我正在尝试通过检查golang TCPConn.Write返回的错误来检测发送失败，但它为nil。我也尝试使用TCPConn.SetWriteDeadline失败。
事情就是这样发生的:
问题:为什么仅向不存在的客户端发送第二条消息会导致错误？案件应如何妥善处理？
代码如下:
package main
import (
"net"
"os"
"bufio"
"fmt"
"time"
)
func AcceptConnections(listener net.Listener, console <- chan string) {
msg := ""
for {
conn, err := listener.Accept()
if err != nil {
panic(err)
}
fmt.Printf("client connected\n")
for {
if msg == "" {
msg = <- console
fmt.Printf("read from console: %s", msg)
}
err = conn.SetWriteDeadline(time.Now().Add(time.Second))
if err != nil {
fmt.Printf("SetWriteDeadline failed: %v\n", err)
}
_, err = conn.Write([]byte(msg))
if err != nil {
// expecting an error after sending a message
// to a non-existing client endpoint
fmt.Printf("failed sending a message to network: %v\n", err)
break
} else {
fmt.Printf("msg sent: %s", msg)
msg = ""
}
}
}
}
func ReadConsole(network chan <- string) {
console := bufio.NewReader(os.Stdin)
for {
line, err := console.ReadString('\n')
if err != nil {
panic(err)
} else {
network <- line
}
}
}
func main() {
listener, err := net.Listen("tcp", "localhost:6666")
if err != nil {
panic(err)
}
println("listening on " + listener.Addr().String())
consoleToNetwork := make(chan string)
go AcceptConnections(listener, consoleToNetwork)
ReadConsole(consoleToNetwork)
}
服务器控制台如下所示:
listening on 127.0.0.1:6666
client connected
hi there!
read from console: hi there!
msg sent: hi there!
this one should fail
read from console: this one should fail
msg sent: this one should fail
this one actually fails
read from console: this one actually fails
failed sending a message to network: write tcp 127.0.0.1:51194: broken pipe
客户看起来像这样:
package main
import (
"net"
"os"
"io"
//"bufio"
//"fmt"
)
func cp(dst io.Writer, src io.Reader, errc chan<- error) {
// -reads from src and writes to dst
// -blocks until EOF
// -EOF is not an error
_, err := io.Copy(dst, src)
// push err to the channel when io.Copy returns
errc <- err
}
func StartCommunication(conn net.Conn) {
//create a channel for errors
errc := make(chan error)
//read connection and print to console
go cp(os.Stdout, conn, errc)
//read user input and write to connection
go cp(conn, os.Stdin, errc)
//wait until nil or an error arrives
err := <- errc
if err != nil {
println("cp error: ", err.Error())
}
}
func main() {
servAddr := "localhost:6666"
tcpAddr, err := net.ResolveTCPAddr("tcp", servAddr)
if err != nil {
println("ResolveTCPAddr failed:", err.Error())
os.Exit(1)
}
conn, err := net.DialTCP("tcp", nil, tcpAddr)
if err != nil {
println("net.DialTCP failed:", err.Error())
os.Exit(1)
}
defer conn.Close()
StartCommunication(conn)
}
编辑:在JimB的建议下，我想出了一个可行的例子。邮件不会再丢失，并且会在新的连接中重新发送。我不太确定在不同的go例程之间使用共享变量(connWrap.IsFaulted)有多安全。
package main
import (
"net"
"os"
"bufio"
"fmt"
)
type Connection struct {
IsFaulted bool
Conn net.Conn
}
func StartWritingToNetwork(connWrap * Connection, errChannel chan <- error, msgStack chan string) {
for {
msg := <- msgStack
if connWrap.IsFaulted {
//put it back for another connection
msgStack <- msg
return
}
_, err := connWrap.Conn.Write([]byte(msg))
if err != nil {
fmt.Printf("failed sending a message to network: %v\n", err)
connWrap.IsFaulted = true
msgStack <- msg
errChannel <- err
return
} else {
fmt.Printf("msg sent: %s", msg)
}
}
}
func StartReadingFromNetwork(connWrap * Connection, errChannel chan <- error){
network := bufio.NewReader(connWrap.Conn)
for (!connWrap.IsFaulted) {
line, err := network.ReadString('\n')
if err != nil {
fmt.Printf("failed reading from network: %v\n", err)
connWrap.IsFaulted = true
errChannel <- err
} else {
fmt.Printf("%s", line)
}
}
}
func AcceptConnections(listener net.Listener, console chan string) {
errChannel := make(chan error)
for {
conn, err := listener.Accept()
if err != nil {
panic(err)
}
fmt.Printf("client connected\n")
connWrap := Connection{false, conn}
go StartReadingFromNetwork(&connWrap, errChannel)
go StartWritingToNetwork(&connWrap, errChannel, console)
//block until an error occurs
<- errChannel
}
}
func ReadConsole(network chan <- string) {
console := bufio.NewReader(os.Stdin)
for {
line, err := console.ReadString('\n')
if err != nil {
panic(err)
} else {
network <- line
}
}
}
func main() {
listener, err := net.Listen("tcp", "localhost:6666")
if err != nil {
panic(err)
}
println("listening on " + listener.Addr().String())
consoleToNetwork := make(chan string)
go AcceptConnections(listener, consoleToNetwork)
ReadConsole(consoleToNetwork)
}
最佳答案
这不是特定于Go的，而是通过底层TCP套接字显示出来的工件。
TCP终止步骤的详细图示在此页面的底部:
http://www.tcpipguide.com/free/t_TCPConnectionTermination-2.htm
简单的版本是，当客户端关闭其套接字时，它将发送FIN，并从服务器接收ACK。然后，它等待服务器执行相同的操作。但是，您不是发送FIN，而是发送更多的数据，这些数据将被丢弃，并且客户端套接字现在假定来自您的任何其他数据都是无效的，因此，下次发送时会收到RST，这就是气泡进入您看到的错误。
回到您的程序，您需要以某种方式进行处理。通常，您可以想到谁负责发起发送，谁负责发起终止，因此您的服务器应假定它可以继续发送直到关闭连接或遇到错误。如果需要更可靠地检测到客户端关闭，则需要在协议(protocol)中具有某种客户端响应。这样，可以在套接字上调用recv并返回0，这将提醒您已关闭的连接。
在执行过程中，这将从连接的Read方法(或您的情况下，在Copy中)返回EOF错误。 SetWriteDeadline不起作用，因为会进行少量写操作，然后将其静默删除，否则客户端最终将以RST响应，从而给您带来错误。