feat: Add experimental traceresponse propagator to Rack instrumentation

instrumentation/rack/example/Gemfile

@@ -4,7 +4,8 @@ source 'https://rubygems.org'
 
 gem 'opentelemetry-api'
 gem 'opentelemetry-common'
-gem 'opentelemetry-instrumentation-rack'
+gem 'opentelemetry-instrumentation-rack', path: '../'
 gem 'opentelemetry-sdk'
+gem 'opentelemetry-sdk-experimental'
 gem 'rack'
 gem 'rack-test'

instrumentation/rack/example/trace_demonstration3.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+# Copyright The OpenTelemetry Authors
+#
+# SPDX-License-Identifier: Apache-2.0
+
+# This uses the response_propagators option. ResponseTextMapPropagator is from the opentelemetry-sdk-experimental gem
+require 'rubygems'
+require 'bundler/setup'
+
+Bundler.require
+
+ENV['OTEL_TRACES_EXPORTER'] = 'console'
+OpenTelemetry::SDK.configure do |c
+  c.use 'OpenTelemetry::Instrumentation::Rack', { response_propagators: [OpenTelemetry::Trace::Propagation::TraceContext::ResponseTextMapPropagator.new] }
+end
+
+# setup fake rack application:
+builder = Rack::Builder.app do
+  # integration should be automatic in web frameworks (like rails),
+  # but for a plain Rack application, enable it in your config.ru, e.g.,
+  use OpenTelemetry::Instrumentation::Rack::Middlewares::TracerMiddleware
+
+  app = ->(_env) { [200, { 'Content-Type' => 'text/plain' }, ['All responses are OK']] }
+  run app
+end
+
+# demonstrate tracing (span output to console):
+res = Rack::MockRequest.new(builder).get('/')
+puts res
+puts res.headers

instrumentation/rack/lib/opentelemetry/instrumentation/rack/instrumentation.rb

@@ -30,6 +30,7 @@ class Instrumentation < OpenTelemetry::Instrumentation::Base
         option :untraced_endpoints,       default: [],    validate: :array
         option :url_quantization,         default: nil,   validate: :callable
         option :untraced_requests,        default: nil,   validate: :callable
+        option :response_propagators,     default: [],    validate: :array
 
         private
 

instrumentation/rack/lib/opentelemetry/instrumentation/rack/middlewares/tracer_middleware.rb

@@ -82,6 +82,7 @@ def call(env) # rubocop:disable Metrics/MethodLength
                 OpenTelemetry::Instrumentation::Rack.with_span(request_span) do
                   @app.call(env).tap do |status, headers, response|
                     set_attributes_after_request(request_span, status, headers, response)
+                    config[:response_propagators].each { |propagator| propagator.inject(headers) }
                   end
                 end
               end

instrumentation/rack/opentelemetry-instrumentation-rack.gemspec

@@ -33,6 +33,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'bundler', '>= 1.17'
   spec.add_development_dependency 'minitest', '~> 5.0'
   spec.add_development_dependency 'opentelemetry-sdk', '~> 1.1'
+  spec.add_development_dependency 'opentelemetry-sdk-experimental', '~> 0.1'
   spec.add_development_dependency 'opentelemetry-test-helpers'
   spec.add_development_dependency 'rack', '~> 2.0.8'
   spec.add_development_dependency 'rack-test', '~> 1.1.0'

instrumentation/rack/test/opentelemetry/instrumentation/rack/middlewares/tracer_middleware_test.rb

@@ -311,6 +311,40 @@
     end
   end
 
+  describe 'config[:response_propagators]' do
+    describe 'with default options' do
+      it 'does not inject the traceresponse header' do
+        res = Rack::MockRequest.new(rack_builder).get('/ping', env)
+        _(res.headers).wont_include('traceresponse')
+      end
+    end
+
+    describe 'with ResponseTextMapPropagator' do
+      let(:config) { default_config.merge(response_propagators: [OpenTelemetry::Trace::Propagation::TraceContext::ResponseTextMapPropagator.new]) }
+
+      it 'injects the traceresponse header' do
+        res = Rack::MockRequest.new(rack_builder).get('/ping', env)
+        _(res.headers).must_include('traceresponse')
+      end
+    end
+
+    describe 'propagator throws' do
+      class MockPropagator < OpenTelemetry::Trace::Propagation::TraceContext::ResponseTextMapPropagator
+        def inject(carrier)
+          raise 'Injection failed'
+        end
+      end
+
+      let(:config) { default_config.merge(response_propagators: [MockPropagator.new]) }
+
+      it 'leads to application errors when there are exceptions' do
+        assert_raises RuntimeError do
+          Rack::MockRequest.new(rack_builder).get('/ping', env)
+        end
+      end
+    end
+  end
+
   describe '#call with error' do
     SimulatedError = Class.new(StandardError)
 

instrumentation/rack/test/test_helper.rb

@@ -7,6 +7,7 @@
 require 'rack'
 
 require 'opentelemetry/sdk'
+require 'opentelemetry-sdk-experimental'
 require 'opentelemetry-test-helpers'
 
 require 'pry'