Re: [Xen-devel] [PATCH v2 1/8] xen: calculate XEN_BUILD_TIME using XEN_BUILD_DATE value

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Wed, Jul 04, 2018 at 04:41:30PM +0100, Ian Jackson wrote:
> Daniel Kiper writes ("Re: [PATCH v2 1/8] xen: calculate XEN_BUILD_TIME using 
> XEN_BUILD_DATE value"):
> > On Wed, Jul 04, 2018 at 02:58:17PM +0100, Ian Jackson wrote:
> > > export XEN_BUILD_POSIX_TIME       ?= $(shell echo 
> > > $${SOURCE_DATE_EPOCH-date +%s})
> >
> > OK, but what if SOURCE_DATE_EPOCH is not defined?
> > "date" command is not a good solution here.
>
> I don't understand why you think not.  I think all the versions of
> date we care about will do something sensible with
>     date +%s
>
> I don't have a FreeBSD host to hand to test, CC'ing Roger.

This is not the date command problem. This is more related to SOURCE_DATE_EPOCH
spec (https://reproducible-builds.org/specs/source-date-epoch/#idm55)
which clearly says:

  The value MUST be reproducible (deterministic) across different
  executions of the build, depending only on the source code. It SHOULD
  be set to the last modification time of the source, incorporating any
  packaging-specific modifications.

So, AIUI, above implies that we should not use the date command. Am I right?

> Although my rune above is wrong and should read
>     export XEN_BUILD_POSIX_TIME       ?= $(shell echo 
> $${SOURCE_DATE_EPOCH-$$(date +%s)})
>
> > If other guys are OK with that I can prepare a patch with your Suggested-by.
>
> How do in intend to choose which wrapper script to use ?

I though that a mechanism to differentiate GNU and BSD systems exists in
the Xen build system. So, I was going to reuse it somehow.

> This pile of stuff will run on awful lot of times during each make so
> keeping it of reasonable size is a good plan.
>
> Maybe it would be better to have a single shell script which outputs a
> whole bunch of variable settings for make to $(eval ) ?

It looks that this variable is referenced once, so, it should not be a problem.

Daniel

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