too many open files error with websocket server in golang

I'm actually using gorilla package to put my websocket server in golang.
If I leave my server on, I have this error every 10 days or so.

2016/11/28 19:22:49 http: Accept error: accept tcp [::]:9001: accept4: too many open files; retrying in 1s
2016/11/28 19:22:50 http: Accept error: accept tcp [::]:9001: accept4: too many open files; retrying in 1s
2016/11/28 19:22:51 http: Accept error: accept tcp [::]:9001: accept4: too many open files; retrying in 1s
2016/11/28 19:22:52 http: Accept error: accept tcp [::]:9001: accept4: too many open files; retrying in 1s
2016/11/28 19:22:53 http: Accept error: accept tcp [::]:9001: accept4: too many open files; retrying in 1s
2016/11/28 19:22:54 http: Accept error: accept tcp [::]:9001: accept4: too many open files; retrying in 1s

this is my code :

func websocketHandler(writer http.ResponseWriter, request *http.Request){
    var socket *websocket.Conn
    user := new(user.User)
    user.Token = getParamURI(request.URL.RequestURI(), "token")

    if user.GetUserByToken() == false {

        errors := api.Error{}
        socket.Close()
        errors.ListErrors = append(errors.ListErrors, "Session doesn't exist")
	    writer.Header().Set("Content-Type", "application/json")
	    writer.WriteHeader(http.StatusNotAcceptable)
	    json.NewEncoder(writer).Encode(errors)
        return

    }

    socket, _ = upgrader.Upgrade(writer, request, nil)

    err := make(chan string)

    go pingSocket(socket, err)
    go handleChangeNotification(socket, user.Id, err)
    go handleChangeMessage(socket, user.Id, err)
    for {
       tmp := <- err
        if len(tmp) > 0 {
            break
        }
    }
   socket.Close()
}

my ulimit configuration :

core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 7902
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 7902
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

The error is rather incomprehensible, I don't understand, because I close the socket well in case of break or error and server is in production but in beta close with 4-5 users, so it's not about limit.

Can someone tell me where the error comes from?

If you have closed all body readeres and covered all other possible reasons, it might be the ulimit problem if you have hight traffic. But usually you can increase that even if you are not the root users since there are two types of limits, hard limit ie. the max, and soft limit. You can check both of them in Linux using:

# ulimit -Sn

# ulimit -Hn

If soft limit (Sn) is lower than hard limit (Hn) (default or set by the admin) you can make it higher up to the hard limit. On all my CentOS installs, as I can see the default is 1024 for Sn and 4096 for Hn, so you can easily max out Sn to 4096, no root user involved.

This function will max out the number of open files if it is possible. Copy/paste and call the maxOpenFile() at the begining of main func and see does it help.

import "syscall"

func maxOpenFiles() {
	var rLimit syscall.Rlimit

    err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rLimit)
    if err != nil {
	    log.Println("Error Getting Rlimit ", err)
    }

    if rLimit.Cur < rLimit.Max {
	    rLimit.Cur = rLimit.Max
	    err = syscall.Setrlimit(syscall.RLIMIT_NOFILE, &rLimit)
	    if err != nil {
		    log.Println("Error Setting Rlimit ", err)
	    }
    }
}

Code is simple, first we read current settings for number of files and if Cur is less than Max, we max it out. This func will set the file limits only to the current app/linux process.