Add user option ‘isAlias’ to allow one user account to alias another

modules/config/users-groups.nix

@@ -86,6 +86,12 @@ let
         ";
       };
 
+      isAlias = mkOption {
+        type = types.bool;
+        default = false;
+        description = "If true, the UID of this user is not required to be unique and can thus alias another user.";
+      };
+
     };
 
     config = {
@@ -122,12 +128,12 @@ let
 
   # Note: the 'X' in front of the password is to distinguish between
   # having an empty password, and not having a password.
-  serializedUser = userName: let u = getAttr userName config.users.extraUsers; in "${u.name}\n${u.description}\n${if u.uid != null then toString u.uid else ""}\n${u.group}\n${toString (concatStringsSep "," u.extraGroups)}\n${u.home}\n${u.shell}\n${toString u.createHome}\n${if u.password != null then "X" + u.password else ""}\n${toString u.isSystemUser}\n${if u.createUser then "yes" else "no"}\n";
+  serializedUser = u: "${u.name}\n${u.description}\n${if u.uid != null then toString u.uid else ""}\n${u.group}\n${toString (concatStringsSep "," u.extraGroups)}\n${u.home}\n${u.shell}\n${toString u.createHome}\n${if u.password != null then "X" + u.password else ""}\n${toString u.isSystemUser}\n${toString u.createUser}\n${toString u.isAlias}\n";
 
-  # keep this extra file so that cat can be used to pass special chars such as "`" which is used in the avahi daemon
   usersFile = pkgs.writeText "users" (
-    concatMapStrings serializedUser (attrNames config.users.extraUsers)
-  );
+    let
+      p = partition (u: u.isAlias) (attrValues config.users.extraUsers);
+    in concatStrings (map serializedUser p.wrong ++ map serializedUser p.right));
 
 in
 
@@ -243,8 +249,9 @@ in
             read password
             read isSystemUser
             read createUser
+            read isAlias
 
-            if ! test "$createUser" = "yes"; then
+            if [ -z "$createUser" ]; then
                 continue
             fi
 
@@ -257,6 +264,7 @@ in
                     --home "$home" \
                     --shell "$shell" \
                     ''${createHome:+--create-home} \
+                    ''${isAlias:+--non-unique} \
                     "$name"
                 if test "''${password:0:1}" = 'X'; then
                     (echo "''${password:1}"; echo "''${password:1}") | ${pkgs.shadow}/bin/passwd "$name"