nixos/auto-upgrade: introduce the concept of desync, for often-failing packages

lib/types.nix

@@ -1082,6 +1082,15 @@ let
           nestedTypes.elemType = elemType;
         };
 
+      extensionFunction = lib.mkOptionType {
+        name = "extensionFunction";
+        description = "extension function";
+        check = lib.isFunction;
+        # A more capable function is possible, see
+        # https://github.com/NixOS/nixpkgs/issues/215486
+        merge = lib.mergeOneOption;
+      };
+
       # A submodule (like typed attribute set). See NixOS manual.
       submodule =
         modules:

nixos/doc/manual/development/option-types.section.md

@@ -139,6 +139,11 @@ merging is handled.
 
 :   A type for the top level Nixpkgs package set.
 
+`types.extensionFunction`
+
+:   An extension function (e.g. `final: prev: { }`). Typically used to overlay
+    the Nixpkgs package set or other "extensible" attrsets.
+
 ### Numeric types {#sec-option-types-numeric}
 
 `types.int`

nixos/doc/manual/release-notes/rl-2511.section.md

@@ -4,6 +4,8 @@
 
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
+- A `desync` option has been added to the `system.autoUpgrade` module, which allows upgrading the system's channels even when not all installed packages can be built. This works by continuing to track the problematic channel's previous revision, which is used for packages that are broken on the newer revision. Having multiple revisions of a channel can potentially lead to other issues, however many of these can be avoided by using declarative containers. See [`system.autoUpgrade.desync`](options.html#opt-system.autoUpgrade.desync) for more details.
+
 - Secure boot support can now be enabled for the Limine bootloader through {option}`boot.loader.limine.secureBoot.enable`. Bootloader install script signs the bootloader, then kernels are hashed during system rebuild and written to a config. This allows Limine to boot only the kernels installed through NixOS system.
 
 ## New Modules {#sec-release-25.11-new-modules}

nixos/modules/misc/nixpkgs.nix

@@ -44,13 +44,6 @@ let
     merge = args: lib.foldr (def: mergeConfig def.value) { };
   };
 
-  overlayType = lib.mkOptionType {
-    name = "nixpkgs-overlay";
-    description = "nixpkgs overlay";
-    check = lib.isFunction;
-    merge = lib.mergeOneOption;
-  };
-
   pkgsType = lib.types.pkgs // {
     # This type is only used by itself, so let's elaborate the description a bit
     # for the purpose of documentation.
@@ -183,7 +176,7 @@ in
           })
         ]
       '';
-      type = lib.types.listOf overlayType;
+      type = lib.types.listOf lib.types.extensionFunction;
       description = ''
         List of overlays to apply to Nixpkgs.
         This option allows modifying the Nixpkgs package set accessed through the `pkgs` module argument.