nixos/prometheus/alertmanager: use DynamicUser instead of nobody

See issue #55370

nixos/doc/manual/release-notes/rl-1909.xml

@@ -101,6 +101,15 @@
       <option>services.prometheus.stateDir</option> at the same time.
     </para>
    </listitem>
+   <listitem>
+    <para>
+      The options <option>services.prometheus.alertmanager.user</option> and
+      <option>services.prometheus.alertmanager.group</option> have been removed
+      because the alertmanager service is now using systemd's <link
+      xlink:href="http://0pointer.net/blog/dynamic-users-with-systemd.html">
+      DynamicUser mechanism</link> which obviates these options.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

nixos/modules/rename.nix

@@ -45,6 +45,8 @@ with lib;
     (mkRemovedOptionModule [ "services" "neo4j" "port" ] "Use services.neo4j.http.listenAddress instead.")
     (mkRemovedOptionModule [ "services" "neo4j" "boltPort" ] "Use services.neo4j.bolt.listenAddress instead.")
     (mkRemovedOptionModule [ "services" "neo4j" "httpsPort" ] "Use services.neo4j.https.listenAddress instead.")
+    (mkRemovedOptionModule [ "services" "prometheus" "alertmanager" "user" ] "The alertmanager service is now using systemd's DynamicUser mechanism which obviates a user setting.")
+    (mkRemovedOptionModule [ "services" "prometheus" "alertmanager" "group" ] "The alertmanager service is now using systemd's DynamicUser mechanism which obviates a group setting.")
     (mkRenamedOptionModule [ "services" "tor" "relay" "portSpec" ] [ "services" "tor" "relay" "port" ])
     (mkRenamedOptionModule [ "services" "vmwareGuest" ] [ "virtualisation" "vmware" "guest" ])
     (mkRenamedOptionModule [ "jobs" ] [ "systemd" "services" ])

nixos/modules/services/monitoring/prometheus/alertmanager.nix

@@ -40,22 +40,6 @@ in {
         '';
       };
 
-      user = mkOption {
-        type = types.str;
-        default = "nobody";
-        description = ''
-          User name under which Alertmanager shall be run.
-        '';
-      };
-
-      group = mkOption {
-        type = types.str;
-        default = "nogroup";
-        description = ''
-          Group under which Alertmanager shall be run.
-        '';
-      };
-
       configuration = mkOption {
         type = types.nullOr types.attrs;
         default = null;
@@ -152,10 +136,8 @@ in {
         wantedBy = [ "multi-user.target" ];
         after    = [ "network.target" ];
         serviceConfig = {
-          User = cfg.user;
-          Group = cfg.group;
           Restart  = "always";
-          PrivateTmp = true;
+          DynamicUser = true;
           WorkingDirectory = "/tmp";
           ExecStart = "${cfg.package}/bin/alertmanager" +
             optionalString (length cmdlineArgs != 0) (" \\\n  " +