pkgs.writeErofsLayers,profiles/ch-runner: MVE

$ nix-build -A examples.dummy.config.debug.closure.erofs.list | xargs
cat | xargs du -h
749M    /nix/store/bzfv5x6lycq6hzhjv6d6vlk1q8fdg9di-base0.erofs
24M     /nix/store/hp41jfq36y0mmjrzqilyh3jfsvqic3kb-nixos.erofs

$ nix run -f . examples.dummy.config.uvms.cloud-hypervisor.runner
...
<<< Welcome to NixOS 25.11pre-git (x86_64) - ttyS0 >>>

nixos login:

The definition of the `pkgs` fixpoint was moved to pkgs/default.nix.
For that, dirToAttrs was moved to lib/, imported ad hoc

profiles/debug-closure.nix

@@ -0,0 +1,85 @@
+{
+  config,
+  lib,
+  pkgs,
+  modulesPath,
+  ...
+}:
+
+let
+  cfg = config.debug.closure;
+  inherit (lib.types) package;
+  inherit (config.system.build) toplevel;
+
+  ps = pkgs.callPackage ../pkgs { };
+  inherit (ps) writeErofsLayers;
+  emptySystem = import (pkgs.path + "/nixos/lib/eval-config.nix") {
+    modules = [
+      (modulesPath + "/profiles/minimal.nix")
+      {
+        system.stateVersion = config.system.stateVersion;
+        fileSystems."/".fsType = "tmpfs";
+        boot.loader.grub.enable = false;
+        networking.hostName = "base";
+        networking.nftables.enable = true;
+        networking.useNetworkd = true;
+        systemd.network.enable = true;
+      }
+    ];
+  };
+in
+{
+  options.debug.closure = {
+    info = lib.mkOption {
+      type = package;
+      description = "closureInfo { rootPaths = [ toplevel ]; }";
+    };
+    write = lib.mkOption {
+      type = package;
+      description = "writeClosure [ toplevel ]";
+    };
+    extraLayers = lib.mkOption {
+      type = lib.types.listOf package;
+      description = "Roots from which to generate `erofs.layers`";
+      defaultText = ''
+        [
+          (nixosSystem { modules = [ ]; }).config.system.build.toplevel
+          # implicitly appended: config.system.build.toplevel
+        ]
+      '';
+    };
+    erofs.layers = lib.mkOption {
+      type = lib.types.listOf package;
+      description = "writeErofs [ ... ]";
+      readOnly = true;
+    };
+    erofs.list = lib.mkOption {
+      type = package;
+      description = "writeClosure [ (writeErofs [ ... ])... ]";
+    };
+  };
+  config.debug.closure = {
+    info = pkgs.closureInfo { rootPaths = [ toplevel ]; };
+    write = pkgs.writeClosure [ toplevel ];
+
+    erofs.list =
+      pkgs.runCommand "layers"
+        {
+          __structuredAttrs = true;
+          inherit (cfg.erofs) layers;
+        }
+        ''
+          printf "%s\n" "''${layers[@]}" > "$out"
+        '';
+    extraLayers = lib.mkMerge [
+      (lib.mkBefore [ emptySystem.config.system.build.toplevel ])
+      # (lib.mkAfter [config.system.build.toplevel])
+    ];
+    erofs.layers = writeErofsLayers {
+      layers = cfg.extraLayers ++ [
+        toplevel
+      ];
+      labelUpper = config.networking.hostName;
+    };
+  };
+}