examples: Add longpoll server and client

This is an example of a race-free long-poll server and client. It uses a
redirection method to signal that the "Watch" is running.

Other race-free methods exist.

examples/longpoll/redirect-server.go

@@ -0,0 +1,56 @@
+// This is an example longpoll server. On client connection it starts a "Watch",
+// and notifies the client with a redirect when that watch is ready. This is
+// important to avoid a possible race between when the client believes the watch
+// is actually ready, and when the server actually is watching.
+package main
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"math/rand"
+	"net/http"
+	"time"
+)
+
+// you can use `wget http://127.0.0.1:12345/hello -O /dev/null`
+// or `go run client.go`
+const (
+	addr = ":12345"
+)
+
+// WatchStart kicks off the initial watch and then redirects the client to
+// notify them that we're ready. The watch operation here is simulated.
+func WatchStart(w http.ResponseWriter, req *http.Request) {
+	log.Printf("Start received...")
+	time.Sleep(time.Duration(5) * time.Second) // 5 seconds to get ready and start *our* watch ;)
+	//started := time.Now().UnixNano() // time since watch is "started"
+	log.Printf("URL: %+v", req.URL)
+
+	token := fmt.Sprintf("%d", rand.Intn(2^32-1))
+	http.Redirect(w, req, fmt.Sprintf("/ready?token=%s", token), http.StatusSeeOther) // TODO: which code should we use ?
+	log.Printf("Redirect sent!")
+}
+
+// WatchReady receives the client connection when it has been notified that the
+// watch has started, and it returns to signal that an event on the watch
+// occurred. The event operation here is simulated.
+func WatchReady(w http.ResponseWriter, req *http.Request) {
+	log.Printf("Ready received")
+	log.Printf("URL: %+v", req.URL)
+
+	//time.Sleep(time.Duration(10) * time.Second)
+	time.Sleep(time.Duration(rand.Intn(10)) * time.Second) // wait until an "event" happens
+
+	io.WriteString(w, "Event happened!\n")
+	log.Printf("Event sent")
+}
+
+func main() {
+	log.Printf("Starting...")
+	//rand.Seed(time.Now().UTC().UnixNano())
+	http.HandleFunc("/watch", WatchStart)
+	http.HandleFunc("/ready", WatchReady)
+	log.Printf("Listening on %s", addr)
+	log.Fatal(http.ListenAndServe(addr, nil))
+}