I'm building a Haskell project with Nix, with CI done by a Hydra server. One thing I would like to do is embed the git SHA in the application itself, that way the SHA can be included in our exception reports and as a header in HTTP responses (to check that the latest server is running).
It's easy to pull the SHA from the .git directory in my dev environment using a package like https://hackage.haskell.org/package/gitrev, but it appears that Hydra doesn't have that directory present after cloning (I added some Template Haskell logging that confirmed that doesDirectoryExist <PWD>.git is false).
Is there a way to get the git SHA from Hydra somehow? Either by including the git directory so I can get the SHA myself, or some other way?
Hydra version: SNAPSHOT-b11789f39993c614e53da39c29711b63cd266291 (using nix-2.0.4)
Edit: I also sshed into the worker server and confirmed there is no .git directory present where the build is being done:
[maximiliantagher@ip-10-0-2-46:/tmp]$ sudo ls -a nix-build-mercury-web-backend-0.0.0.drv-0/szjq4m1zklbydw7wrz7dbbxl9ndw0d0f-source
. app db Dockerfile Gemfile.lock .ghcid Makefile one-off-task README.md shell.nix stack.yaml
.. config .dir-locals.el docs gemset.nix .gitignore mercury-web-backend.cabal package.yaml release.nix sql-scripts templates
API-Spec data dist Gemfile .ghci .hlint.yaml .nix Rakefile Setup src test
By default Hydra will delete the .git folder, preventing you from reading the git SHA. You can tell it not to do this by setting the NIX_PREFETCH_GIT_LEAVE_DOT_GIT to "1".
If you are building Hydra using the release.nix file in its repo, you would add --set NIX_PREFETCH_GIT_LEAVE_DOT_GIT "1" after the other --sets in the file: https://github.com/NixOS/hydra/blob/1c44de1779d0e315615b072049fb2d2ce333fb32/release.nix#L155-L157