nixos/hydra postgresql: Fix #27314 and add test case

nixos/modules/services/continuous-integration/hydra/default.nix

···
 
           ${optionalString haveLocalDB ''
             if ! [ -e ${baseDir}/.db-created ]; then
-              ${config.services.postgresql.package}/bin/createuser hydra
-              ${config.services.postgresql.package}/bin/createdb -O hydra hydra
+              ${pkgs.sudo}/bin/sudo -u ${config.services.postgresql.superUser} ${config.services.postgresql.package}/bin/createuser hydra
+              ${pkgs.sudo}/bin/sudo -u ${config.services.postgresql.superUser} ${config.services.postgresql.package}/bin/createdb -O hydra hydra
               touch ${baseDir}/.db-created
             fi
           ''}

nixos/modules/services/databases/postgresql.nix

···
 
   pre84 = versionOlder (builtins.parseDrvName postgresql.name).version "8.4";
 
-  # NixOS traditionally used `root` as superuser, most other distros use `postgres`. From 17.09
-  # we also try to follow this standard
-  superuser = (if versionAtLeast config.system.stateVersion "17.09" then "postgres" else "root");
 
 in
 
···
           Contents of the <filename>recovery.conf</filename> file.
         '';
       };
+      superUser = mkOption {
+        type = types.str;
+        default= if versionAtLeast config.system.stateVersion "17.09" then "postgres" else "root";
+        internal = true;
+        description = ''
+          NixOS traditionally used `root` as superuser, most other distros use `postgres`.
+          From 17.09 we also try to follow this standard. Internal since changing this value
+          would lead to breakage while setting up databases.
+        '';
+        };
     };
 
   };
···
           ''
             # Initialise the database.
             if ! test -e ${cfg.dataDir}/PG_VERSION; then
-              initdb -U ${superuser}
+              initdb -U ${cfg.superUser}
               # See postStart!
               touch "${cfg.dataDir}/.first_startup"
             fi
···
         # Wait for PostgreSQL to be ready to accept connections.
         postStart =
           ''
-            while ! ${pkgs.sudo}/bin/sudo -u ${superuser} psql --port=${toString cfg.port} -d postgres -c "" 2> /dev/null; do
+            while ! ${pkgs.sudo}/bin/sudo -u ${cfg.superUser} psql --port=${toString cfg.port} -d postgres -c "" 2> /dev/null; do
                 if ! kill -0 "$MAINPID"; then exit 1; fi
                 sleep 0.1
             done
 
             if test -e "${cfg.dataDir}/.first_startup"; then
               ${optionalString (cfg.initialScript != null) ''
-                ${pkgs.sudo}/bin/sudo -u ${superuser} psql -f "${cfg.initialScript}" --port=${toString cfg.port} -d postgres
+                ${pkgs.sudo}/bin/sudo -u ${cfg.superUser} psql -f "${cfg.initialScript}" --port=${toString cfg.port} -d postgres
               ''}
               rm -f "${cfg.dataDir}/.first_startup"
             fi

nixos/tests/hydra.nix

···
+import ./make-test.nix ({ pkgs, ...} : {
+  name = "hydra-init-localdb";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ pstn ];
+  };
+
+  machine =
+    { config, pkgs, ... }:
+
+    {
+      services.hydra = {
+        enable = true;
+
+        #Hydra needs those settings to start up, so we add something not harmfull.
+        hydraURL = "example.com";
+        notificationSender = "[e-mail]";
+      };
+    };
+
+  testScript =
+    ''
+      # let the system boot up
+      $machine->waitForUnit("multi-user.target");
+      # test whether the database is running
+      $machine->succeed("systemctl status postgresql.service");
+      # test whether the actual hydra daemons are running
+      $machine->succeed("systemctl status hydra-queue-runner.service");
+      $machine->succeed("systemctl status hydra-init.service");
+      $machine->succeed("systemctl status hydra-evaluator.service");
+      $machine->succeed("systemctl status hydra-send-stats.service");
+     '';
+})