How to know TCP connection is closed in net package? - tcp

Question

I'm implementing a small TCP server. How do I know if one of my clients closed? Should I just try to read or write and check if err is nil?

Answer 1

That thread "Best way to reliably detect that a TCP connection is closed", using net.Conn for 'c' (also seen in utils/ping.go or locale-backend/server.go or many other instances):
one := make([]byte, 1)
c.SetReadDeadline(time.Now())
if _, err := c.Read(one); err == io.EOF {
l.Printf(logger.LevelDebug, "%s detected closed LAN connection", id)
c.Close()
c = nil
} else {
var zero time.Time
c.SetReadDeadline(time.Now().Add(10 * time.Millisecond))
}
For detecting a timeout, it suggests:
if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
...
Update 2019: tuxedo25 mentions in the comments:
In go 1.7+, zero byte reads return immediately and will never return an error.
You must read at least one byte.
See commit 5bcdd63 and go issue 15735
net: don't return io.EOF from zero byte reads

Answer 2

Just try to read from it, and it will throw an error if it's closed. Handle gracefully if you wish!
For risk of giving away too much:
func Read(c *net.Conn, buffer []byte) bool {
bytesRead, err := c.Read(buffer)
if err != nil {
c.Close()
log.Println(err)
return false
}
log.Println("Read ", bytesRead, " bytes")
return true
}
Here is a nice introduction to using the net package to make a small TCP "chat server":
"Golang Away: TCP Chat Server"

Answer 3

After struggling for a while on this, here is a POSIX solution that uses MSG_PEEK to prevent draining the buffer and causing race conditions. This lets you check the whether the READ half of a TCP socket is still open from another goroutine:
func connCheck(conn net.Conn) error {
var sysErr error = nil
rc, err := conn.(syscall.Conn).SyscallConn()
if err != nil { return err }
err = rc.Read(func(fd uintptr) bool {
var buf []byte = []byte{0}
n, _, err := syscall.Recvfrom(int(fd), buf, syscall.MSG_PEEK | syscall.MSG_DONTWAIT)
switch {
case n == 0 && err == nil:
sysErr = io.EOF
case err == syscall.EAGAIN || err == syscall.EWOULDBLOCK:
sysErr = nil
default:
sysErr = err
}
return true
})
if err != nil { return err }
return sysErr
}
This is based on the above mysql#connCheck, but that one does a 1-byte read syscall, which can potentially conflict with other goroutines attempting to read a stream.

Answer 4

April 2019 status:
Reading the threads and posts on https://github.com/golang/go/issues/15735
There is a general lack of official support to detect whether a server closed a TCP connection if the server does not write on the connection (reading will fail, but writing won't).
There is a solution provided by https://github.com/methane that works only on Linux and is expensive as it does some allocations -
This is available here: https://github.com/go-sql-driver/mysql/blob/master/conncheck.go
I found this to be working but not being cross platform is blocking me from adopting it -

Answer 5

My five cents, I figured it is better to write to the connection and get an error, thanks for the answers above, here is my working solution
func isTCPWorking(c net.Conn) bool {
_, err := c.Write([]byte("OK"))
if err != nil {
c.Close() // close if problem
return false
}
return true
}

Answer 6

_, err := conn.Read(make([]byte, 0))
if err!=io.EOF{
// this connection is invalid
logger.W("conn closed....",err)
}else{
byt, _:= ioutil.ReadAll(conn);
}