go - 选择 block 如何等待 ctx.Done() 操作？

Question

在下面的代码中:

    ctx, cancel := context.WithTimeout(req.Context(), 5000*time.Second)

    // Wait for the response or timeout
    select {
    case <-ctx.Done():
        log.Println("timeout, cancel work...")

        // Cancel the request and wait for it to complete
        // this will shutdown the go-routine immediately
        tr.CancelRequest(req)
        log.Println(<-ch)

    case err := <-ch:
        // do something
    }

select同时等待两个接收操作。一个接收操作( <-ch)是一个 block 操作

在执行 select 中 block ，是 ctx.Done()在 select 中多次调用 block ，验证，如果 ctx.Done()返回一个 channel ？直到 <-ch被封锁了……

Answer 1

Spec: Select statements:

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.

select来电 ctx.Done()只有一次。它返回一个 channel ，并监视来自该 channel 的接收操作是否可以继续。

如果 5000*time.Second超时到期，或者如果父上下文( req.Context() )被取消(例如超时或它的 cancel() 函数被调用)，那么由 ctx.Done() 返回的 channel 将被关闭，因此可以继续从它接收(它不再是阻塞操作)。 Spec: 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.