Useful pattern from "Concurrency in Go" for merging "done" channels

or_chan.go

// Returns a channel that will be closed whenever ANY of the input channels are
// closed (or written to).
func or(channels ...<-chan struct{}) <-chan struct{} {
    switch len(channels) {
    case 0:
        return nil
    case 1:
        return channels[0]
    }
    orDone := make(chan struct{})
    go func() {
        defer close(orDone)
        switch len(channels) {
        case 2:
            select {
            case <-channels[0]:
            case <-channels[1]:
            }
        default:
            select {
            case <-channels[0]:
            case <-channels[1]:
            case <-channels[2]:
            case <-or(append(channels[3:], orDone)...):
            }
        }
    }()
    return orDone
}