uwsgi-service: Add user/group for uwsgi service. Also add a uwsgi directory under /run (defaulting to /run/uwsgi) where the uwsgi user can place sockets.

nixos/modules/misc/ids.nix

···
       ripple-rest = 198;
       nix-serve = 199;
       tvheadend = 200;
+      uwsgi = 201;
 
       # When adding a uid, make sure it doesn't match an existing gid. And don't use uids above 399!
 
···
       #ripple-rest = 198; #unused
       #nix-serve = 199; #unused
       #tvheadend = 200; #unused
+      uwsgi = 201;
 
       # When adding a gid, make sure it doesn't match an existing
       # uid. Users and groups with the same name should have equal

nixos/modules/services/web-servers/uwsgi.nix

···
 
   options = {
     services.uwsgi = {
-      
+
       enable = mkOption {
         type = types.bool;
         default = false;
         description = "Enable uWSGI";
       };
 
+      runDir = mkOption {
+        type = types.string;
+        default = "/run/uwsgi";
+        description = "Where uWSGI communication sockets can live";
+      };
+
       instance = mkOption {
         type = types.attrs;
         default = {
···
               moin = {
                 type = "normal";
                 python2Packages = self: with self; [ moinmoin ];
-                socket = "/run/uwsgi.sock";
+                socket = "${config.services.uwsgi.runDir}/uwsgi.sock";
               };
             };
           }
···
         description = "Plugins used with uWSGI";
       };
 
-    };
+      user = mkOption {
+        type = types.str;
+        default = "uwsgi";
+        description = "User account under which uwsgi runs.";
+      };
 
+      group = mkOption {
+        type = types.str;
+        default = "uwsgi";
+        description = "Group account under which uwsgi runs.";
+      };
+    };
   };
 
   config = mkIf cfg.enable {
-
     systemd.services.uwsgi = {
       wantedBy = [ "multi-user.target" ];
-      
+      preStart = ''
+        mkdir -p ${cfg.runDir}
+        chown ${cfg.user}:${cfg.group} ${cfg.runDir}
+      '';
       serviceConfig = {
         Type = "notify";
-        ExecStart = "${uwsgi}/bin/uwsgi --json ${pkgs.writeText "uwsgi.json" (buildCfg cfg.instance)}";
+        ExecStart = "${uwsgi}/bin/uwsgi --uid ${cfg.user} --gid ${cfg.group} --json ${pkgs.writeText "uwsgi.json" (buildCfg cfg.instance)}";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
         ExecStop = "${pkgs.coreutils}/bin/kill -INT $MAINPID";
         NotifyAccess = "main";
         KillSignal = "SIGQUIT";
       };
+    };
 
-    };
+    users.extraUsers = optionalAttrs (cfg.user == "uwsgi") (singleton
+      { name = "uwsgi";
+        group = cfg.group;
+        uid = config.ids.uids.uwsgi;
+      });
+
+    users.extraGroups = optionalAttrs (cfg.group == "uwsgi") (singleton
+      { name = "uwsgi";
+        gid = config.ids.gids.uwsgi;
+      });
   };
 }