客户端禁用Keepalive，服务端开启Keepalive，会怎么样？

最近部署的web程序，在服务器上出现不少time_wait的连接状态，会占用tcp端口，费了几天时间排查。

之前我有结论：HTTP keep-alive 是在应用层对TCP连接的滑动续约复用，如果客户端、服务器稳定续约，就成了名副其实的长连接。

目前所有的HTTP网络库(不论是客户端、服务端)都默认开启了HTTP Keep-Alive，通过Request/Response的Connection标头来协商复用连接。

非常规做法导致的短连接

我手上有个项目，由于历史原因，客户端禁用了Keep-Alive，服务端默认开启了Keep-Alive，如此一来协商复用连接失败，客户端每次请求会使用新的TCP连接，也就是回退为短连接。

客户端强制禁用Keep-Alive

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"time"
)

func main() {
	tr := http.Transport{
		DisableKeepAlives: true,
	}
	client := &http.Client{
		Timeout:   10 * time.Second,
		Transport: &tr,
	}
	for {
		requestWithClose(client)
		time.Sleep(time.Second * 1)
	}
}

func requestWithClose(client *http.Client) {
	resp, err := client.Get("http://10.100.219.9:8081")
	if err != nil {
		fmt.Printf("error occurred while fetching page, error: %s", err.Error())
		return
	}
	defer resp.Body.Close()
	c, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		log.Fatalf("Couldn't parse response body. %+v", err)
	}

	fmt.Println(string(c))
}

web服务端默认开启Keep-Alive

package main

import (
	"fmt"
	"log"
	"net/http"
)

// 根据RemoteAddr 知道客户端使用的持久连接
func IndexHandler(w http.ResponseWriter, r *http.Request) {
	fmt.Println("receive a request from:", r.RemoteAddr, r.Header)
	w.Write([]byte("ok"))
}

func main() {
	fmt.Printf("Starting server at port 8081\n")
	// net/http 默认开启持久连接
	if err := http.ListenAndServe(":8081", http.HandlerFunc(IndexHandler)); err != nil {
		log.Fatal(err)
	}
}

从服务端的日志看，确实是短连接。

receive a request from: 10.22.38.48:54722 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54724 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54726 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54728 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54731 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54733 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54734 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54738 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54740 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54741 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54743 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54744 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]
receive a request from: 10.22.38.48:54746 map[Accept-Encoding:[gzip] Connection:[close] User-Agent:[Go-http-client/1.1]]

谁是主动断开方？

我想当然的以为客户端是主动断开方，被现实啪啪打脸。

某一天服务器上超过300的time_wait报警告诉我这tmd是服务器主动终断连接。

常规的TCP4次挥手，主动断开方会进入time_wait状态，等待2MSL后释放占用的SOCKET

以下是从服务器上tcpdump抓取的tcp连接信息。

红框2,3部分明确提示是从 Server端发起TCP的FIN消息, 之后Client回应ACK确认收到Server的关闭通知；之后Client再发FIN消息，告知现在可以关闭了， Server端最终发ACK确认收到，并进入Time_WAIT状态，等待2MSL的时间关闭Socket。

特意指出，红框1表示TCP双端同时关闭，此时会在Client,Server同时留下time_wait痕迹，发生概率较小。

没有源码说个串串

此种情况是服务端主动关闭，我们往回翻一翻golang httpServer的源码

http.ListenAndServe(":8081")
server.ListenAndServe()
srv.Serve(ln)
go c.serve(connCtx) 使用go协程来处理每个请求

服务器连接处理请求的简略源码如下：

func (c *conn) serve(ctx context.Context) {
	c.remoteAddr = c.rwc.RemoteAddr().String()
	ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr())
	defer func() {
    if !c.hijacked() {
			c.close()
			c.setState(c.rwc, StateClosed, runHooks)
		}
	}()

  ......
	// HTTP/1.x from here on.

	ctx, cancelCtx := context.WithCancel(ctx)
	c.cancelCtx = cancelCtx
	defer cancelCtx()

	c.r = &connReader{conn: c}
	c.bufr = newBufioReader(c.r)
	c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)

	for {
		w, err := c.readRequest(ctx)
		......
		serverHandler{c.server}.ServeHTTP(w, w.req)
		w.cancelCtx()
		if c.hijacked() {
			return
		}
		w.finishRequest()
		if !w.shouldReuseConnection() {
			if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
				c.closeWriteAndWait()
			}
			return
		}
		c.setState(c.rwc, StateIdle, runHooks)
		c.curReq.Store((*response)(nil))

		if !w.conn.server.doKeepAlives() {
			// We're in shutdown mode. We might've replied
			// to the user without "Connection: close" and
			// they might think they can send another
			// request, but such is life with HTTP/1.1.
			return
		}

		if d := c.server.idleTimeout(); d != 0 {
			c.rwc.SetReadDeadline(time.Now().Add(d))
			if _, err := c.bufr.Peek(4); err != nil {
				return
			}
		}
		c.rwc.SetReadDeadline(time.Time{})
	}
}

我们需要关注

① for循环，表示尝试复用该conn，用于处理迎面而来的请求

② w.shouldReuseConnection() = false，表明读取到ClientConnection：Close标头，设置closeAfterReply=true，跳出dor循环，协程即将结束，结束之前执行defer函数，defer函数内close该连接

c.close()
......
// Close the connection.
func (c *conn) close() {
	c.finalFlush()
	c.rwc.Close()
}

③ 如果 w.shouldReuseConnection() = true，则将该连接状态置为idle，并继续走for循环，处理后续请求。

我的收获

tcp 4次挥手的八股文
短连接在服务器上的效应，time_wait，占用可用的SOCKET，根据实际业务看是否需要切换为长连接
golang http keep-alive复用tcp连接的源码级分析
tcpdump抓包的姿势

------ 2022/2/24 补充-------

本文作为tcp的八股文，当客户端和服务端的Keep-alive的策略不匹配是，到底谁是主动终断方 ？

从上文源码看，服务端有能力从客户端标头拿到Connection：Close, 也可以找到服务端自己的Keep-Alive策略，所以

如果客户端开启Keep-Alive，服务端禁用Keep-Alive，服务端依旧是主动断开方，当然也存在少量客户端和服务端同时关闭的情况