How this select statement executes?

Issue


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"?

Solution

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.

Answered By – mkopriva

Answer Checked By – Candace Johnson (GoLangFix Volunteer)

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