Priority of case versus default in golang select statements

Question

I have an application with multiple goroutines that are running for loops, and need a way to signal these for loops to break, and to test whether the timeout case occurred. I was looking into using a shared channel with select statements to accomplish this as follows:

// elsewhere in the code, this channel is created, and passed below
done := make(chan struct{})
time.AfterFunc(timeout, func() { close(done) })
...
go func() {
Loop:
  for {
    select {
    case <-done:
      break Loop
    default:
      foo()
      time.Sleep(1 * time.Second)
    }
  }
  select {
  case <-done:
    panic("timed out!")
  default:
    // ok
  }
}()

Is this a valid way to accomplish this? What I'm most concerned about is that the branch of a select that is chosen could be non-deterministic, so that default may be chosen even if one of the cases is ready. Is this possible? Is there any documentation that states that a matching case is guaranteed to have preference over a default. The concern is that the for loop above could loop several times after done is closed and/or report success even though a timeout occurred.

Answer 1

The Go Programming Language Specification

Select statements

Execution of a "select" statement proceeds in several steps:

1. For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the "select" statement. The result is a set of channels to receive from or send to, and the corresponding values to send. Any side effects in that evaluation will occur irrespective of which (if any) communication operation is selected to proceed. Expressions on the left-hand side of a RecvStmt with a short variable declaration or assignment are not yet evaluated.
2. If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the "select" statement blocks until at least one of the communications can proceed.
3. Unless the selected case is the default case, the respective communication operation is executed.
4. If the selected case is a RecvStmt with a short variable declaration or an assignment, the left-hand side expressions are evaluated and the received value (or values) are assigned.
5. The statement list of the selected case is executed.

---

"What I'm most concerned about is that the branch of a select that is chosen could be non-deterministic, so that default may be chosen even if one of the cases is ready. Is this possible?"

No. See step 2 of the select specification.