# frozen_string_literal: true

module Gitlab
  module Redis
    class MultiStore
      include Gitlab::Utils::StrongMemoize

      class ReadFromPrimaryError < StandardError
        def message
          'Value not found on the redis primary store. Read from the redis secondary store successful.'
        end
      end
      class MethodMissingError < StandardError
        def message
          'Method missing. Falling back to execute method on the redis secondary store.'
        end
      end

      attr_reader :primary_store, :secondary_store, :instance_name

      FAILED_TO_READ_ERROR_MESSAGE = 'Failed to read from the redis primary_store.'
      FAILED_TO_WRITE_ERROR_MESSAGE = 'Failed to write to the redis primary_store.'

      READ_COMMANDS = %i(
        get
        mget
        smembers
        scard
      ).freeze

      WRITE_COMMANDS = %i(
        set
        setnx
        setex
        sadd
        srem
        del
        pipelined
        flushdb
      ).freeze

      def initialize(primary_store, secondary_store, instance_name = nil)
        @primary_store = primary_store
        @secondary_store = secondary_store
        @instance_name = instance_name

        validate_stores!
      end

      READ_COMMANDS.each do |name|
        define_method(name) do |*args, &block|
          if multi_store_enabled?
            read_command(name, *args, &block)
          else
            secondary_store.send(name, *args, &block) # rubocop:disable GitlabSecurity/PublicSend
          end
        end
      end

      WRITE_COMMANDS.each do |name|
        define_method(name) do |*args, &block|
          if multi_store_enabled?
            write_command(name, *args, &block)
          else
            secondary_store.send(name, *args, &block) # rubocop:disable GitlabSecurity/PublicSend
          end
        end
      end

      def method_missing(...)
        return @instance.send(...) if @instance # rubocop:disable GitlabSecurity/PublicSend

        log_method_missing(...)

        secondary_store.send(...) # rubocop:disable GitlabSecurity/PublicSend
      end

      def respond_to_missing?(command_name, include_private = false)
        true
      end

      # This is needed because of Redis::Rack::Connection is requiring Redis::Store
      # https://github.com/redis-store/redis-rack/blob/a833086ba494083b6a384a1a4e58b36573a9165d/lib/redis/rack/connection.rb#L15
      # Done similarly in https://github.com/lsegal/yard/blob/main/lib/yard/templates/template.rb#L122
      def is_a?(klass)
        return true if klass == secondary_store.class

        super(klass)
      end
      alias_method :kind_of?, :is_a?

      def to_s
        if multi_store_enabled?
          primary_store.to_s
        else
          secondary_store.to_s
        end
      end

      private

      def log_method_missing(command_name, *_args)
        log_error(MethodMissingError.new, command_name)
        increment_method_missing_count(command_name)
      end

      def read_command(command_name, *args, &block)
        if @instance
          send_command(@instance, command_name, *args, &block)
        else
          read_one_with_fallback(command_name, *args, &block)
        end
      end

      def write_command(command_name, *args, &block)
        if @instance
          send_command(@instance, command_name, *args, &block)
        else
          write_both(command_name, *args, &block)
        end
      end

      def read_one_with_fallback(command_name, *args, &block)
        begin
          value = send_command(primary_store, command_name, *args, &block)
        rescue StandardError => e
          log_error(e, command_name,
            multi_store_error_message: FAILED_TO_READ_ERROR_MESSAGE)
        end

        value ||= fallback_read(command_name, *args, &block)

        value
      end

      def fallback_read(command_name, *args, &block)
        value = send_command(secondary_store, command_name, *args, &block)

        if value
          log_error(ReadFromPrimaryError.new, command_name)
          increment_read_fallback_count(command_name)
        end

        value
      end

      def write_both(command_name, *args, &block)
        begin
          send_command(primary_store, command_name, *args, &block)
        rescue StandardError => e
          log_error(e, command_name,
            multi_store_error_message: FAILED_TO_WRITE_ERROR_MESSAGE)
        end

        send_command(secondary_store, command_name, *args, &block)
      end

      def multi_store_enabled?
        Feature.enabled?(:use_multi_store, default_enabled: :yaml) && !same_redis_store?
      end

      def same_redis_store?
        strong_memoize(:same_redis_store) do
          # <Redis client v4.4.0 for redis:///path_to/redis/redis.socket/5>"
          primary_store.inspect == secondary_store.inspect
        end
      end

      # rubocop:disable GitlabSecurity/PublicSend
      def send_command(redis_instance, command_name, *args, &block)
        if block_given?
          # Make sure that block is wrapped and executed only on the redis instance that is executing the block
          redis_instance.send(command_name, *args) do |*params|
            with_instance(redis_instance, *params, &block)
          end
        else
          redis_instance.send(command_name, *args)
        end
      end
      # rubocop:enable GitlabSecurity/PublicSend

      def with_instance(instance, *params)
        @instance = instance

        yield(*params)
      ensure
        @instance = nil
      end

      def increment_read_fallback_count(command_name)
        @read_fallback_counter ||= Gitlab::Metrics.counter(:gitlab_redis_multi_store_read_fallback_total, 'Client side Redis MultiStore reading fallback')
        @read_fallback_counter.increment(command: command_name, instance_name: instance_name)
      end

      def increment_method_missing_count(command_name)
        @method_missing_counter ||= Gitlab::Metrics.counter(:gitlab_redis_multi_store_method_missing_total, 'Client side Redis MultiStore method missing')
        @method_missing_counter.increment(command: command_name, innamece_name: instance_name)
      end

      def validate_stores!
        raise ArgumentError, 'primary_store is required' unless primary_store
        raise ArgumentError, 'secondary_store is required' unless secondary_store
        raise ArgumentError, 'invalid primary_store' unless primary_store.is_a?(::Redis)
        raise ArgumentError, 'invalid secondary_store' unless secondary_store.is_a?(::Redis)
      end

      def log_error(exception, command_name, extra = {})
        Gitlab::ErrorTracking.log_exception(
          exception,
          command_name: command_name,
          extra: extra.merge(instance_name: instance_name))
      end
    end
  end
end