Fix evaluation of release-20.03-aarch64 jobset

[bennofs]

Motivation for this change

The nixos-20.03-aarch64 jobset on hydra currently fails to eval. This is because the constituents of the tested job are hardcoded to refer to the x86_64-linux tests, which aren't available in that jobset.

Proposed solution

This PR uses helper functions to only generate tested constituents for the available platforms. I also checked which tests are supported on aarch64, and limited everything else to x86_64-linux.

Variant for master: https://github.com/bennofs/nixpkgs/tree/fix-nixos-aarch64-eval-master

Things done

1. verify that tested.constituents did stay the same before and after the change: diff of the output from nix-instantiate --eval --strict --json -A tested.constituents nixos/release-combined.nix | jq before and after the change:

-  "nixos.iso_minimal.aarch64-linux",
-  "nixos.iso_minimal.i686-linux",
   "nixos.iso_minimal.x86_64-linux",
+  "nixos.iso_minimal.i686-linux",
+  "nixos.iso_minimal.aarch64-linux",

2. verify that all constituents of the tested job can be instantiated for aarch64:

#!/usr/bin/env fish
nix-instantiate --eval --strict --json -A tested.constituents --arg 'limitedSupportedSystems' '[]' --arg 'supportedSystems' '["aarch64-linux"]' nixos/release-combined.nix |
  jq -r '.[]' |
  while read attr;
      echo + instantiate $attr; 
      nix-instantiate -A $attr --arg 'limitedSupportedSystems' '[]' --arg 'supportedSystems' '["aarch64-linux"]' nixos/release-combined.nix;
  end

[samueldr]

(Recording the change discussed on IRC here)

This change should first be made on master, then backported (though the backport will not be a trivial cherry-pick).

[bennofs]

There is a version of this PR for master on https://github.com/bennofs/nixpkgs/tree/fix-nixos-aarch64-eval-master. Note however that the situation for master is a little bit different, as we don't have split jobsets for x86_64-linux and aarch64-linux there. Thus, if this change is applied to master, aarch64-linux tests will become part of the tested job on trunk-combined.

[bennofs]

Is it OK to keep this PR for discussion? Or should I create a new one against master?

[worldofpeace]

There is a version of this PR for master on https://github.com/bennofs/nixpkgs/tree/fix-nixos-aarch64-eval-master. Note however that the situation for master is a little bit different, as we don't have split jobsets for x86_64-linux and aarch64-linux there. Thus, if this change is applied to master, aarch64-linux tests will become part of the tested job on trunk-combined.

I guess that means we'd need two PRs, with this one merged just for 20.03.

[bennofs]

@worldofpeace ok, I created a second PR for master.

[worldofpeace]

I believe this is fine, though I'm not sure how I feel about the function names (can't suggest better right now I'm afraid).

[andir]

I did some prior work on a related topic. When the hydra eval changed we had to apply the conversion to strings on release-19.09 as well:
https://github.com/NixOS/nixpkgs/blob/release-19.09/nixos/release-combined.nix#L47-L149

I tried to be as close to the "old" style definition as possible to make it more familiar and reduce conflicts if we would be cherry-picking changes.

No real opinion just felt like that might be a good piece of information.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/go-no-go-meeting-nixos-20-03-markhor/6495/16