Defer statements execute when the surrounding function returns or encounters a panic. Yours is inside an infinite for loop so it will never execute the close. You should move this logic to your handle function.

https://golang.org/ref/spec#Defer_statements

@bruston Thank you first, defer just an example.
My purpose is Close the Connection, but there is no way to close when TCP connection close by accident.
I think if there have a way to listen the connection status, then use a channel to close it.

As far as I know, at least on Linux and Mac, there is no way to get notified when the connection is closed, except reading from the socket and checking for a 0 byte read (which is converted to a (0, io.EOF) return value in Go).

You could enable keep alive on each connection via TCPConn.SetKeepAlive. This would ensure that the socket will be closed by the OS after some time, but you would still need to detect this by reading from the socket and closing the Conn in your code.

If the client closes the connection properly it should work just fine so long as you check and handle your read errors. If the connection is not properly closed (internet connection dies, router gets unplugged, etc) things get a bit more tricky.

You can set timeout deadlines for your read operations: https://golang.org/pkg/net/#TCPConn.SetReadDeadline and then close the connection yourself on a timeout error.

You might also be able to use KeepAlive: https://golang.org/pkg/net/#TCPConn.SetKeepAlive or implement your own kind of health check.

This doesn't seem like a bug, rather just the way TCP works. If you have more questions you're more than welcome to ask for assistance on any of the mediums listed here: https://github.com/golang/go/wiki/Questions

I don't see any actual bug here, so closing.

@ianlancetaylor yes, actually there is not a bug, but I want Close those TCP closed connection automatically.
Now, I just use exec.Command to run lsof to check all CLOSE_WAIT connection and close it.

Here is the code to clean CLOSE_WAIT, thank you for your advices.

package main

import (
	"fmt"
	"net"
	"os"
	"os/exec"
	"strconv"
	"regexp"
	"time"
	"strings"
	"github.com/liuzheng/golog"
)

var lsofout = ""

func main() {
	l, err := net.Listen("tcp", ":1234")
	if err != nil {
		fmt.Println("Error listening: ", err)
		os.Exit(1)
	}
	defer l.Close()

	ticker := time.NewTicker(time.Second * 5)
	go func() {
		for range ticker.C {
			lsof := exec.Command("lsof", "-p", strconv.Itoa(os.Getpid()), "-a", "-i", "tcp")
			grep := exec.Command("grep", "CLOSE_WAIT")
			lsofOut, _ := lsof.StdoutPipe()
			lsof.Start()
			grep.Stdin = lsofOut
			out, _ := grep.Output()
			lsofout = string(out)
		}
	}()

	for {
		conn, err := l.Accept()
		if err != nil {
			fmt.Println("Error accepting: ", err)
			os.Exit(1)
		}

		fmt.Printf("Start to receive message %s -> %s \n", conn.RemoteAddr(), conn.LocalAddr())

		go handle(conn)

		go func() {
			for range ticker.C {
				if strings.Index(lsofout, regexp.MustCompile(":[0-9]+$").FindAllStringSubmatch(conn.RemoteAddr().String(), -1)[0][0]+" ") > -1 {
					golog.Info("close", "Close the connection in %v", conn.RemoteAddr())
					conn.Close()
					return
				}
			}
		}()
	}
}
func handle(conn net.Conn) {
	// do something
}

same problem.
I will take your way to solve that problem.
by the way, any other idea? @liuzheng

by the way, your code have a bug. every time accept will make new go func and only if strings.Index(lsofout, regexp.MustCompile(":[0-9]+$").FindAllStringSubmatch(conn.RemoteAddr().String(), -1)[0][0]+" ") > -1 will return.

@suutaku sorry, I cannot understand that, in which case cause it become a bug?
This go func() is made for a watchdog to close the connection conn.Close().
I know in each connection will create go func is a dirty way, and the regular expression cause a lot CPU time, but there always have solutions to solve it.
We can use some event engine, but this is out of the topic.