nixos/slurm: run ctld as user and fix spool dir

* run as user 'slurm' per default instead of root
* add user/group slurm to ids.nix
* fix default location for the state dir of slurmctld:
  (/var/spool -> /var/spool/slurmctld)
* Update release notes with the above changes

nixos/modules/services/computing/slurm/slurm.nix

@@ -6,9 +6,14 @@ let
 
   cfg = config.services.slurm;
   # configuration file can be generated by http://slurm.schedmd.com/configurator.html
+
+  defaultUser = "slurm";
+
   configFile = pkgs.writeTextDir "slurm.conf"
     ''
       ClusterName=${cfg.clusterName}
+      StateSaveLocation=${cfg.stateSaveLocation}
+      SlurmUser=${cfg.user}
       ${optionalString (cfg.controlMachine != null) ''controlMachine=${cfg.controlMachine}''}
       ${optionalString (cfg.controlAddr != null) ''controlAddr=${cfg.controlAddr}''}
       ${optionalString (cfg.nodeName != null) ''nodeName=${cfg.nodeName}''}
@@ -159,6 +164,25 @@ in
         '';
       };
 
+      stateSaveLocation = mkOption {
+        type = types.str;
+        default = "/var/spool/slurmctld";
+        description = ''
+          Directory into which the Slurm controller, slurmctld, saves its state.
+        '';
+      };
+
+      user = mkOption {
+        type = types.str;
+        default = defaultUser;
+        description = ''
+          Set this option when you want to run the slurmctld daemon
+          as something else than the default slurm user "slurm".
+          Note that the UID of this user needs to be the same
+          on all nodes.
+        '';
+      };
+
       extraConfig = mkOption {
         default = "";
         type = types.lines;
@@ -226,6 +250,15 @@ in
 
     services.munge.enable = mkDefault true;
 
+    # use a static uid as default to ensure it is the same on all nodes
+    users.users.slurm = mkIf (cfg.user == defaultUser) {
+      name = defaultUser;
+      group = "slurm";
+      uid = config.ids.uids.slurm;
+    };
+
+    users.groups.slurm.gid = config.ids.uids.slurm;
+
     systemd.services.slurmd = mkIf (cfg.client.enable) {
       path = with pkgs; [ wrappedSlurm coreutils ]
         ++ lib.optional cfg.enableSrunX11 slurm-spank-x11;
@@ -261,6 +294,11 @@ in
         PIDFile = "/run/slurmctld.pid";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
       };
+
+      preStart = ''
+        mkdir -p ${cfg.stateSaveLocation}
+        chown -R ${cfg.user}:slurm ${cfg.stateSaveLocation}
+      '';
     };
 
   };