module NulogyMessageBusProducer
  module Subscriptions
    # This is a custom AST::Analyzer. It attempts to prevent:
    #   1. Expansions that could take too long
    #   2. Expansions that could be difficult to test
    #
    # We resolved that the causes of (1) would mostly be unbound lists
    # and (2) could be arguments passed to nodes.
    #
    # Thus, as a blunt first step, this analyzer prevents queries from
    #   1. expanding list fields
    #   2. using arguments on nodes
    #
    # While this is fairly strict, it did not affect any existing queries at the time.
    # These could be relaxed in the future but require more-sophisticated techniques.
    # For example, a bounded lists of elements may be required.
    class RiskySubscriptionBlocker < GraphQL::Analysis::AST::Analyzer
      def initialize(query_or_multiplex)
        super
        @nodes_using_arguments = Set.new
        @nodes_are_lists = Set.new
      end

      PERMITTED_ARGUMENTS = Set.new(%w[subscriptionId subscriptionGroupId topicName])

      def analyze?
        subject.subscription?
      end

      def on_enter_argument(argument, parent, _visitor)
        @nodes_using_arguments << parent unless PERMITTED_ARGUMENTS.include?(argument.name)
      end

      def on_enter_field(node, _parent, visitor)
        @nodes_are_lists << node if list?(node, visitor)
      end

      def result
        error_messages = []

        if @nodes_using_arguments.any?
          node_names = @nodes_using_arguments.map(&:name).join("\n")
          error_messages << "Arguments may not be used:\n#{node_names}"
        end

        GraphQL::AnalysisError.new(error_messages.join("\n\n")) if error_messages.any?
      end

      private

      def list?(node, visitor)
        field_definition = visitor.field_definition
        nested_type = field_definition.type

        loop do
          @nodes_are_lists << node if nested_type.is_a?(GraphQL::Schema::List)

          break unless nested_type.respond_to?(:of_type)

          nested_type = nested_type.of_type
        end
      end
    end
  end
end