golang 2 go routines if one terminates terminate the other one

Issue

so I am using gorillas websocket library and I’m building a websocket server when I receive a connection I create 2 go routines one to read incoming messages from the client and another 1 to listen for in coming messages sent to the channel and then send them to the client.

func (p *Player) EventLoop() {
    l4g.Info("Starting player %s event loop", p)
    go p.readFromSocket()
    go p.writeToSocket()

    //blocks until we receive an interrupt from the read channel
    <-p.closeEventChan
    p.cleanup() //nothing else to do so lets cleanup
}

func (p *Player) writeToSocket() {
    for m := range p.writeChan {
        if p.conn == nil {
            break
        }

        if reflect.DeepEqual(network.Packet{}, m) == true {
            break
        }

        if err := p.conn.WriteJSON(m); err != nil {
            break
        }
    }
    p.closeEventChan <- true
}

func (p *Player) readFromSocket() {
    for {
        if p.conn == nil {
            break
        }

        m := network.Packet{}

        if err := p.conn.ReadJSON(m); err != nil {
            break
        }
    }
    p.closeEventChan <- true
}

func (p *Player) Cleanup() {

    //make sure the channels get a close message which will break us out of the go routines
    close(p.writeChan)
    close(p.closeEventChan)

    //only close the connection if we have a connection
    if p.conn != nil {
        p.conn.Close()
        p.conn = nil
    }
}

my problem is if we leave the readFromSocket() loop Cleanup() is called however we never leave the writeToSocket() loop ! this problem can be simpler demonstrated in this go playground https://play.golang.org/p/49bh7bbbG-

how can we fix this so if we leave the writeToSocket() loop we also leave the readFromSocket() loop and vice vesa?

I was under the impression that this would work as if you call close on a channel (close(p.writeChan)) the default value that the channel accepts will be sent

Solution

You can usually do this with a shared quit channel, and some counting.

func (p *Player) writeToSocket(quit <-chan struct{})
    defer func() {
        p.closeEventChan <- true
    }()
    for {
        select {
        case <-quit:
            return
        case m := <-p.writeChan:
            // Do stuff
        // Case where writeChan is closed, but quit isn't
        default:
            return
        }
    }
}


func (p *Player) readFromSocket(quit <-chan struct{})
    defer func() {
        p.closeEventChan <- true
    }()
    for {
        select {
        case <-quit:
            return
        default:
            // Do stuff
        }
    }
}

func (p *Player) EventLoop() {
    l4g.Info("Starting player %s event loop", p)
    quit := make(chan struct{})
    go p.readFromSocket(quit)
    go p.writeToSocket(quit)

    //blocks until we receive an interrupt from the read channel
    <-p.closeEventChan
    close(quit)
    // This is superfluous here, but in a different case
    // you loop over the remaining number of workers using a NumWorkers variable
    for i := 0; i < 1; i++ {
        <-p.closeEventChan
    }
    p.cleanup() //nothing else to do so lets cleanup
}

The idea here is that:

  1. All workers notify the central routine upon close via a channel before returning.
  2. All workers return upon a quit message being broadcast (via close).
  3. The central loop closes the quit channel after the first worker exits, then waits for the rest to exit.

Answered By – Linear

Answer Checked By – Gilberto Lyons (GoLangFix Admin)

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