{ lib, ... }:
{

  name = "activation-etc-overlay-mutable";

  meta.maintainers = with lib.maintainers; [ nikstur ];

  nodes.machine =
    { pkgs, ... }:
    {
      system.etc.overlay.enable = true;
      system.etc.overlay.mutable = true;

      # Prerequisites
      boot.initrd.systemd.enable = true;
      boot.kernelPackages = pkgs.linuxPackages_latest;

      specialisation.new-generation.configuration = {
        environment.etc."newgen".text = "newgen";
      };
      specialisation.newer-generation.configuration = {
        environment.etc."newergen".text = "newergen";
      };
    };

  testScript = # python
    ''
      newergen = machine.succeed("realpath /run/current-system/specialisation/newer-generation/bin/switch-to-configuration").rstrip()

      with subtest("/run/etc-metadata/ is mounted"):
        print(machine.succeed("mountpoint /run/etc-metadata"))

      with subtest("No temporary files leaked into stage 2"):
        machine.succeed("[ ! -e /etc-metadata-image ]")
        machine.succeed("[ ! -e /etc-basedir ]")

      with subtest("/etc is mounted as an overlay"):
        machine.succeed("findmnt --kernel --type overlay /etc")

      with subtest("switching to the same generation"):
        machine.succeed("/run/current-system/bin/switch-to-configuration test")

      with subtest("the initrd didn't get rebuilt"):
        machine.succeed("test /run/current-system/initrd -ef /run/current-system/specialisation/new-generation/initrd")

      with subtest("switching to a new generation"):
        machine.fail("stat /etc/newgen")
        machine.succeed("echo -n 'mutable' > /etc/mutable")

        # Directory
        machine.succeed("mkdir /etc/mountpoint")
        machine.succeed("mount -t tmpfs tmpfs /etc/mountpoint")
        machine.succeed("touch /etc/mountpoint/extra-file")

        # File
        machine.succeed("touch /etc/filemount")
        machine.succeed("mount --bind /dev/null /etc/filemount")

        machine.succeed("/run/current-system/specialisation/new-generation/bin/switch-to-configuration switch")

        assert machine.succeed("cat /etc/newgen") == "newgen"
        assert machine.succeed("cat /etc/mutable") == "mutable"

        print(machine.succeed("findmnt /etc/mountpoint"))
        print(machine.succeed("stat /etc/mountpoint/extra-file"))
        print(machine.succeed("findmnt /etc/filemount"))

        machine.succeed(f"{newergen} switch")
        assert machine.succeed("cat /etc/newergen") == "newergen"

        tmpMounts = machine.succeed("find /tmp -maxdepth 1 -type d -regex '/tmp/nixos-etc\\..*' | wc -l").rstrip()
        metaMounts = machine.succeed("find /tmp -maxdepth 1 -type d -regex '/tmp/nixos-etc-metadata\\..*' | wc -l").rstrip()

        assert tmpMounts == "0", f"Found {tmpMounts} remaining tmpmounts"
        assert metaMounts == "1", f"Found {metaMounts} remaining metamounts"
    '';
}