nixos/users: require one of users.users.name.{isSystemUser,isNormalUser}

As the only consequence of isSystemUser is that if the uid is null then
it's allocated below 500, if a user has uid = something below 500 then
we don't require isSystemUser to be set.

Motivation: https://github.com/NixOS/nixpkgs/issues/112647

nixos/modules/config/pulseaudio.nix

@@ -306,6 +306,7 @@ in {
         description = "PulseAudio system service user";
         home = stateDir;
         createHome = true;
+        isSystemUser = true;
       };
 
       users.groups.pulse.gid = gid;

nixos/modules/config/users-groups.nix

@@ -92,6 +92,8 @@ let
           the user's UID is allocated in the range for system users
           (below 500) or in the range for normal users (starting at
           1000).
+          Exactly one of <literal>isNormalUser</literal> and
+          <literal>isSystemUser</literal> must be true.
         '';
       };
 
@@ -107,6 +109,8 @@ let
           <option>useDefaultShell</option> to <literal>true</literal>,
           and <option>isSystemUser</option> to
           <literal>false</literal>.
+          Exactly one of <literal>isNormalUser</literal> and
+          <literal>isSystemUser</literal> must be true.
         '';
       };
 
@@ -521,6 +525,7 @@ in {
       };
       nobody = {
         uid = ids.uids.nobody;
+        isSystemUser = true;
         description = "Unprivileged account (don't use!)";
         group = "nogroup";
       };
@@ -608,17 +613,28 @@ in {
           Neither the root account nor any wheel user has a password or SSH authorized key.
           You must set one to prevent being locked out of your system.'';
       }
-    ] ++ flip mapAttrsToList cfg.users (name: user:
-      {
+    ] ++ flatten (flip mapAttrsToList cfg.users (name: user:
+      [
+        {
         assertion = (user.hashedPassword != null)
-                    -> (builtins.match ".*:.*" user.hashedPassword == null);
+        -> (builtins.match ".*:.*" user.hashedPassword == null);
         message = ''
-          The password hash of user "${user.name}" contains a ":" character.
-          This is invalid and would break the login system because the fields
-          of /etc/shadow (file where hashes are stored) are colon-separated.
-          Please check the value of option `users.users."${user.name}".hashedPassword`.'';
-      }
-    );
+            The password hash of user "${user.name}" contains a ":" character.
+            This is invalid and would break the login system because the fields
+            of /etc/shadow (file where hashes are stored) are colon-separated.
+            Please check the value of option `users.users."${user.name}".hashedPassword`.'';
+          }
+          {
+            assertion = let
+              xor = a: b: a && !b || b && !a;
+              isEffectivelySystemUser = user.isSystemUser || (user.uid != null && user.uid < 500);
+            in xor isEffectivelySystemUser user.isNormalUser;
+            message = ''
+              Exactly one of users.users.${user.name}.isSystemUser and users.users.${user.name}.isNormalUser must be set.
+            '';
+          }
+        ]
+    ));
 
     warnings =
       builtins.filter (x: x != null) (

nixos/modules/services/backup/borgbackup.nix

@@ -169,6 +169,7 @@ let
         (map (mkAuthorizedKey cfg false) cfg.authorizedKeys
         ++ map (mkAuthorizedKey cfg true) cfg.authorizedKeysAppendOnly);
       useDefaultShell = true;
+      isSystemUser = true;
     };
     groups.${cfg.group} = { };
   };

nixos/modules/services/databases/pgmanage.nix

@@ -197,6 +197,7 @@ in {
         group = pgmanage;
         home  = cfg.sqlRoot;
         createHome = true;
+        isSystemUser = true;
       };
       groups.${pgmanage} = {
         name = pgmanage;

nixos/modules/services/misc/bazarr.nix

@@ -64,6 +64,7 @@ in
 
     users.users = mkIf (cfg.user == "bazarr") {
       bazarr = {
+        isSystemUser = true;
         group = cfg.group;
         home = "/var/lib/${config.systemd.services.bazarr.serviceConfig.StateDirectory}";
       };

nixos/modules/services/misc/nix-daemon.nix

@@ -21,6 +21,7 @@ let
          calls in `libstore/build.cc', don't add any supplementary group
          here except "nixbld".  */
       uid = builtins.add config.ids.uids.nixbld nr;
+      isSystemUser = true;
       group = "nixbld";
       extraGroups = [ "nixbld" ];
     };

nixos/modules/services/monitoring/tuptime.nix

@@ -34,7 +34,10 @@ in {
 
     users = {
       groups._tuptime.members = [ "_tuptime" ];
-      users._tuptime.description = "tuptime database owner";
+      users._tuptime = {
+        isSystemUser = true;
+        description = "tuptime database owner";
+      };
     };
 
     systemd = {

nixos/modules/services/networking/bird.nix

@@ -73,6 +73,7 @@ let
           users.${variant} = {
             description = "BIRD Internet Routing Daemon user";
             group = variant;
+            isSystemUser = true;
           };
           groups.${variant} = {};
         };

nixos/modules/services/networking/ncdns.nix

@@ -243,8 +243,10 @@ in
         xlog.journal = true;
     };
 
-    users.users.ncdns =
-      { description = "ncdns daemon user"; };
+    users.users.ncdns = {
+      isSystemUser = true;
+      description = "ncdns daemon user";
+    };
 
     systemd.services.ncdns = {
       description = "ncdns daemon";

nixos/modules/services/networking/pixiecore.nix

@@ -93,6 +93,7 @@ in
     users.users.pixiecore = {
       description = "Pixiecore daemon user";
       group = "pixiecore";
+      isSystemUser = true;
     };
 
     networking.firewall = mkIf cfg.openFirewall {

nixos/modules/services/networking/pleroma.nix

@@ -75,6 +75,7 @@ in {
         description = "Pleroma user";
         home = cfg.stateDir;
         extraGroups = [ cfg.group ];
+        isSystemUser = true;
       };
       groups."${cfg.group}" = {};
     };

nixos/modules/services/security/privacyidea.nix

@@ -264,6 +264,7 @@ in
 
       users.users.privacyidea = mkIf (cfg.user == "privacyidea") {
         group = cfg.group;
+        isSystemUser = true;
       };
 
       users.groups.privacyidea = mkIf (cfg.group == "privacyidea") {};
@@ -294,6 +295,7 @@ in
 
       users.users.pi-ldap-proxy = mkIf (cfg.ldap-proxy.user == "pi-ldap-proxy") {
         group = cfg.ldap-proxy.group;
+        isSystemUser = true;
       };
 
       users.groups.pi-ldap-proxy = mkIf (cfg.ldap-proxy.group == "pi-ldap-proxy") {};

nixos/modules/services/web-apps/nextcloud.nix

@@ -607,6 +607,7 @@ in {
         home = "${cfg.home}";
         group = "nextcloud";
         createHome = true;
+        isSystemUser = true;
       };
       users.groups.nextcloud.members = [ "nextcloud" config.services.nginx.user ];