nixos/hydra: incorporate upstream changes and update test

During the last update, `hydra-notify` was rewritten as a daemon which
listens to postgresql notifications for each build[1]. The module
uses the `hydra-notify.service` unit from upstream's Hydra module and
the VM test ensures that email notifications are sent properly.

Also updated `hydra-init.service` to install `pg_trgm` on a local
database if needed[2].

[1] https://github.com/NixOS/hydra/commit/c7861b85c4c3cc974b27147bbf3cc258b9fe9cc3
[2] https://github.com/NixOS/hydra/commit/8a0a5ec3a3200d4f4d4d38f87d0afdb49f092b39

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

···
               ${pkgs.sudo}/bin/sudo -u ${config.services.postgresql.superUser} ${config.services.postgresql.package}/bin/createdb -O hydra hydra
               touch ${baseDir}/.db-created
             fi
+            echo "create extension if not exists pg_trgm" | ${pkgs.sudo}/bin/sudo -u ${config.services.postgresql.superUser} -- ${config.services.postgresql.package}/bin/psql hydra
           ''}
 
           if [ ! -e ${cfg.gcRootsDir} ]; then
···
           };
       };
 
+    systemd.services.hydra-notify =
+      { wantedBy = [ "multi-user.target" ];
+        requires = [ "hydra-init.service" ];
+        after = [ "hydra-init.service" ];
+        restartTriggers = [ hydraConf ];
+        environment = env // {
+          PGPASSFILE = "${baseDir}/pgpass-queue-runner";
+        };
+        serviceConfig =
+          { ExecStart = "@${cfg.package}/bin/hydra-notify hydra-notify";
+            # FIXME: run this under a less privileged user?
+            User = "hydra-queue-runner";
+            Restart = "always";
+            RestartSec = 5;
+          };
+      };
+
     # If there is less than a certain amount of free disk space, stop
     # the queue/evaluator to prevent builds from failing or aborting.
     systemd.services.hydra-check-space =
···
         hydra-users hydra-queue-runner hydra
         hydra-users hydra-www hydra
         hydra-users root hydra
+        # The postgres user is used to create the pg_trgm extension for the hydra database
+        hydra-users postgres postgres
       '';
 
     services.postgresql.authentication = optionalString haveLocalDB

nixos/tests/hydra/create-trivial-project.sh

···
   "enabled": "1",
   "visible": "1",
   "keepnr": "1",
+  "enableemail": true,
+  "emailoverride": "hydra@localhost",
   "nixexprinput": "trivial",
   "nixexprpath": "trivial.nix",
   "inputs": {

nixos/tests/hydra/default.nix

···
               notificationSender = "[e-mail]";
 
               package = pkgs.hydra.override { inherit nix; };
+
+              extraConfig = ''
+                email_notification = 1
+              '';
             };
+            services.postfix.enable = true;
             nix = {
               buildMachines = [{
                 hostName = "localhost";
···
           # let the system boot up
           $machine->waitForUnit("multi-user.target");
           # test whether the database is running
-          $machine->succeed("systemctl status postgresql.service");
+          $machine->waitForUnit("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");
+          $machine->waitForUnit("hydra-init.service");
+          $machine->requireActiveUnit("hydra-queue-runner.service");
+          $machine->requireActiveUnit("hydra-evaluator.service");
+          $machine->requireActiveUnit("hydra-notify.service");
 
           $machine->succeed("hydra-create-user admin --role admin --password admin");
 
···
           $machine->succeed("create-trivial-project.sh");
 
           $machine->waitUntilSucceeds('curl -L -s http://localhost:3000/build/1 -H "Accept: application/json" |  jq .buildstatus | xargs test 0 -eq');
+
+          $machine->waitUntilSucceeds('journalctl -eu hydra-notify.service -o cat | grep -q "sending mail notification to hydra@localhost"');
         '';
       })));