Getting value from Go channel - select

Question

I have a go-routine which is listening for TCP connections and send these on a channel back to the main loop. The reason I'm doing this in a go-routine is to make this listening non-blocking and be able to handle active connections simultaneously.
I have implemented this with a select statement with an empty default case like this:
go pollTcpConnections(listener, rawConnections)
for {
// Check for new connections (non-blocking)
select {
case tcpConn := <-rawConnections:
currentCon := NewClientConnection()
pendingConnections.PushBack(currentCon)
fmt.Println(currentCon)
go currentCon.Routine(tcpConn)
default:
}
// ... handle active connections
}
Here is my pollTcpConnections routine:
func pollTcpConnections(listener net.Listener, rawConnections chan net.Conn) {
for {
conn, err := listener.Accept() // this blocks, afaik
if(err != nil) {
checkError(err)
}
fmt.Println("New connection")
rawConnections<-conn
}
}
The problem is that I never recieve these connections. If I do it in a blocking way, like this:
for {
tcpConn := <-rawConnections
// ...
}
I recieve the connections, but it blocks... I have tried buffering the channel as well, but the same thing happens. What am I missing here?

Answer 1

it's a little hard to tell why you're not seeing any connections based on the existing code. One problem with your sample is that you have an empty default case in a select statement, and then we can't see what else is happening in this for loop. The way you've written it, that loop might never yield to the scheduler. You're basically saying "get a thing from the channel. don't have one? ok, start over. get a thing from the channel!", but you never actually wait. When you do some action that blocks your goroutine, that goroutine yields to the scheduler. So when you do a channel read in the normal fashion, if there's no value to be read, that goroutine is blocked reading. Since it's blocked, it also yields to the scheduler to allow other goroutines to continue executing on the underlying thread. I'm fairly certain this is why your select with an empty default is breaking; you're causing that goroutine to loop infinitely on the for loop without ever yielding to the scheduler.
It's not clear what the role of pendingConnections is, or whether it's needed at all.
The other thing that's impossible to tell from the behavior is what your checkError function does. It doesn't, for example, continue to the top of the for loop, or bail.
Anyway, it looks like this is more complicated than it needs to be. Just have a function that takes the new connection as it's one parameter, and then launch that in a new goroutine when it connects. I always write it like this:
func handleConnection(c net.Conn) {
// do something with your connection here.
}
for {
// Wait for a connection.
conn, err := l.Accept()
if err != nil {
// do something with your error. You probably want to break or return here.
break
}
// handle each connection in a new goroutine
go handleConnection(conn)
}
this is more or less exactly what they do in the documentation.