package systems, before and after the core-updates merge

[Christopher Baines <mail@cbaines.net>]

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Hey,

One thing that I was aware of before the recent core-updates merge was
that the Guix Data Service [1] didn't generate derivations for systems
other than x86_64-linux and i686-linux, at least to the same extent as
the master branch before the recent core-updates merge.

1: http://data.guix.gnu.org/revision/2fa55c72476c73211cbb2d6b29c05a1ad58a6cf9
   (see the Derivations table)

I put this down to a bug I wasn't seeing, but now that the core-updates
branch has been merged, and now that this effect is showing up on the
master branch, I've investigated a little more.

I think this is down to the use of package-transitive-supported-systems
within the Guix Data Service, but the output of this function for a
package can also be seen by running guix package --show.

Before the recent core-updates merge:

  ./pre-inst-env guix package --show=hello
  …
  systems: x86_64-linux i686-linux armhf-linux aarch64-linux mips64el-linux

After the recent core-updates merge:

  → guix package --show=hello
  …
  systems: x86_64-linux i686-linux

Looking at the implementation of package-transitive-supported-systems,
I'm pretty sure the change in behaviour is to do with the
supported-systems for the new packages involved in the bootstrap
process.

So in terms of questions I now have, what's the "systems: " output by
guix package --show meant to mean, and what does the recent change
removing the 3 systems above from most packages in Guix mean?

Also, for the Guix Data Service, all I want to know is for a given
package, is for which systems and targets a derivation can be reasonably
computed. Maybe it is wrong to use package-transitive-supported-systems
for this.

Thanks,

Chris

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