Go - Cancel Job by close channel

• Close a channel to broadcast cancellation
• No way to know a channel is closed or not, so need another channel to indicate a channel is closed
• This example shows when "done" channel is closed, there is still some work not done, some new messages are sent to channel
  So need consume all messages from channel when a "done channel" is closed
• 

Go - Select Channel

• Declare

select {

    case <- ch:

        ...

    case x := <- ch2:

        ...

    case ch3 <- y:

        ...

    default:

        ...

}

• Select wait until some case is ready to proceed
• Select with no case, wait forever

select {}

Here is example, please notice if more than 1 channel can be process, Go will pick one to process randomly