support cabal.project source-repository-package commit

[brainrake]

Support using a commit hash instead of tag on cabal.project source-repository-package.

If one puts the commit has in the tag: field instead, cabal-project-parser works, but cabal crashes while trying to download a tag instead of a commit.

[hamishmack]

Is there an example somewhere of the problem this fixes?

[brainrake]

Anytime commit is used in source-repository-package in cabal.project.
Here's a demo:
https://github.com/brainrape/haskell-nix-source-repository-package-commit-example

If I remove --sha256 and build with nix develop --impure I get

fetching Git repository 'https://github.com/input-output-hk/cardano-prelude'fatal: couldn't find remote ref refs/heads/c7fc9fba236972c27a55e0f3d34c2758cf616bfc

Maybe that should be looked into as well.

[brainrake]

Hm, looks like cabal accepts commits in tag: as well, I'll need to look into this further.

[purefn]

The "couldn't find remote ref" error is tracked as #1288. The simple solution is to change ref to rev here

haskell.nix/lib/call-cabal-project-to-nix.nix

Line 202 in d420709

let drv = builtins.fetchGit { inherit (repoData) url ref; };

.

commit is not a supported attribute for source-repository-package. tag is intended to be the revision, but it was named tag to try and be VCS agnostic, which just results in confusion. Discussion of that can be found here haskell/cabal#6888.

I think your example only works by accident because the revision you've specified for commit happens to be the HEAD of the default branch. Otherwise, you would see that it is ignored.

[brainrake]

Thanks for clearing that up