A git buildpack

[genevieve]

While trying to build a go app with the go buildpack, we hit a snag trying to inject the git sha at build-time. In a dockerfile it might look something like:

RUN GIT_COMMIT=$(git rev-list -1 HEAD) && \
  go build -ldflags "-X main.GitCommit=$GIT_COMMIT"

The go buildpack supports buildtime flags for the -ldflags ... part but there isn't an easy way to extract the first part where the sha is deduced. Would this be something y'all would be interesting in creating?

[ryanmoran]

I think this make sense to support, but maybe not directly in the go-build buildpack. Instead, I could imagine a git buildpack that will optionally participate in a build and provide git metadata as build-time environment variables, making them accessible to a number of different buildpacks. This would make this type of feature accessible across all of the language-families.

[ForestEckhardt]

Could you provide me some other uses cases for this kind of buildpack? I am not opposed to the idea I am just struggling to think how else this might be utilized.

[genevieve]

Besides the usecase for go build, I've also seen multiple Rails applications that want a "version" (in non-release cases, they want the git sha) for different purposes at runtime.

[arjun024]

Just curious - does your build process create commits? Wondering why injecting the value before build-time wouldn't be sufficient? like,
pack build app1 -p <path> --env BP_GO_BUILD_FLAGS="-ldflags=\"-X main.GitCommit=$(git -C <path> rev-list -1 HEAD)\""

[genevieve]

Hey! Our pack build command is in the circleci orb and used by more apps than the golang one that I'm working on so it's not easy to introduce a custom flag arg. Additionally, I'm not sure this would work when we move to kpack and are using the image/build resources with env var configuration.

[arjun024]

Thanks for the background. In case you're interested to make a quick buildpack to provide the variable, you should be able to do so in a few lines since git is provided by the paketo full and base images.

‣ cat bin/detect
#!/usr/bin/env bash

‣ cat bin/build
#!/usr/bin/env bash
mkdir -p "${1}/layer1/env.build"
echo -n "build = true" > "${1}/layer1.toml"
git rev-list -1 HEAD | tr -d "\n" > "${1}/layer1/env.build/GIT_COMMIT.default"

‣ cat buildpack.toml
api = "0.4"
[buildpack]
  id = "gitvar"
  version = "0.1"
[[stacks]]
  id = "io.buildpacks.stacks.bionic"

[genevieve]

RFC here: paketo-buildpacks/rfcs#39