graphite service: store PID files under /run and configure systemd to use them

The advantage of putting the PID file under the ephemeral /run is that
when the machine crashes /run gets cleared allowing graphite to start
once the machine is rebooted.

We also set the PIDFile systemd option so that systemd knows the correct
PID and enables systemd to remove the file after service shut down.

nixos/modules/services/monitoring/graphite.nix

@@ -41,8 +41,15 @@ let
   };
 
   carbonOpts = name: with config.ids; ''
-    --nodaemon --syslog --prefix=${name} --pidfile ${dataDir}/${name}.pid ${name}
+    --nodaemon --syslog --prefix=${name} --pidfile /run/${name}/${name}.pid ${name}
   '';
+
+  mkPidFileDir = name: ''
+    mkdir -p /run/${name}
+    chmod 0700 /run/${name}
+    chown -R graphite:graphite /run/${name}
+  '';
+
   carbonEnv = {
     PYTHONPATH = "${pkgs.python27Packages.carbon}/lib/python2.7/site-packages";
     GRAPHITE_ROOT = dataDir;
@@ -370,18 +377,20 @@ in {
 
   config = mkMerge [
     (mkIf cfg.carbon.enableCache {
-      systemd.services.carbonCache = {
+      systemd.services.carbonCache = let name = "carbon-cache"; in {
         description = "Graphite Data Storage Backend";
         wantedBy = [ "multi-user.target" ];
         after = [ "network-interfaces.target" ];
         environment = carbonEnv;
         serviceConfig = {
-          ExecStart = "${pkgs.twisted}/bin/twistd ${carbonOpts "carbon-cache"}";
+          ExecStart = "${pkgs.twisted}/bin/twistd ${carbonOpts name}";
           User = "graphite";
           Group = "graphite";
           PermissionsStartOnly = true;
+          PIDFile="/run/${name}/${name}.pid";
         };
-        preStart = ''
+        preStart = mkPidFileDir name + ''
+
           mkdir -p ${cfg.dataDir}/whisper
           chmod 0700 ${cfg.dataDir}/whisper
           chown -R graphite:graphite ${cfg.dataDir}
@@ -390,31 +399,35 @@ in {
     })
 
     (mkIf cfg.carbon.enableAggregator {
-      systemd.services.carbonAggregator = {
+      systemd.services.carbonAggregator = let name = "carbon-aggregator"; in {
         enable = cfg.carbon.enableAggregator;
         description = "Carbon Data Aggregator";
         wantedBy = [ "multi-user.target" ];
         after = [ "network-interfaces.target" ];
         environment = carbonEnv;
         serviceConfig = {
-          ExecStart = "${pkgs.twisted}/bin/twistd ${carbonOpts "carbon-aggregator"}";
+          ExecStart = "${pkgs.twisted}/bin/twistd ${carbonOpts name}";
           User = "graphite";
           Group = "graphite";
+          PIDFile="/run/${name}/${name}.pid";
         };
+        preStart = mkPidFileDir name;
       };
     })
 
     (mkIf cfg.carbon.enableRelay {
-      systemd.services.carbonRelay = {
+      systemd.services.carbonRelay = let name = "carbon-relay"; in {
         description = "Carbon Data Relay";
         wantedBy = [ "multi-user.target" ];
         after = [ "network-interfaces.target" ];
         environment = carbonEnv;
         serviceConfig = {
-          ExecStart = "${pkgs.twisted}/bin/twistd ${carbonOpts "carbon-relay"}";
+          ExecStart = "${pkgs.twisted}/bin/twistd ${carbonOpts name}";
           User = "graphite";
           Group = "graphite";
+          PIDFile="/run/${name}/${name}.pid";
         };
+        preStart = mkPidFileDir name;
       };
     })