I have a slice of channels that all receive the same message:
func broadcast(c <-chan string, chans []chan<- string) {
for msg := range c {
for _, ch := range chans {
ch <- msg
}
}
}
However, since each of the channels in chans are potentially being read at a different rate, I don't want to block the other channels when I get a slow consumer. I've solved this with goroutines:
func broadcast(c <-chan string, chans []chan<- string) {
for msg := range c {
for _, ch := range chans {
go func() { ch <- msg }()
}
}
}
However, the order of the messages that get passed to each channel is important. I looked to the spec to see if channels preserve order when blocked, and all I found was this:
If the capacity is greater than zero, the channel is asynchronous: communication operations succeed without blocking if the buffer is not full (sends) or not empty (receives), and elements are received in the order they are sent.
To me, if a write is blocked, then it is not "sent", but waiting to be sent. With that assumption, the above says nothing about order of sending when multiple goroutines are blocked on writing.
Are there any guarantees about the order of sends after a channel becomes unblocked?
No, there are no guarantees.
Even when the channel is not full, if two goroutines are started at about the same time to send to it, I don't think there is any guarantee that the goroutine that was started first would actually execute first. So you can't count on the messages arriving in order.