`SOURCE_DATE_EPOCH` should not affect vergen git outputs

[cherryman]

from the spec for SOURCE_DATE_EPOCH, seems this env var's purpose is to ensure reproducibility for builds. using VERGEN_GIT_COMMIT_{DATE,TIMESTAMP} is technically already reproducible, since the git commit date should be fixed, unlike VERGEN_BUILD_TIMESTAMP and the like.

[cherryman]

actually same goes for VERGEN_IDEMPOTENT overriding the commit date and timestamp but not the git hash?

[CraZySacX]

You are correct that those git based values would be fixed. The main intent for SOURCE_DATE_EPOCH is to allow package maintainers to set the output of those particular variables when they are building from outside of a git repository.

The idempotent feature builds on the package maintainer use case, allowing them the set an env variable at build time (VERGEN_IDEMPOTENT) that overrides all generated environment variables with a common value. This ensures the environment variables are set to something in case env! was used instead of option_env! on use in the source and they can complete a build.

I'll update the docs as the use case for this isn't very clear.

[unprofessor]

The main intent for SOURCE_DATE_EPOCH is to allow package maintainers to set the output of those particular variables when they are building from outside of a git repository.

Is it reasonable to check for access to a git repository? Currently, whenever SOURCE_DATE_EPOCH is set for other non-reproducible dates, e.g. when building with GNU Guix, I have to work around this behavior, somewhat defeating the purpose of vergen:

let gitcl = GitclBuilder::default()
    .all()
    .describe(true, true, Some("v*.*.*"))
    .commit_timestamp(false) // workaround: see below
    .commit_date(false) // workaround: see below
    .build()?;

// ...

let ts_out = Command::new("git")
    .args(["log", "-1", "--format=%cI"])
    .output()?;
let ts = String::from_utf8_lossy(&ts_out.stdout).trim().to_string();
println!("cargo:rustc-env=VERGEN_GIT_COMMIT_TIMESTAMP={}", ts);

let d_out = Command::new("git")
    .args(["log", "-1", "--format=%cs"])
    .output()?;
let d = String::from_utf8_lossy(&d_out.stdout).trim().to_string();
println!("cargo:rustc-env=VERGEN_GIT_COMMIT_DATE={}", d);

[cherryman]

@exfed we ended up dropping vergen precisely because this bizarre behaviour breaks during nix, where git info is available but we have no way of including it. we have our own impl: https://github.com/n1xyz/gitver