Tracing Net::HTTP in ruby

lib.rb

# Taken from Jon Anderson (FireHydrant) on Honeycomb slack
# https://honeycombpollinators.slack.com/archives/CJR134U2F/p1626469086124100

# frozen_string_literal: true

require "net/http"

module Net
  class HTTP
    alias_method :original_request, :request

    def request(req, body = nil, &block)
      return original_request(req, body, &block) if should_call_untraced_original?(req)

      Honeycomb.start_span(name: "http_client") do |span|
        _hc_annotate_request(req, span) rescue nil

        response = original_request(req, body, &block)

        _hc_annotate_response(response, span) rescue nil

        response
      end
    end

    private

    # This is brittle and I hate it but I can't think of a better way to not
    # double up faraday spans with net/http as the adapter.
    def should_call_untraced_original?(req)
      current_span = Honeycomb.current_span
      event = current_span && current_span.instance_variable_get(:@event)

      event && event.data["meta.package"] == "faraday" || invalid_host?(req)
    rescue
      true
    end

    # This is some weird magic that we may be able to remove once we no longer
    # send spans to Datadog. We were seeing Net::HTTP requests with no host
    # but still going somewhere, but only when running in combined tracer mode.
    def invalid_host?(req)
      host = req.respond_to?(:uri) ? req.uri&.host : @host

      host.to_s.empty?
    end

    def _hc_annotate_request(req, span)
      span.add_field("meta.package", "net/http")
      span.add_field("meta.type", "http_client")

      span.add_field("request.method", req.method.to_s.upcase)
      span.add_field("request.content_type", req["content-type"])
      span.add_field("request.accept", req["accept"])

      if req.respond_to?(:uri) && req.uri
        span.add_field("request.host", req.uri.host)
        span.add_field("request.port", req.uri.port.to_s)
        span.add_field("request.path", req.uri.path)
        span.add_field("request.scheme", req.uri.scheme)
      else
        span.add_field("request.host", @request)
        span.add_field("request.port", @port)
      end
    end

    def _hc_annotate_response(response, span)
      return if response.nil?

      span.add_field("response.status_code", response.code.to_i)
      span.add_field("response.content_type", response["content-type"])
    end
  end
end