module: append application_name to HYDRA_DBI

This will make it easier to track specifically where queries are being
made from (assuming a `log_line_prefix` that includes `%a` in the
postgres configuration).

hydra-module.nix

@@ -58,6 +58,11 @@ in
         example = "dbi:Pg:dbname=hydra;host=postgres.example.org;user=foo;";
         description = ''
           The DBI string for Hydra database connection.
+
+          NOTE: Attempts to set `application_name` will be overridden by
+          `hydra-TYPE` (where TYPE is e.g. `evaluator`, `queue-runner`,
+          etc.) in all hydra services to more easily distinguish where
+          queries are coming from.
         '';
       };
 
@@ -248,7 +253,9 @@ in
       { wantedBy = [ "multi-user.target" ];
         requires = optional haveLocalDB "postgresql.service";
         after = optional haveLocalDB "postgresql.service";
-        environment = env;
+        environment = env // {
+          HYDRA_DBI = "${env.HYDRA_DBI};application_name=hydra-init";
+        };
         path = [ pkgs.utillinux ];
         preStart = ''
           mkdir -p ${baseDir}
@@ -304,7 +311,9 @@ in
       { wantedBy = [ "multi-user.target" ];
         requires = [ "hydra-init.service" ];
         after = [ "hydra-init.service" ];
-        environment = serverEnv;
+        environment = serverEnv // {
+          HYDRA_DBI = "${serverEnv.HYDRA_DBI};application_name=hydra-server";
+        };
         restartTriggers = [ hydraConf ];
         serviceConfig =
           { ExecStart =
@@ -326,6 +335,7 @@ in
         environment = env // {
           PGPASSFILE = "${baseDir}/pgpass-queue-runner"; # grrr
           IN_SYSTEMD = "1"; # to get log severity levels
+          HYDRA_DBI = "${env.HYDRA_DBI};application_name=hydra-queue-runner";
         };
         serviceConfig =
           { ExecStart = "@${cfg.package}/bin/hydra-queue-runner hydra-queue-runner -v";
@@ -345,7 +355,9 @@ in
         restartTriggers = [ hydraConf ];
         after = [ "hydra-init.service" "network.target" ];
         path = with pkgs; [ nettools cfg.package jq ];
-        environment = env;
+        environment = env // {
+          HYDRA_DBI = "${env.HYDRA_DBI};application_name=hydra-evaluator";
+        };
         serviceConfig =
           { ExecStart = "@${cfg.package}/bin/hydra-evaluator hydra-evaluator";
             ExecStopPost = "${cfg.package}/bin/hydra-evaluator --unlock";
@@ -358,7 +370,9 @@ in
     systemd.services.hydra-update-gc-roots =
       { requires = [ "hydra-init.service" ];
         after = [ "hydra-init.service" ];
-        environment = env;
+        environment = env // {
+          HYDRA_DBI = "${env.HYDRA_DBI};application_name=hydra-update-gc-roots";
+        };
         serviceConfig =
           { ExecStart = "@${cfg.package}/bin/hydra-update-gc-roots hydra-update-gc-roots";
             User = "hydra";
@@ -369,7 +383,9 @@ in
     systemd.services.hydra-send-stats =
       { wantedBy = [ "multi-user.target" ];
         after = [ "hydra-init.service" ];
-        environment = env;
+        environment = env // {
+          HYDRA_DBI = "${env.HYDRA_DBI};application_name=hydra-send-stats";
+        };
         serviceConfig =
           { ExecStart = "@${cfg.package}/bin/hydra-send-stats hydra-send-stats";
             User = "hydra";
@@ -383,6 +399,7 @@ in
         restartTriggers = [ hydraConf ];
         environment = env // {
           PGPASSFILE = "${baseDir}/pgpass-queue-runner"; # grrr
+          HYDRA_DBI = "${env.HYDRA_DBI};application_name=hydra-notify";
         };
         serviceConfig =
           { ExecStart = "@${cfg.package}/bin/hydra-notify hydra-notify";