如何使用 Channel 或 Context 等待多个子协程执行完毕？

import "math/rand"
import "time"

func main() {
    const num = 10
    ch := make(chan int, num)

    for i := 0; i < num; i++ {
        go func() {
            fmt.println("goroutine start: ", i)
            t := rand.intn(5) + 1
            time.sleep(time.duration(t) * time.second)
            fmt.println("goroutine end: ", i)
            ch <- 0
        }()
    }

    fmt.println("main start")
    for i := 0; i < num; i++ {
        <-ch
        fmt.println("main get: ", i)
    }
    fmt.println("main end")
}

import "context"

func main() {
    const num = 10

    ctx, cancel := context.WithCancel(context.Background())
    ch := make(chan int, num)

    for i := 0; i < num; i++ {
        go func(ctx context.Context) {
            fmt.Println("goroutine start: ", i)
            select {
            case <-ctx.Done():
                fmt.Println("goroutine cancelled: ", i)
                return
            default:
            }
            t := rand.Intn(5) + 1
            time.Sleep(time.Duration(t) * time.Second)
            fmt.Println("goroutine end: ", i)
            ch <- 0
        }(ctx)
    }

    fmt.Println("main start")
    for i := 0; i < num; i++ {
        <-ch
        fmt.Println("main get: ", i)
    }
    cancel()
    fmt.Println("main end")
}