incoming := make(chan int)
done := make(chan struct{})

...
close(done)
...

select {
    case incoming <- order:
        // logic
    case <-done:
        return 
    }

Let's say you close the done channel before the select statement:

1.The case <-done will be selected.

2.Is this because when close(done) executes, the done channel will be in a state, in which the select statement will consider the "case <-done" a match over the writing case "incoming <- order"?

CodePudding user response：

The select statement blocks until one or more of the communication operations in its cases can proceed. As soon as one or more communication operations can proceed select will randomly choose one of them.

A receive operation on a closed channel can proceed immediately.

Select Statement:

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.

Receive operator:

A receive operation on a closed channel can always proceed immediately, yielding the element type's zero value after any previously sent values have been received.