Double read from socket - sockets

Question

Go version: go1.14.4 darwin/amd64
TCP server
package main
import (
"fmt"
"io"
"net"
"time"
)
const (
maxBufLen int = 4 * 1024
)
type Server struct {
}
func (s *Server) Start() {
listener, err := net.Listen("tcp", "localhost:9001")
if err != nil {
panic(err)
}
var connections []net.Conn
defer func() {
for _, conn := range connections {
// ignore
_ = conn.Close()
}
}()
// loop...
for {
connected, err := listener.Accept()
if err != nil {
if ne, ok := err.(net.Error); ok && ne.Temporary() {
// temporary err will bi ignored
continue
} else {
fmt.Println(err)
return
}
}
go handleConn(connected, true)
connections = append(connections, connected)
}
}
func handleConn(conn net.Conn, server bool) {
buf := make([]byte, maxBufLen)
for {
// read
setTimeout(conn)
_, err := conn.Read(buf)
if err != nil {
if ne, ok := err.(net.Error); ok && (ne.Timeout() || ne.Temporary()) {
fmt.Println("need continue...")
continue
}
if err == io.EOF {
fmt.Println("EOF")
break
}
// other...
panic(err)
}
// handle recv msg.
s := string(buf)
if server {
//fmt.Println("server recv req ", s)
} else {
fmt.Println("client recv resp ", s)
}
if server {
output := "hi " + s
ob := []byte(output)
_, err := conn.Write(ob)
if err != nil {
fmt.Println(err)
break
}
}
}
}
func setTimeout(conn net.Conn) {
setErr := conn.SetReadDeadline(time.Now().Add(20 * time.Second))
if setErr != nil {
panic(setErr)
}
}
TCP client
package main
import (
"net"
"time"
)
type Client struct {
Exit chan struct{}
}
func (c *Client) Start() {
conn, err := net.Dial("tcp", "localhost:9001")
if err != nil {
panic(err)
}
defer conn.Close()
go handleWrite(conn)
go handleConn(conn, false)
<-c.Exit
}
func handleWrite(conn net.Conn) {
for {
input := "carryxyh"
_, err := conn.Write([]byte(input))
if err != nil {
panic(err)
}
<-time.After(100 * time.Second)
}
}
Main function
package main
import (
"fmt"
"os"
"os/signal"
"syscall"
)
type Starter interface {
Start()
}
func main() {
var s Server
var c Client
go s.Start()
go c.Start()
sigs := make(chan os.Signal)
signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)
<-sigs
c.Exit <- struct{}{}
fmt.Println("exit")
}
Running package main will print :
client recv resp hi carryxyh
client recv resp carryxyh
Printed twice. But from the program point of view, the server only writes information to the client once, and the content of the information should be hi carryxyh. But the client printed carryxyh in addition to hi carryxyh, which made me very confused.
During the investigation, I accidentally modified a line of code, and the program immediately returned to normal:
modeifycode
As shown above, I modified the server response information: output := "hi "+ s => output := "hi ", at this time the program only prints client recv resp hi.
This makes me completely confused, can anyone help me solve this problem? It would be better if there is a troubleshooting idea.

Answer 1

enter image description here
这里你忽略了读取的字节长度，可能返回0字节，你又把buf的内容 又回写给client了.
_, err := conn.Read(buf) 改成 bytesRead, err := conn.Read(buf) ;
if bytesRead <= 0 { // 没有buf 可以读取 }
--
eg:
if bytesRead == 0 && err == nil {
err = io.EOF
log.Errorf("[network] ReadOnce maybe always return (0, nil) and causes dead loop, Connection = %d, Local Address = %+v, Remote Address = %+v",
c.id, c.rawConnection.LocalAddr(), c.RemoteAddr())
}