nixosTest: Fix infinite recursion involving hasContext testScript whe…

…n useNixStoreImage = true

The involved test was nixosTests.nextcloud.basic21.

It has a testScript that is strict in nodes.nextcloud.config.system.build.vm,
in assertions about imagemagick being in the system closure.

The recursion was introduced in 329a446 from
NixOS#140792

nixos/lib/testing-python.nix

@@ -129,6 +129,32 @@ rec {
             extraConfigurations = extraConfigurations ++ (pkgs.lib.optional (qemu_pkg != null)
               {
                 virtualisation.qemu.package = qemu_pkg;
+
+                # Make sure all derivations referenced by the test
+                # script are available on the nodes. When the store is
+                # accessed through 9p, this isn't important, since
+                # everything in the store is available to the guest,
+                # but when building a root image it is, as all paths
+                # that should be available to the guest has to be
+                # copied to the image.
+                virtualisation.additionalPaths =
+                  lib.optional
+                    # A testScript may evaluate nodes, which has caused
+                    # infinite recursions. The demand cycle involves:
+                    #   testScript -->
+                    #   nodes -->
+                    #   toplevel -->
+                    #   additionalPaths -->
+                    #   hasContext testScript' -->
+                    #   testScript (ad infinitum)
+                    # If we don't need to build an image, we can break this
+                    # cycle by short-circuiting when useNixStoreImage is false.
+                    (config.virtualisation.useNixStoreImage && builtins.hasContext testScript')
+                    (pkgs.writeStringReferencesToFile testScript');
+
+                # Ensure we do not use aliases. Ideally this is only set
+                # when the test framework is used by Nixpkgs NixOS tests.
+                nixpkgs.config.allowAliases = false;
               }
             );
           };