modules/common: add hjem.linker option

modules/common.nix

@@ -10,7 +10,7 @@
   inherit (lib.modules) mkIf mkDefault mkDerivedConfig;
   inherit (lib.options) mkOption mkEnableOption literalExpression;
   inherit (lib.strings) hasPrefix;
-  inherit (lib.types) bool submodule str path attrsOf nullOr lines listOf package;
+  inherit (lib.types) attrsOf bool lines listOf nullOr package path str submodule;
 
   clobberOpt = config.clobberFiles;
 

modules/nixos/default.nix

@@ -9,7 +9,7 @@
   inherit (lib.lists) filter map flatten concatLists;
   inherit (lib.attrsets) filterAttrs mapAttrs' attrValues mapAttrsToList;
   inherit (lib.trivial) flip;
-  inherit (lib.types) bool attrsOf submoduleWith listOf raw attrs;
+  inherit (lib.types) attrs attrsOf bool enum listOf raw submoduleWith;
 
   cfg = config.hjem;
   enabledUsers = filterAttrs (_: u: u.enable) cfg.users;
@@ -82,6 +82,27 @@ in {
         to be passed down to to all imported modules.
       '';
     };
+
+    linker = mkOption {
+      default =
+        if pkgs.stdenv.hostPlatform.isLinux
+        then "tmpfiles"
+        else "manifest";
+      description = ''
+        Method to use to link files.
+
+        `"tmpfiles"` will use `systemd-tmpfiles`, which is only supported on Linux.
+        This is the default file linker on Linux, as it is the more mature linker, but it has the downside of leaving
+        behind symlinks that may not get invalidated until the next GC, if an entry is removed from {option}`hjem.<user>.files`.
+
+        `"manifest"` will use Hjem's custom file linker based on an internally-generated manifest.
+        The custom file linker will use this manifest to create or remove links as needed, by comparing the
+        manifest of the currently activated system with that of the new system.
+        This prevents dangling symlinks when an entry is removed from {option}`hjem.<user>.files`.
+        This linker is currently experimental; once it matures, it may become the default in the future.
+      '';
+      type = enum ["manifest" "tmpfiles"];
+    };
   };
 
   config = mkMerge [
@@ -90,7 +111,9 @@ in {
         inherit name;
         value.packages = packages;
       }) (filterAttrs (_: u: u.packages != []) enabledUsers);
+    }
 
+    (mkIf (cfg.linker == "tmpfiles") {
       systemd.user.tmpfiles.users = mapAttrs' (name: {files, ...}: {
         inherit name;
         value.rules = map (
@@ -107,7 +130,11 @@ in {
             "${mode} '${file.target}' - - - - ${file.source}"
         ) (filter (f: f.enable && f.source != null) (attrValues files));
       }) (filterAttrs (_: u: u.files != {}) enabledUsers);
-    }
+    })
+
+    (mkIf (cfg.linker == "manifest") {
+
+    })
 
     (mkIf (enabledUsers != {}) {
       warnings = flatten (flip mapAttrsToList enabledUsers (user: config: