Select on time.After results deadlock

Question

package main

import "time"

func main() {
    chan_never_used := make(chan int, 10)

    c := make(chan int, 10)
    for {
        select {
        case <-time.After(time.Second):
            c <- 0
        case c <- <-chan_never_used:
        }

        println(<-c)
    }
}

https://play.golang.org/p/7hZMdITecg

The code above results in fatal error: all goroutines are asleep - deadlock!.

But if I change one line:

package main

import "time"

func main() {
    chan_never_used := make(chan int, 10)

    c := make(chan int, 10)
    for {
        select {
        case <-time.After(time.Second):
            c <- 0
        case n := <-chan_never_used: // This line changed
            c <- n                   // Add this line
        }

        println(<-c)
    }
}

It works well.

Why the first version of code results in deadlock and why could this change make the code work?

Answer 1

As per the standard the select statement contains of a bunch of Send or Receive statements.

In this case it is the send statement.

Send statement is defined as:

SendStmt = Channel "<-" Expression .
Channel  = Expression .

Looking at your code:

case c <- <-chan_never_used:

The Channel fraction is c, the Expression fraction is <-chan_never_used.

So, the select semantics of the Send Statement of being (potentially) non-blocking is not applicable because it is the expression that blocks. And before the Send Statements semantics is applied the Expression part must be completely evaluated.