Automatically pass SOURCE_DATE_EPOCH to containers

[dalcinl]

Many build tools downstream cibuildwheel have support for reproducible builds via SOURCE_DATE_EPOCH.

Would it make sense passing this environment variable automatically to Linux containers (or any other OS containers cibuildwheel may support in the future)?

[joerick]

Yeah, I'd be interested in this. What would make sense for us, I wonder? Typically file mtimes in CI environments have the time of git checkout IIRC, but I suppose the right (reproducible) value would be the timestamp of the git commit that we're building.

That introduces a coupling to git, which we don't have currently. But we could easily add it to the cibuildwheel github action.

[dalcinl]

@joerick I'm not asking for cibuildwheel to determine (via git or whatever) and set the SOURCE_DATE_EPOCH environment variable. I'm just asking whether to pass it down to Linux containers. This is something that cibuildwheel supports if users explitily list SOURCE_DATE_EPOCH within the environment-pass option. I'm just asking to make this process automatic.

[joerick]

ah, right. Yeah, sounds good to me!

We could just modify this to add passthrough of SOURCE_DATE_EPOCH via the --env option to the container engine.

cibuildwheel/cibuildwheel/oci_container.py

Lines 113 to 128 in 18ff2c5

	subprocess.run(
	[
	self.engine.name,
	"create",
	"--env=CIBUILDWHEEL",
	f"--name={self.name}",
	"--interactive",
	"--volume=/:/host", # ignored on CircleCI
	*network_args,
	*self.engine.create_args,
	self.image,
	*shell_args,
	],
	check=True,
	)

[dalcinl]

@joerick Implementation looks indeed trivial. Where would you suggest this new feature should be documented?

[joerick]

A note in the environment-pass documentation would be sufficient, I think. https://cibuildwheel.readthedocs.io/en/stable/options/#environment-pass