nixos/tests/hostname: init (check system's host name)

NixOS currently has issues with setting the FQDN of a system in a way
where standard tools work. In order to help with experimentation and
avoid regressions, add a test that checks that the hostname is
reported as the user wanted it to be.

Co-authored-by: Michael Weiss <dev.primeos@gmail.com>

nixos/tests/all-tests.nix

@@ -134,6 +134,7 @@ in
   hitch = handleTest ./hitch {};
   hocker-fetchdocker = handleTest ./hocker-fetchdocker {};
   home-assistant = handleTest ./home-assistant.nix {};
+  hostname = handleTest ./hostname.nix {};
   hound = handleTest ./hound.nix {};
   hydra = handleTest ./hydra {};
   hydra-db-migration = handleTest ./hydra/db-migration.nix {};

nixos/tests/hostname.nix

@@ -0,0 +1,66 @@
+{ system ? builtins.currentSystem,
+  config ? {},
+  pkgs ? import ../.. { inherit system config; }
+}:
+
+with import ../lib/testing-python.nix { inherit system pkgs; };
+with pkgs.lib;
+
+let
+  makeHostNameTest = hostName: domain:
+    let
+      fqdn = hostName + (optionalString (domain != null) ".${domain}");
+    in
+      makeTest {
+        name = "hostname-${fqdn}";
+        meta = with pkgs.stdenv.lib.maintainers; {
+          maintainers = [ primeos blitz ];
+        };
+
+        machine = { lib, ... }: {
+          networking.hostName = hostName;
+          networking.domain = domain;
+
+          environment.systemPackages = with pkgs; [
+            inetutils
+          ];
+        };
+
+        testScript = ''
+          start_all()
+
+          machine = ${hostName}
+
+          machine.wait_for_unit("network-online.target")
+
+          # The FQDN, domain name, and hostname detection should work as expected:
+          assert "${fqdn}" == machine.succeed("hostname --fqdn").strip()
+          assert "${optionalString (domain != null) domain}" == machine.succeed("dnsdomainname").strip()
+          assert (
+              "${hostName}"
+              == machine.succeed(
+                  'hostnamectl status | grep "Static hostname" | cut -d: -f2'
+              ).strip()
+          )
+
+          # 127.0.0.1 and ::1 should resolve back to "localhost":
+          assert (
+              "localhost" == machine.succeed("getent hosts 127.0.0.1 | awk '{print $2}'").strip()
+          )
+          assert "localhost" == machine.succeed("getent hosts ::1 | awk '{print $2}'").strip()
+
+          # 127.0.0.2 should resolve back to the FQDN and hostname:
+          fqdn_and_host_name = "${optionalString (domain != null) "${hostName}.${domain} "}${hostName}"
+          assert (
+              fqdn_and_host_name
+              == machine.succeed("getent hosts 127.0.0.2 | awk '{print $2,$3}'").strip()
+          )
+        '';
+      };
+
+in
+{
+  noExplicitDomain = makeHostNameTest "ahost" null;
+
+  explicitDomain = makeHostNameTest "ahost" "adomain";
+}