Wrapper around mux to allow runtime route changes

We want to be able to update routes without restarting the http
server.  mutableRouter is a very thin wrapper around
github.com/gorilla/mux that enables safely switching to a new mux
router without pausing requests.

src/router/mutablemux.go

@@ -0,0 +1,54 @@
+/*
+Copyright 2016 The Fission Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package router
+
+import (
+	"net/http"
+	"sync/atomic"
+	"github.com/gorilla/mux"
+	"log"
+)
+
+// 
+// mutableRouter wraps the mux router, and allows the router to be
+// atomically changed.
+//
+
+type mutableRouter struct {
+	router atomic.Value // mux.Router
+}
+
+func NewMutableRouter(handler *mux.Router) (*mutableRouter) {
+	mr := mutableRouter{};
+	mr.router.Store(handler)
+	return &mr
+}
+
+func (mr *mutableRouter) ServeHTTP(responseWriter http.ResponseWriter, request *http.Request) {
+	// Atomically grab the underlying mux router and call it.
+	routerValue := mr.router.Load()
+	router, ok := routerValue.(*mux.Router)
+	if (!ok) {
+		log.Panic("Invalid router type")
+	}
+	router.ServeHTTP(responseWriter, request)
+}
+
+func (mr *mutableRouter) UpdateRouter(newHandler *mux.Router) {
+	log.Print("Updating router")
+	mr.router.Store(newHandler)
+}

src/router/mutablemux_test.go

@@ -0,0 +1,107 @@
+/*
+Copyright 2016 The Fission Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package router
+
+import (
+	"testing"
+	"net/http"
+	"github.com/gorilla/mux"
+	"log"
+	"io/ioutil"
+	"time"
+)
+
+func OldHandler(responseWriter http.ResponseWriter, request *http.Request) {
+	responseWriter.Write([]byte("old handler"))
+}
+func NewHandler(responseWriter http.ResponseWriter, request *http.Request) {
+	responseWriter.Write([]byte("new handler"))
+}
+
+func verifyRequest(expectedResponse string) {
+	resp, err := http.Get("http://localhost:3333")
+	if (err != nil) {
+		log.Panic("failed make get request")
+	}
+	defer resp.Body.Close()
+
+	body, err := ioutil.ReadAll(resp.Body)
+	if (err != nil) {
+		log.Panic("failed to read response")
+	}
+
+	bodyStr := string(body)
+	log.Printf("Server responded with %v", bodyStr)
+	if (bodyStr != expectedResponse) {
+		log.Panic("Unexpected response")
+	}
+}
+
+func startServer(mr *mutableRouter) {
+	http.ListenAndServe(":3333", mr)
+}
+
+func spamServer() {
+	i := 0
+	for {
+		i = i + 1
+		resp, err := http.Get("http://localhost:3333")
+		if (err != nil) {
+			log.Panicf("failed make get request %v", i)
+		}
+		resp.Body.Close()
+		log.Printf("request count = %v", i)
+	}
+}
+
+func TestMutableMux(t *testing.T) {
+	// make a simple mutable router
+	log.Print("Create mutable router")
+	muxRouter := mux.NewRouter()
+	muxRouter.HandleFunc("/", OldHandler)
+	mr := NewMutableRouter(muxRouter)
+	
+	// start http server
+	log.Print("Start http server")
+	go startServer(mr)
+	
+	// continuously make requests, panic if any fails
+	go spamServer()
+	go spamServer()
+	go spamServer()
+
+	time.Sleep(5 * time.Millisecond)
+
+	// connect and verify old handler
+	log.Print("Verify old handler")
+	verifyRequest("old handler")
+
+	// change the muxer
+	log.Print("Change mux router")
+	newMuxRouter := mux.NewRouter()
+	newMuxRouter.HandleFunc("/", NewHandler)
+	mr.UpdateRouter(newMuxRouter)
+
+	// connect and verify the new handler
+	log.Print("Verify new handler")
+	verifyRequest("new handler")
+	
+	time.Sleep(5 * time.Millisecond)
+
+	// all done
+	log.Print("ok")
+}