nixos/etc: support direct symlinks with etc overlay

nixos/modules/system/etc/build-composefs-dump.py

···
                 paths[glob_target] = composefs_path
                 add_leading_directories(glob_target, attrs, paths)
         else:  # Without globbing
-            if mode == "symlink":
+            if mode == "symlink" or mode == "direct-symlink":
                 composefs_path = ComposefsPath(
                     attrs,
                     # A high approximation of the size of a symlink
···
                     mode="0777",
                     payload=source,
                 )
+            elif os.path.isdir(source):
+                composefs_path = ComposefsPath(
+                    attrs,
+                    size=4096,
+                    filetype=FileType.directory,
+                    mode=mode,
+                    payload=source,
+                )
             else:
-                if os.path.isdir(source):
-                    composefs_path = ComposefsPath(
-                        attrs,
-                        size=4096,
-                        filetype=FileType.directory,
-                        mode=mode,
-                        payload=source,
-                    )
-                else:
-                    composefs_path = ComposefsPath(
-                        attrs,
-                        size=os.stat(source).st_size,
-                        filetype=FileType.file,
-                        mode=mode,
-                        # payload needs to be relative path in this case
-                        payload=target.lstrip("/"),
-                    )
+                composefs_path = ComposefsPath(
+                    attrs,
+                    size=os.stat(source).st_size,
+                    filetype=FileType.file,
+                    mode=mode,
+                    # payload needs to be relative path in this case
+                    payload=target.lstrip("/"),
+                )
             paths[target] = composefs_path
             add_leading_directories(target, attrs, paths)
 

nixos/modules/system/etc/etc.nix

···
     ]) etc'}
   '';
 
-  etcHardlinks = filter (f: f.mode != "symlink") etc';
+  etcHardlinks = filter (f: f.mode != "symlink" && f.mode != "direct-symlink") etc';
 
   build-composefs-dump = pkgs.runCommand "build-composefs-dump.py"
     {

nixos/tests/activation/etc-overlay-immutable.nix

···
     users.mutableUsers = false;
     boot.initrd.systemd.enable = true;
     boot.kernelPackages = pkgs.linuxPackages_latest;
+    time.timeZone = "Utc";
 
     specialisation.new-generation.configuration = {
       environment.etc."newgen".text = "newgen";
···
   testScript = ''
     with subtest("/etc is mounted as an overlay"):
       machine.succeed("findmnt --kernel --type overlay /etc")
+
+    with subtest("direct symlinks point to the target without indirection"):
+      assert machine.succeed("readlink -n /etc/localtime") == "/etc/zoneinfo/Utc"
 
     with subtest("switching to the same generation"):
       machine.succeed("/run/current-system/bin/switch-to-configuration test")