Correct way to save memory using write buffer pool and freeing net.http default buffers (#761)

**Summary of Changes**

1. Add an example that uses the write buffer pool

The loop process of the websocket connection is inner the http handler
at existing examples， This usage will cause the 8k buffer(4k read buffer
+ 4k write buffer) allocated by net.http can't be GC(Observed by heap
profiling, see picture below) . The purpose of saving memory is not
achieved even if the WriteBufferPool is used.

In example bufferpool, server process websocket connection in a new
goroutine, and the goroutine created by the net.http will exit, then the
8k buffer will be GC.


![heap](https://user-images.githubusercontent.com/12793501/148676918-872d1a6d-ce10-4146-ba01-7de114db09f5.png)

Co-authored-by: hakunaliu <hakunaliu@tencent.com>
Co-authored-by: Corey Daley <cdaley@redhat.com>

README.md

@@ -14,6 +14,7 @@ Gorilla WebSocket is a [Go](http://golang.org/) implementation of the
 * [Command example](https://github.com/gorilla/websocket/tree/master/examples/command)
 * [Client and server example](https://github.com/gorilla/websocket/tree/master/examples/echo)
 * [File watch example](https://github.com/gorilla/websocket/tree/master/examples/filewatch)
+* [Write buffer pool example](https://github.com/gorilla/websocket/tree/master/examples/bufferpool)
 
 ### Status
 

examples/bufferpool/client.go

@@ -0,0 +1,89 @@
+//go:build ignore
+// +build ignore
+
+package main
+
+import (
+	"flag"
+	"log"
+	"net/url"
+	"os"
+	"os/signal"
+	"sync"
+	"time"
+
+	"github.com/gorilla/websocket"
+)
+
+var addr = flag.String("addr", "localhost:8080", "http service address")
+
+func runNewConn(wg *sync.WaitGroup) {
+	defer wg.Done()
+
+	interrupt := make(chan os.Signal, 1)
+	signal.Notify(interrupt, os.Interrupt)
+
+	u := url.URL{Scheme: "ws", Host: *addr, Path: "/ws"}
+	log.Printf("connecting to %s", u.String())
+	c, _, err := websocket.DefaultDialer.Dial(u.String(), nil)
+	if err != nil {
+		log.Fatal("dial:", err)
+	}
+	defer c.Close()
+
+	done := make(chan struct{})
+
+	go func() {
+		defer close(done)
+		for {
+			_, message, err := c.ReadMessage()
+			if err != nil {
+				log.Println("read:", err)
+				return
+			}
+			log.Printf("recv: %s", message)
+		}
+	}()
+
+	ticker := time.NewTicker(time.Minute * 5)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-done:
+			return
+		case t := <-ticker.C:
+			err := c.WriteMessage(websocket.TextMessage, []byte(t.String()))
+			if err != nil {
+				log.Println("write:", err)
+				return
+			}
+		case <-interrupt:
+			log.Println("interrupt")
+
+			// Cleanly close the connection by sending a close message and then
+			// waiting (with timeout) for the server to close the connection.
+			err := c.WriteMessage(websocket.CloseMessage, websocket.FormatCloseMessage(websocket.CloseNormalClosure, ""))
+			if err != nil {
+				log.Println("write close:", err)
+				return
+			}
+			select {
+			case <-done:
+			case <-time.After(time.Second):
+			}
+			return
+		}
+	}
+}
+
+func main() {
+	flag.Parse()
+	log.SetFlags(0)
+	wg := &sync.WaitGroup{}
+	for i := 0; i < 1000; i++ {
+		wg.Add(1)
+		go runNewConn(wg)
+	}
+	wg.Wait()
+}

examples/bufferpool/server.go

@@ -0,0 +1,55 @@
+//go:build ignore
+// +build ignore
+
+package main
+
+import (
+	"flag"
+	"log"
+	"net/http"
+	"sync"
+
+	_ "net/http/pprof"
+
+	"github.com/gorilla/websocket"
+)
+
+var addr = flag.String("addr", "localhost:8080", "http service address")
+
+var upgrader = websocket.Upgrader{
+	ReadBufferSize:  256,
+	WriteBufferSize: 256,
+	WriteBufferPool: &sync.Pool{},
+}
+
+func process(c *websocket.Conn) {
+	defer c.Close()
+	for {
+		_, message, err := c.ReadMessage()
+		if err != nil {
+			log.Println("read:", err)
+			break
+		}
+		log.Printf("recv: %s", message)
+	}
+}
+
+func handler(w http.ResponseWriter, r *http.Request) {
+	c, err := upgrader.Upgrade(w, r, nil)
+	if err != nil {
+		log.Print("upgrade:", err)
+		return
+	}
+
+	// Process connection in a new goroutine
+	go process(c)
+
+	// Let the http handler return, the 8k buffer created by it will be garbage collected
+}
+
+func main() {
+	flag.Parse()
+	log.SetFlags(0)
+	http.HandleFunc("/ws", handler)
+	log.Fatal(http.ListenAndServe(*addr, nil))
+}