不同goroutine中的频道消费顺序

Question

package main

import (
    "fmt"
    "time"
)

func main() {
    done := make(chan bool)

    go func() {
        for {
            select {
            case <-done:
                fmt.Println("here")
            }
        }
    }()

    go func() {
        q := time.NewTimer(time.Second)
        <-q.C
        done <- true
    }()

    <-done
    fmt.Println("closing")
}

My question is if done channel is consumed by goroutine not main goroutine. the program will be blocked.

But the fact is that the program never be blocked.main goroutine seem to be consumed the done channel everytime.

Why?

Answer 1

Although, I didn't found answer in docs, looks like receives value the routine which first tried to read from the channel. In our case, always main method is almost always reaching <-done (99.0%), because routines starts asynchronous and takes a while to run. But anyway, I would strongly suggest you do not rely on this, as this is not a guaranteed.

To demonstrate it, look at the following example:

package main

import (
    "fmt"
    "time"
)

func main() {
    done := make(chan bool)

    n := 5

    for i := 0; i < n; i++ {
        go func(i int) {
            fmt.Println(i, "Waiting to read")
            <-done
            fmt.Println(i, "DONE")
        }(i)
        time.Sleep(time.Second)
    }

    time.Sleep(time.Second)

    for i := 0 ; i < n; i++ {
        time.Sleep(time.Second)
        done <- true
    }

    time.Sleep(time.Second)
}

Which will produce output

0 Waiting to read
1 Waiting to read
2 Waiting to read
3 Waiting to read
4 Waiting to read
0 DONE
1 DONE
2 DONE
3 DONE
4 DONE