flake:
{ pkgs
, name ? "sandbox"
, user ? "dummy"
, config ? { }
, tools ? []
, envVars ? { }
, restrictNetwork ? true  # to be replaced with virtualisation.restrictNetwork
, patchQemu9p ? false  # until qemu 7.2.0 becomes available in nixpkgs
}@params:

let
  shellLib = flake.lib.shell { inherit pkgs; };

  print = rec {
    printSharedDir = name: { source, target }: ''
      echo -en ${shellLib.fmt.keyword target}": "
      echo ${pkgs.lib.escapeShellArg source}
    '';
    printSharedDirs = sharedDirs: ''
      ${shellLib.fmt.printSectionTitle "SHARED DIRECTORIES (guest: host)"}
      ${shellLib.mapAttrsToLines printSharedDir sharedDirs}
    '';

    formatAddrPort = { address, port }: "${address}:${toString port}";
    printForwardedPort = { from, proto, host, guest, }:
      if from == "host" then ''
        echo -n ${proto}' '
        echo -en ${shellLib.fmt.keyword (formatAddrPort host)}
        echo -n ' -> '
        echo ${formatAddrPort guest}
      '' else ''
        echo -n ${proto}' '
        echo -n ${formatAddrPort host}
        echo -n ' <- '
        echo -e ${shellLib.fmt.keyword (formatAddrPort guest)}
      '';
    printForwardedPorts = portForwards: ''
      ${shellLib.fmt.printSectionTitle "FORWARDED PORTS (host <-> guest)"}
      ${pkgs.lib.concatMapStringsSep "\n" printForwardedPort portForwards}
    '';

    printNetworkRestricted = isRestricted:
      if isRestricted then "Restricted" else "Unrestricted";
    printNetworkAllowed = isRestricted:
      if isRestricted then "disallowed" else "allowed";
    printRestrictedNetwork = isRestricted: ''
      ${shellLib.fmt.printSectionTitle "NETWORK ACCESS"}
      echo -en ${shellLib.fmt.keyword (printNetworkRestricted isRestricted)}
      echo -n ': local network and internet access '
      echo ${printNetworkAllowed isRestricted}.
    '';
  };

in rec {

  nixosConfigurations.${name} = pkgs.nixos
  ({ modulesPath, config, lib, pkgs, ... }: {
    imports = [
      (modulesPath + "/profiles/minimal.nix")
      { environment.noXlibs = false; }  # avoid mass rebuild

      (modulesPath + "/profiles/qemu-guest.nix")
      (modulesPath + "/virtualisation/qemu-vm.nix")

      params.config
    ];

    system.stateVersion = lib.mkDefault lib.trivial.release;

    networking = {
      hostName = name;
      firewall.enable = lib.mkDefault false;
    };

    users.users.${user} = {
      isNormalUser = lib.mkDefault true;
      password = lib.mkDefault "";
      extraGroups = lib.mkDefault [ "wheel" ];
    };

    security.sudo.wheelNeedsPassword = lib.mkDefault false;

    services.getty = {
      autologinUser = lib.mkDefault user;
      helpLine = lib.mkDefault ''
        Press <CTRL-a> <x> to terminate the virtual machine.
      '';
    };

    environment = {
      variables = envVars;
      systemPackages = tools;

      interactiveShellInit = lib.mkBefore ''
        ${shellLib.ifSomeAttrs envVars shellLib.printEnvVars}
        ${shellLib.ifSomeList tools shellLib.printBins}
        ${shellLib.ifSomeAttrs config.virtualisation.sharedDirectories
          print.printSharedDirs}
        ${shellLib.ifSomeList config.virtualisation.forwardPorts
          print.printForwardedPorts}
        ${print.printRestrictedNetwork restrictNetwork}
      '';
    };

    virtualisation = {
      graphics = lib.mkDefault false;
      diskImage = lib.mkDefault "$TMP_DISK";

      sharedDirectories.host = {
        source = "$SHARED_CWD";
        target = "/mnt";
      };

      # Uncomment when this is merged:
      # https://github.com/NixOS/nixpkgs/pull/200225
      #restrictNetwork = lib.mkDefault true;

      # Patched QEMU to fix slow 9p file share.
      # https://linus.schreibt.jetzt/posts/qemu-9p-performance.html
      qemu.package = lib.mkDefault (
        if patchQemu9p then
          assert !(pkgs.lib.versionAtLeast pkgs.qemu_kvm.version "7.2.0");
          pkgs.qemu_kvm.overrideAttrs (o: {
            patches = o.patches ++ [ (pkgs.fetchpatch {
              name = "qemu-9p-performance-fix.patch";
              url = "https://github.com/qemu/qemu/commit/f5265c8.patch";
              sha256 = "sha256-PSOv0dhiEq9g6B1uIbs6vbhGr7BQWCtAoLHnk4vnvVg=";
            }) ];
          })
        else pkgs.qemu_kvm
      );

    };
  });

  packages.${name} = nixosConfigurations.${name}.config.system.build.vm;

  apps.${name} = {
    type = "app";
    program = toString (pkgs.writeShellScript "sandbox-vm" (
    (pkgs.lib.optionalString restrictNetwork ''
      # Isolate from network
      # Stopgap solution until this is merged:
      # https://github.com/NixOS/nixpkgs/pull/200225
      QEMU_NET_OPTS="restrict=yes,''${QEMU_NET_OPTS:+,$QEMU_NET_OPTS}"
      export QEMU_NET_OPTS
    '') + ''
      # Save current directory for mounting in VM
      SHARED_CWD=$PWD
      export SHARED_CWD

      TMP_DISK="$(mktemp).qcow2"
      export TMP_DISK
      trap "rm -f \"$TMP_DISK\"" EXIT

      ${packages.${name}}/bin/run-${name}-vm

      reset
      echo "Exited virtual machine."
    ''));
  };

}