require 'thread'
require 'ddtrace/ext/cache'

module Datadog
  module Contrib
    module Rails
      # Code used to create and handle 'rails.cache' spans.
      module ActiveSupport
        def self.instrument
          # patch Rails core components
          Datadog::RailsCachePatcher.patch_cache_store()

          # subscribe when a cache read starts being processed
          ::ActiveSupport::Notifications.subscribe('!datadog.start_cache_tracing.active_support') do |*args|
            start_trace_cache(*args)
          end

          # subscribe when a cache read has been processed
          ::ActiveSupport::Notifications.subscribe('!datadog.finish_cache_tracing.active_support') do |*args|
            finish_trace_cache(*args)
          end
        end

        def self.start_trace_cache(_name, _start, _finish, _id, payload)
          tracer = ::Rails.configuration.datadog_trace.fetch(:tracer)
          tracing_context = payload.fetch(:tracing_context)

          # In most of the cases Rails ``fetch()`` and ``read()`` calls are nested.
          # This check ensures that two reads are not nested since they don't provide
          # interesting details.
          # NOTE: the ``finish_trace_cache()`` is fired but it already has a safe-guard
          # to avoid any kind of issue.
          current_span = tracer.active_span
          return if current_span.try('name') == 'rails.cache' &&
                    current_span.try('resource') == 'GET' &&
                    payload[:action] == 'GET'

          # create a new ``Span`` and add it to the tracing context
          service = ::Rails.configuration.datadog_trace.fetch(:default_cache_service)
          type = Datadog::Ext::CACHE::TYPE
          span = tracer.trace('rails.cache', service: service, span_type: type)
          span.resource = payload.fetch(:action)
          tracing_context[:dd_cache_span] = span
        rescue StandardError => e
          Datadog::Tracer.log.debug(e.message)
        end

        def self.finish_trace_cache(_name, _start, _finish, _id, payload)
          # retrieve the tracing context and continue the trace
          tracing_context = payload.fetch(:tracing_context)
          span = tracing_context[:dd_cache_span]
          return unless span && !span.finished?

          begin
            # discard parameters from the cache_store configuration
            store, = *Array.wrap(::Rails.configuration.cache_store).flatten
            span.set_tag('rails.cache.backend', store)
            span.set_tag('rails.cache.key', payload.fetch(:key))
            span.set_error(payload[:exception]) if payload[:exception]
          ensure
            span.finish()
          end
        rescue StandardError => e
          Datadog::Tracer.log.debug(e.message)
        end
      end
    end
  end
end