nixos/switchable-system: improve switch-inhibitor pre-check (#510514)

      preSwitchChecks.switchInhibitors =

        let

          realpath = lib.getExe' pkgs.coreutils "realpath";

          mktemp = lib.getExe' pkgs.coreutils "mktemp";

          rm = lib.getExe' pkgs.coreutils "rm";

          jq = lib.getExe' pkgs.jq "jq";

          jq = lib.getExe pkgs.jq;

          empty = pkgs.writeText "empty-inhibitors" "{}";

        in

        # bash

        ''

          incoming="''${1-}"

          action="''${2-}"

          if [ "$action" == "boot" ]; then

          case "$action" in

            boot|dry-activate)

              echo "Not checking switch inhibitors (action = $action)"

              exit

          fi

              ;;

          esac

          echo -n "Checking switch inhibitors..."

          # Create a temporary file that we use in case a generation does not have

          # the switch-inhibitors file.

          empty="$(${mktemp} -t switch_inhibit.XXXX)"

          # shellcheck disable=SC2329

          clean_up() {

            ${rm} -f "$empty"

          }

          trap clean_up EXIT

          echo "{}" > "$empty"

          current_inhibitors="$(${realpath} /run/current-system)/switch-inhibitors"

          current_inhibitors="/run/current-system/switch-inhibitors"

          if [ ! -f "$current_inhibitors" ]; then

            current_inhibitors="$empty"

            current_inhibitors="${empty}"

          fi

          new_inhibitors="$(${realpath} "$incoming")/switch-inhibitors"

          new_inhibitors="$incoming/switch-inhibitors"

          if [ ! -f "$new_inhibitors" ]; then

            new_inhibitors="$empty"

            new_inhibitors="${empty}"

          fi

          diff="$(

              --rawfile current "$current_inhibitors" \

              --rawfile newgen "$new_inhibitors" \

            '

              $current | try fromjson catch {} as $old |

              $newgen | try fromjson catch {} as $new |

              ($current | fromjson) as $old |

              ($newgen | fromjson) as $new |

              $old |

              to_entries |

              map(

          assert_contains(out, "baz")

          # Confirm that we can set that same generation as the new boot default

          switch_to_specialisation("${machine}", "inhibitors_changed", action="boot")

          # Confirm that dry-activate is not blocked by inhibitors

          out = switch_to_specialisation("${machine}", "inhibitors_changed", action="dry-activate")

          assert_contains(out, "Not checking switch inhibitors")

          # Check that we can switch into a new generation with new inhibitors, but same values for existing ones

          switch_to_specialisation("${machine}", "inhibitors_new", action="switch")

          # Check that we can switch back into a generation without inhibitors