ci/eval: eval nixosTests.simple

We can't eval all nixos tests, this would be way too expensive. But we
can evaluate `nixosTests.simple` as an indication whether the nixos test
driver has changed. If that's the case, this means that *all* NixOS
tests need to be rebuilt.

ci/eval/outpaths.nix

@@ -14,7 +14,8 @@
 }:
 let
   lib = import (path + "/lib");
-  hydraJobs =
+
+  nixpkgsJobs =
     import (path + "/pkgs/top-level/release.nix")
       # Compromise: accuracy vs. resources needed for evaluation.
       {
@@ -59,9 +60,15 @@ let
           __allowFileset = false;
         };
       };
+
+  nixosJobs = import (path + "/nixos/release.nix") {
+    inherit attrNamesOnly;
+    supportedSystems = if systems == null then [ builtins.currentSystem ] else systems;
+  };
+
   recurseIntoAttrs = attrs: attrs // { recurseForDerivations = true; };
 
-  # hydraJobs leaves recurseForDerivations as empty attrmaps;
+  # release-lib leaves recurseForDerivations as empty attrmaps;
   # that would break nix-env and we also need to recurse everywhere.
   tweak = lib.mapAttrs (
     name: val:
@@ -87,4 +94,9 @@ let
   ];
 
 in
-tweak (builtins.removeAttrs hydraJobs blacklist)
+tweak (
+  (builtins.removeAttrs nixpkgsJobs blacklist)
+  // {
+    nixosTests.simple = nixosJobs.tests.simple;
+  }
+)

nixos/release.nix

@@ -12,6 +12,16 @@ with import ../lib;
     "aarch64-linux"
   ],
   configuration ? { },
+
+  # This flag, if set to true, causes the resulting tree of attributes
+  # to *not* have a ".${system}" suffixed upon every job name like Hydra
+  # expects. So far, this is only implemented for `tests`.
+  #
+  # This flag exists mainly for use by ci/eval/attrpaths.nix; see
+  # that file for full details.  The exact behavior of this flag
+  # may change; it should be considered an internal implementation
+  # detail of ci/eval.
+  attrNamesOnly ? false,
 }:
 
 with import ../pkgs/top-level/release-lib.nix { inherit supportedSystems; };
@@ -31,7 +41,12 @@ let
     import ./tests/all-tests.nix {
       inherit system;
       pkgs = import ./.. { inherit system; };
-      callTest = config: {
+      callTest =
+        config:
+        if attrNamesOnly then
+          hydraJob config.test
+        else
+          {
             ${system} = hydraJob config.test;
           };
     }
@@ -41,13 +56,20 @@ let
       allDrivers = import ./tests/all-tests.nix {
         inherit system;
         pkgs = import ./.. { inherit system; };
-        callTest = config: {
+        callTest =
+          config:
+          if attrNamesOnly then
+            hydraJob config.test
+          else
+            {
               ${system} = hydraJob config.driver;
             };
       };
     };
 
-  allTests = foldAttrs recursiveUpdate { } (map allTestsForSystem supportedSystems);
+  allTests = foldAttrs recursiveUpdate { } (
+    map allTestsForSystem (if attrNamesOnly then [ (head supportedSystems) ] else supportedSystems)
+  );
 
   pkgs = import ./.. { system = "x86_64-linux"; };