网络通信中，为了防止长时间无响应的情况，经常会用到网络连接超时、读写超时的设置。

本文结合例子简介golang的连接超时和读写超时设置。

1.超时设置

1.1 连接超时

func DialTimeout(network, address string, timeout time.Duration) (Conn, error) 

第三个参数timeout可以用来设置连接超时设置。
如果超过timeout的指定的时间，连接没有完成，会返回超时错误。

1.2 读写超时

在Conn定义中，包括读写的超时时间设置。

type Conn interface {
    // SetDeadline sets the read and write deadlines associated
    // with the connection. It is equivalent to calling both
    // SetReadDeadline and SetWriteDeadline.
    //
    ... ...
    SetDeadline(t time.Time) error

    // SetReadDeadline sets the deadline for future Read calls
    // and any currently-blocked Read call.
    // A zero value for t means Read will not time out.
    SetReadDeadline(t time.Time) error

    // SetWriteDeadline sets the deadline for future Write calls
    // and any currently-blocked Write call.
    // Even if write times out, it may return n > 0, indicating that
    // some of the data was successfully written.
    // A zero value for t means Write will not time out.
    SetWriteDeadline(t time.Time) error
}

通过上面的函数说明，可以得知，这里的参数t是一个未来的时间点，所以每次读或写之前，都要调用SetXXX重新设置超时时间，

如果只设置一次，就会出现总是超时的问题。

2.例子

2.1 server

server端监听连接，如果收到连接请求，就是创建一个goroutine负责这个连接的数据收发。

为了测试超时，我们在写操作之前，sleep 3s。

package main

import (
        "net"
        "log"
        "time"
)

func main() {
        addr := "0.0.0.0:8080"

        tcpAddr, err := net.ResolveTCPAddr("tcp",addr)

        if err != nil {
                log.Fatalf("net.ResovleTCPAddr fail:%s", addr)
        }

        listener, err := net.ListenTCP("tcp", tcpAddr)
        if err != nil {
                log.Fatalf("listen %s fail: %s", addr, err)
        } else {
                log.Println("listening", addr)
        }


        for {
                conn, err := listener.Accept()
                if err != nil {
                        log.Println("listener.Accept error:", err)
                        continue
                }

                go handleConnection(conn)
        }

}


func handleConnection(conn net.Conn) {
        defer conn.Close()

        var buffer []byte = []byte("You are welcome. I'm server.")


        for {
                time.Sleep(3*time.Second)// sleep 3s
                n, err := conn.Write(buffer)
                if err != nil {
                        log.Println("Write error:", err)
                        break
                }
                log.Println("send:", n)
        }

        log.Println("connetion end")

}

2.2 client

client建立连接时，使用的超时时间是3s。

创建连接成功后，设置连接的读超时。
每次读之前，都重新设置超时时间。

package main

import (
        "log"
        "net"
        "os"
        "time"
)

func main() {
        connTimeout := 3*time.Second
        conn, err := net.DialTimeout("tcp", "127.0.0.1:8080", connTimeout)  // 3s timeout
        if err != nil {
                log.Println("dial failed:", err)
                os.Exit(1)
        }
        defer conn.Close()

        readTimeout := 2*time.Second

        buffer := make([]byte, 512)

        for {
                err = conn.SetReadDeadline(time.Now().Add(readTimeout)) // timeout
                if err != nil {
                        log.Println("setReadDeadline failed:", err)
                }

                n, err := conn.Read(buffer)
                if err != nil {
                        log.Println("Read failed:", err)
                        //break
                }

                log.Println("count:", n, "msg:", string(buffer))
        }
  
}

输出结果

2019/05/12 16:18:19 Read failed: read tcp 127.0.0.1:51718->127.0.0.1:8080: i/o timeout
2019/05/12 16:18:19 count: 0 msg:
2019/05/12 16:18:20 count: 28 msg: You are welcome. I'm server.
2019/05/12 16:18:22 Read failed: read tcp 127.0.0.1:51718->127.0.0.1:8080: i/o timeout
2019/05/12 16:18:22 count: 0 msg: You are welcome. I'm server.
2019/05/12 16:18:23 count: 28 msg: You are welcome. I'm server.
2019/05/12 16:18:25 Read failed: read tcp 127.0.0.1:51718->127.0.0.1:8080: i/o timeout
2019/05/12 16:18:25 count: 0 msg: You are welcome. I'm server.
2019/05/12 16:18:26 count: 28 msg: You are welcome. I'm server.