How can I stop a goroutine that is reading from UDP?

Question

I have a go program that uses a goroutine to read UDP packets. I wanted to use a select clause and a "stopping" channel to close the goroutine to shut down as soon as it is not needed anymore.

Here is a simple code example for the goroutine:

func Run(c chan string, q chan bool, conn *net.UDPConn) {

    defer close(c)

    buf := make([]byte, 1024)

    for {
        select {
            case <- q:
                return
            default:
                n, _, err := conn.ReadFromUDP(buf)
                c <- string(buf[0:n])
                fmt.Println("Received ", string(buf[0:n]))

                if err != nil {
                    fmt.Println("Error: ", err)
                }
        }
    }
}

The connection is created as:

    conn, err := net.ListenUDP("udp",addr.Addr)

And the goroutine is supposed to terminate using:

    close(q)

After closing the "stopping" channel ("q") the goroutine does not immediately stop. I need to send one more string via the UDP connection. When doing so the goroutine stops. I simply do not understand this behaviour and I would be grateful if somebody could enlighten me.

Thank you in advance!

Answer 1

Your program is likely stopped at this line when you close the channel:

n, _, err := conn.ReadFromUDP(buf)

Because execution is blocked at a ReadFrom method, the select statement is not being evaluated, therefore the close on channel q is not immediately detected. When you do another send on the UDP connection, ReadFrom unblocks and (once that loop iteration finishes) control moves to the select statement: at that point the close on q is detected.

You can close the connection to unblock ReadFrom, as was suggested in a comment. See the PacketConn documentation in the net package, especially "Any blocked ReadFrom or WriteTo operations will be unblocked and return errors":

// Close closes the connection.
// Any blocked ReadFrom or WriteTo operations will be unblocked and return errors.
Close() error

Depending on your needs a timeout might be an option as well, see again PacketConn documentation in the net package:

 // ReadFrom reads a packet from the connection,
 // copying the payload into b. It returns the number of
 // bytes copied into b and the return address that
 // was on the packet.
 // ReadFrom can be made to time out and return
 // an Error with Timeout() == true after a fixed time limit;
 // see SetDeadline and SetReadDeadline.
 ReadFrom(b []byte) (n int, addr Addr, err error)