永久循环读取两个通道的输出

Question

I am working on the tree exercise of tour.golang. I have tried to implement the same function as written below.

func Same(t1, t2 *tree.Tree) bool {
    ch1 := make(chan int)
    ch2 := make(chan int)
    go Walk(t1, ch1);
    go Walk(t2, ch2);

    for c := range ch1 {
        d := <- ch2
        if c-d !=0 {
            return false
        }   

    }
    return true
}

Using the forever loop, I would like to compare if an output from ch1 is different from that of ch2. But the following is throwing this error:

fatal error: all goroutines are asleep - deadlock!

live version

Answer 1

You are seeing a deadlock for a very simple reason: you are ranging over ch1, but never closing it, so the for loop never terminates.

You could fix this by manually iterating over each tree only a certain number of times like your 0..10 loop in main():

// Same determines whether the trees
// t1 and t2 contain the same values.
func Same(t1, t2 *tree.Tree) bool {
    ch1 := make(chan int)
    ch2 := make(chan int)
    go Walk(t1, ch1)
    go Walk(t2, ch2)

    for i := 0; i < 10; i++ {
        c := <-ch1
        d := <-ch2
        if c-d != 0 {
            return false
        }

    }
    return true
}

Playground

Alternatively, you can alter the signature of Walk to accept a waitgroup argument that is incremented by the caller of Walk and decremented when each Walk returns along with a goroutine to close the channel once you're done walking:

// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk(t *tree.Tree, ch chan int, wg *sync.WaitGroup) {
    defer wg.Done()
    if t.Left != nil {
        wg.Add(1)
        Walk(t.Left, ch, wg)
    }
    ch <- t.Value
    if t.Right != nil {
        wg.Add(1)
        Walk(t.Right, ch, wg)
    }

}

// Same determines whether the trees
// t1 and t2 contain the same values.
func Same(t1, t2 *tree.Tree) bool {
    ch1 := make(chan int)
    ch2 := make(chan int)

    var wg1 sync.WaitGroup
    wg1.Add(1)
    go Walk(t1, ch1, &wg1)
    go func() {
        wg1.Wait()
        close(ch1)
    }()

    var wg2 sync.WaitGroup
    wg2.Add(1)
    go Walk(t2, ch2, &wg2)
    go func() {
        // not strictly necessary, since we're not ranging over ch2, but here for completeness
        wg2.Wait()
        close(ch2)
    }()

    for c := range ch1 {
        d := <-ch2
        if c-d != 0 {
            return false
        }

    }
    return true
}

Playground