Fix Rack http_server.queue spans missing from distributed traces

lib/ddtrace/contrib/rack/middlewares.rb

@@ -43,6 +43,13 @@ def call(env)
           # retrieve integration settings
           tracer = configuration[:tracer]
 
+          # Extract distributed tracing context before creating any spans,
+          # so that all spans will be added to the distributed trace.
+          if configuration[:distributed_tracing]
+            context = HTTPPropagator.extract(env)
+            tracer.provider.context = context if context.trace_id
+          end
+
           # [experimental] create a root Span to keep track of frontend web servers
           # (i.e. Apache, nginx) if the header is properly set
           frontend_span = compute_queue_time(env, tracer)
@@ -53,11 +60,6 @@ def call(env)
             span_type: Datadog::Ext::HTTP::TYPE
           }
 
-          if configuration[:distributed_tracing]
-            context = HTTPPropagator.extract(env)
-            tracer.provider.context = context if context.trace_id
-          end
-
           # start a new request span and attach it to the current Rack environment;
           # we must ensure that the span `resource` is set later
           request_span = tracer.trace(Ext::SPAN_REQUEST, trace_options)

spec/ddtrace/contrib/rack/distributed_spec.rb

@@ -88,6 +88,32 @@
       context 'with distributed tracing headers' do
         include_context 'distributed tracing headers'
         it_behaves_like 'a Rack request with distributed tracing'
+
+        context 'and request_queuing is enabled' do
+          let(:rack_options) { super().merge(request_queuing: true, web_service_name: web_service_name) }
+          let(:web_service_name) { 'frontend_web_server' }
+
+          let(:server_span) { spans.first }
+          let(:rack_span) { spans.last }
+
+          before(:each) do
+            header 'X-Request-Start', "t=#{Time.now.to_f}"
+          end
+
+          it 'contains a request_queuing span that belongs to the distributed trace' do
+            is_expected.to be_ok
+            expect(spans).to have(2).items
+
+            expect(server_span.name).to eq('http_server.queue')
+            expect(server_span.trace_id).to eq(trace_id)
+            expect(server_span.parent_id).to eq(parent_id)
+            expect(server_span.get_metric(Datadog::Ext::DistributedTracing::SAMPLING_PRIORITY_KEY)).to eq(sampling_priority)
+
+            expect(rack_span.name).to eq('rack.request')
+            expect(rack_span.trace_id).to eq(trace_id)
+            expect(rack_span.parent_id).to eq(server_span.span_id)
+          end
+        end
       end
 
       context 'without distributed tracing headers' do