在Go select构造中，是否可以在两个case语句中向非缓冲通道发送和接收？

Question

I am expecting this code to go in an infinite loop, sending and receiving the message. But it does not seem to either send or receive. Why?

go func() {
    for {
        select {
        case ch1 <- 1:
            println("sent 1 ")
        case c := <-ch1:
            println(" received ", c)
        }
        time.Sleep(time.Second)
    }
}()

Answer 1

The channel documentation says:

If the capacity is zero or absent, the channel is unbuffered and communication succeeds only when both a sender and receiver are ready.

The select documentation says:

If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection.

The receive case cannot proceed because there's not a ready sender. The send case cannot proceed because there's not a ready receiver. The program deadlocks in the select.

A single goroutine cannot make the send and receive both ready because the goroutine can only execute one branch of the select at a time.

The program will loop forever if the channel is buffered (playground example).