mkhelp.pl: support reproducible build

[jurobystricky]

If the environment contains SOURCE_DATE_EPOCH, generate output such as:

• Generation time: reproducible build, date unspecified

Instead of the usual output based on the current date such as:

• Generation time: Tue Oct-24 18:01:41 2017

This will improve reproducibility. The generated string is only
part of a comment, so there should be no adverse consequences.

Signed-off-by: Juro Bystricky juro.bystricky@intel.com

[jay]

If the environment contains SOURCE_DATE_EPOCH, generate output such as:

Generation time: reproducible build, date unspecified

From what I'm reading isn't that variable part of a spec that is supposed to have the seconds in it. See https://reproducible-builds.org/specs/source-date-epoch/

I don't see "date unspecified" covered (though I just did some cursory searching). Also note we have an SHA256 generated in the comment area that may do what you want. (Edit: ignore the SHA256 comment I was thinking of mk-ca-bundle)

[jurobystricky]

Yes, SOURCE_DATE_EPOCH does contain an "epoch" value. So it is tempting to print that value instead of the build time. However, I replicated the approach I have seen in openssl, where they issue an identical message: "reproducible build: date unspecified". Claiming that the "Generation time" is the time specified by SOURCE_DATE_EPOCH would not be entirely true.

[jurobystricky]

Another approach would be not to generate the whole line "Generation time: ..." at all.

[bagder]

I don't think the time string serves any good purpose there so I think the simpler option of just removing it is the better one here!

[jurobystricky]

Remove unconditionally or only when SOURCE_DATE_EPOCH is present?
If there is no good purpose, then unconditional removal would be probably best.

[bagder]

I say remove unconditionally

[jurobystricky]

OK then. I will create a new pull request.

[bagder]

Excellent, thank you!