draft: test for non-flake build regressions

this adds a nixos vm test doing a deploy on a nix which does not have
flakes enabled, to guard against this breaking as it has done before [1].

The existing test infrastructure is changed slightly to make enabling
flakes configurable inside the vm's config.

[1] #272

nix/tests/common.nix

@@ -2,11 +2,12 @@
 #
 # SPDX-License-Identifier: MPL-2.0
 
-{inputs, pkgs, ...}: {
+{inputs, pkgs, flakes, ...}: {
   nix = {
     registry.nixpkgs.flake = inputs.nixpkgs;
+    nixPath = [ "nixpkgs=${inputs.nixpkgs}" ];
     extraOptions = ''
-      experimental-features = nix-command flakes
+      experimental-features = ${if flakes then "nix-command flakes" else "nix-command"}
     '';
     settings = {
       trusted-users = [ "root" "@wheel" ];

nix/tests/default.nix

@@ -20,12 +20,12 @@ let
     done <$refs
   '';
 
-  mkTest = { name ? "", user ? "root", isLocal ? true, deployArgs }: let
+  mkTest = { name ? "", user ? "root", flakes ? true, isLocal ? true, deployArgs }: let
     nodes = {
       server = { nodes, ... }: {
         imports = [
          ./server.nix
-         (import ./common.nix { inherit inputs pkgs; })
+         (import ./common.nix { inherit inputs pkgs flakes; })
         ];
         virtualisation.additionalPaths = lib.optionals (!isLocal) [
           pkgs.hello
@@ -35,7 +35,7 @@ let
         ];
       };
       client = { nodes, ... }: {
-        imports = [ (import ./common.nix { inherit inputs pkgs; }) ];
+        imports = [ (import ./common.nix { inherit inputs pkgs flakes; }) ];
         environment.systemPackages = [ pkgs.deploy-rs.deploy-rs ];
         virtualisation.additionalPaths = lib.optionals isLocal [
           pkgs.hello
@@ -56,11 +56,24 @@ let
       systems.url = "${inputs.utils.inputs.systems}";
       flake-compat.url = "${inputs.flake-compat}";
       flake-compat.flake = false;
+
+      enable-flakes.url = "${builtins.toFile "use-flakes" (if flakes then "true" else "false")}";
+      enable-flakes.flake = false;
     '';
 
     flake = builtins.toFile "flake.nix"
       (lib.replaceStrings [ "##inputs##" ] [ flakeInputs ] (builtins.readFile ./deploy-flake.nix));
 
+    flakeCompat = builtins.toFile "default.nix" ''
+      (import
+        (
+          let lock = builtins.fromJSON (builtins.readFile ./flake.lock); in
+          lock.nodes.flake-compat.locked.path
+        )
+        { src = ./.; }
+      ).defaultNix
+    '';
+
   in pkgs.nixosTest {
     inherit nodes name;
 
@@ -73,11 +86,13 @@ let
       # Prepare
       client.succeed("mkdir tmp && cd tmp")
       client.succeed("cp ${flake} ./flake.nix")
+      client.succeed("cp ${flakeCompat} ./default.nix")
       client.succeed("cp ${./server.nix} ./server.nix")
       client.succeed("cp ${./common.nix} ./common.nix")
       client.succeed("cp ${serverNetworkJSON} ./network.json")
-      client.succeed("nix flake lock")
+      client.succeed("nix --extra-experimental-features flakes flake lock")
 
+      client.succeed("cat flake.lock")
 
       # Setup SSH key
       client.succeed("mkdir -m 700 /root/.ssh")
@@ -96,7 +111,7 @@ let
       server.fail("su ${user} -l -c 'hello | figlet'")
 
       # Deploy to the server
-      client.succeed("deploy ${deployArgs}")
+      client.succeed("deploy ${deployArgs}", timeout=9000)
 
       # Make sure packages are present after deployment
       server.succeed("su ${user} -l -c 'hello | figlet' >&2")
@@ -131,4 +146,10 @@ in {
     user = "deploy";
     deployArgs = "-s .#profile -- --offline";
   };
+  # Deployment using a non-flake nix
+  non-flake-build = mkTest {
+    name = "local-build";
+    flakes = false;
+    deployArgs = "-s .#server";
+  };
 }

nix/tests/deploy-flake.nix

@@ -15,7 +15,7 @@
   in {
     nixosConfigurations.server = nixpkgs.lib.nixosSystem {
       inherit system pkgs;
-      specialArgs = { inherit inputs; };
+      specialArgs = { inherit inputs; flakes = import inputs.enable-flakes; };
       modules = [
         ./server.nix
         ./common.nix