[Mic92]

Well now with flakes nixpkgs the git is copied to nix store now every time anyway.

[edolstra]

There is an issue about that: NixOS/nix#3121