add VM test for nix upgrade-nix

This commit adds a new NixOS VM test, which tests that `nix upgrade-nix`
works on both kinds of profiles (manifest.nix and manifest.json).

Done as a separate commit from 831d18a13, since it relies on the
--store-path argument from 026c90e5f as well.

Change-Id: I5fc94b751d252862cb6cffb541a4c072faad9f3b

tests/nixos/default.nix

@@ -141,6 +141,8 @@ in
 
   nix-copy = runNixOSTestFor "x86_64-linux" ./nix-copy.nix;
 
+  nix-upgrade-nix = runNixOSTestFor "x86_64-linux" ./nix-upgrade-nix.nix;
+
   nssPreload = runNixOSTestFor "x86_64-linux" ./nss-preload.nix;
 
   githubFlakes = runNixOSTestFor "x86_64-linux" ./github-flakes.nix;

tests/nixos/nix-upgrade-nix.nix

@@ -0,0 +1,80 @@
+{ lib, config, ... }:
+
+/**
+ * Test that nix upgrade-nix works regardless of whether /nix/var/nix/profiles/default
+ * is a nix-env style profile or a nix profile style profile.
+ */
+
+let
+  pkgs = config.nodes.machine.nixpkgs.pkgs;
+
+  lix = pkgs.nix;
+  lixVersion = lib.getVersion lix;
+
+  newNix = pkgs.nixVersions.unstable;
+  newNixVersion = lib.getVersion newNix;
+
+in {
+  name = "nix-upgrade-nix";
+
+  nodes = {
+    machine = { config, lib, pkgs, ... }: {
+      virtualisation.writableStore = true;
+      virtualisation.additionalPaths = [ pkgs.hello.drvPath ];
+      nix.settings.substituters = lib.mkForce [ ];
+      nix.settings.experimental-features = [ "nix-command" "flakes" ];
+      services.getty.autologinUser = "root";
+
+    };
+  };
+
+  testScript = { nodes }: ''
+    # fmt: off
+
+    start_all()
+
+    machine.succeed("nix --version >&2")
+
+    # Install Lix into the default profile, overriding /run/current-system/sw/bin/nix,
+    # and thus making Lix think we're not on NixOS.
+    machine.succeed("nix-env --install '${lib.getBin lix}' --profile /nix/var/nix/profiles/default >&2")
+
+    # Make sure that correctly got inserted into our PATH.
+    default_profile_nix_path = machine.succeed("command -v nix")
+    print(default_profile_nix_path)
+    assert default_profile_nix_path.strip() == "/nix/var/nix/profiles/default/bin/nix", \
+      f"{default_profile_nix_path.strip()=} != /nix/var/nix/profiles/default/bin/nix"
+
+    # And that it's the Nix we specified.
+    default_profile_version = machine.succeed("nix --version")
+    assert "${lixVersion}" in default_profile_version, f"${lixVersion} not in {default_profile_version}"
+
+    # Upgrade to a different version of Nix, and make sure that also worked.
+
+    machine.succeed("nix upgrade-nix --store-path ${newNix} >&2")
+    default_profile_version = machine.succeed("nix --version")
+    print(default_profile_version)
+    assert "${newNixVersion}" in default_profile_version, f"${newNixVersion} not in {default_profile_version}"
+
+    # Now 'break' this profile -- use nix profile on it so nix-env will no longer work on it.
+    machine.succeed(
+      "nix profile install --profile /nix/var/nix/profiles/default '${pkgs.hello.drvPath}^*' >&2"
+    )
+
+    # Confirm that nix-env is broken.
+    machine.fail(
+      "nix-env --query --installed --profile /nix/var/nix/profiles/default >&2"
+    )
+
+    # And use nix upgrade-nix one more time, on the `nix profile` style profile.
+    # (Specifying Lix by full path so we can use --store-path.)
+    machine.succeed(
+      "${lib.getBin lix}/bin/nix upgrade-nix --store-path '${lix}' >&2"
+    )
+
+    default_profile_version = machine.succeed("nix --version")
+    print(default_profile_version)
+    assert "${lixVersion}" in default_profile_version, f"${lixVersion} not in {default_profile_version}"
+  '';
+
+}