Fix MuxHandler initialization race (#1626)

Fix race condition that can occur when calling handlers returned from
Controller.MuxHandler where the Service's slice of middleware functions
is written to in a way that is not thread-safe.

The race will only occur when the capacity of the Service's middleware
slice is greater than its length and when the number of Controller
middlewares is more than 0. The functions returned by MuxHandler (that
are all from the same Controller object), if called in parallel, could
cause the race to occur.

Makefile

@@ -62,7 +62,7 @@ cyclo:
 	fi
 
 test:
-	@ginkgo -r --randomizeAllSpecs --failOnPending --randomizeSuites -skipPackage vendor
+	@ginkgo -r --randomizeAllSpecs --failOnPending --randomizeSuites -race -skipPackage vendor
 	go test ./_integration_tests
 
 goagen:

service.go

@@ -310,7 +310,8 @@ func (ctrl *Controller) MuxHandler(name string, hdlr Handler, unm Unmarshaler) M
 				}
 				return nil
 			}
-			chain := append(ctrl.Service.middleware, ctrl.middleware...)
+			mwLen := len(ctrl.Service.middleware)
+			chain := append(ctrl.Service.middleware[:mwLen:mwLen], ctrl.middleware...)
 			ml := len(chain)
 			for i := range chain {
 				handler = chain[ml-i-1](handler)

service_test.go

@@ -11,6 +11,8 @@ import (
 
 	"context"
 
+	"sync"
+
 	"github.com/goadesign/goa"
 	. "github.com/onsi/ginkgo"
 	. "github.com/onsi/gomega"
@@ -185,6 +187,48 @@ var _ = Describe("Service", func() {
 			Ω(muxHandler).ShouldNot(BeNil())
 		})
 
+		Context("with multiple instances and middlewares", func() {
+			var ctrl *goa.Controller
+			var handlers []goa.MuxHandler
+			var rws []*TestResponseWriter
+			var reqs []*http.Request
+			var p url.Values
+			var wg sync.WaitGroup
+
+			BeforeEach(func() {
+				nopHandler := func(context.Context, http.ResponseWriter, *http.Request) error {
+					return nil
+				}
+				ctrl = s.NewController("test")
+				for i := 0; i < 5; i++ {
+					ctrl.Service.Use(func(goa.Handler) goa.Handler {
+						return nopHandler
+					})
+				}
+				ctrl.Use(func(goa.Handler) goa.Handler { return nopHandler })
+				for i := 0; i < 10; i++ {
+					tmp := ctrl.MuxHandler("test", nopHandler, nil)
+					handlers = append(handlers, tmp)
+					rws = append(rws, &TestResponseWriter{})
+					req, _ := http.NewRequest("GET", "", nil)
+					reqs = append(reqs, req)
+				}
+				p = url.Values{}
+				wg = sync.WaitGroup{}
+				wg.Add(10)
+			})
+
+			It("doesn't race with parallel handler calls", func() {
+				for i := range handlers {
+					go func(j int) {
+						handlers[j](rws[j], reqs[j], p)
+						wg.Done()
+					}(i)
+				}
+				wg.Wait()
+			})
+		})
+
 		Context("with a request", func() {
 			var rw http.ResponseWriter
 			var r *http.Request