Don't instrument response bodies twice

If multiple rack instrumentation middleware are present in the
middleware stack, don't instrument the response body multiple times.

Once it detects the response body is already a BodyWrapper subclass,
pass it through without change.

lib/appsignal/rack/abstract_middleware.rb

@@ -93,8 +93,14 @@ def instrument_app_call(env, transaction)
 
       def call_app(env, transaction)
         status, headers, obody = @app.call(env)
-        # Instrument response body and closing of the response body
-        [status, headers, Appsignal::Rack::BodyWrapper.wrap(obody, transaction)]
+        body =
+          if obody.is_a? Appsignal::Rack::BodyWrapper
+            obody
+          else
+            # Instrument response body and closing of the response body
+            Appsignal::Rack::BodyWrapper.wrap(obody, transaction)
+          end
+        [status, headers, body]
       end
 
       # Instrument the request fully. This is used by the top instrumentation

spec/lib/appsignal/rack/abstract_middleware_spec.rb

@@ -331,8 +331,10 @@ def filtered_params
       end
 
       context "with parent instrumentation" do
+        let(:transaction) { http_request_transaction }
         before do
-          env[Appsignal::Rack::APPSIGNAL_TRANSACTION] = http_request_transaction
+          env[Appsignal::Rack::APPSIGNAL_TRANSACTION] = transaction
+          set_current_transaction(transaction)
         end
 
         it "uses the existing transaction" do
@@ -341,6 +343,35 @@ def filtered_params
           expect { make_request }.to_not(change { created_transactions.count })
         end
 
+        it "wraps the response body in a BodyWrapper subclass" do
+          _status, _headers, body = make_request
+          expect(body).to be_kind_of(Appsignal::Rack::BodyWrapper)
+
+          body.to_ary
+          response_events =
+            last_transaction.to_h["events"].count do |event|
+              event["name"] == "process_response_body.rack"
+            end
+          expect(response_events).to eq(1)
+        end
+
+        context "when response body is already a BodyWrapper subclass" do
+          let(:body) { Appsignal::Rack::BodyWrapper.wrap(["hello!"], transaction) }
+          let(:app) { DummyApp.new { [200, {}, body] } }
+
+          it "doesn't wrap the body again" do
+            _status, _headers, body = make_request
+            expect(body).to eq(body)
+
+            body.to_ary
+            response_events =
+              last_transaction.to_h["events"].count do |event|
+                event["name"] == "process_response_body.rack"
+              end
+            expect(response_events).to eq(1)
+          end
+        end
+
         context "with error" do
           let(:app) { lambda { |_env| raise ExampleException, "error message" } }
 

spec/support/mocks/dummy_app.rb

@@ -8,7 +8,7 @@ def call(env)
     if @app
       @app&.call(env)
     else
-      [200, {}, "body"]
+      [200, {}, ["body"]]
     end
   ensure
     @called = true