为什么我需要使用新的子例程运行Walk？

Question

I’m writing the Walk function in the go tutorial that basically traverses a tree in-order. What I have works:

package main

import (
    "fmt"
    "code.google.com/p/go-tour/tree"
)

// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk__helper(t *tree.Tree, ch chan int) {
    if (t == nil) {
        return
    }

    Walk__helper(t.Left, ch)
    ch <- t.Value
    Walk__helper(t.Right, ch)
}

func Walk(t *tree.Tree, ch chan int) {
    Walk__helper(t, ch)
    close(ch)
}

func main() {
    ch := make(chan int)
    go Walk(tree.New(1), ch)
    for v := range ch {
        fmt.Println(v)   
    }
}

Why must I use go Walk(tree.New(1), ch) instead of just Walk(tree.New(1), ch)?

I was under the impression that the go keyword basically spawns a new thread. In that case, we’d run into issues since the for loop might run before the subroutine completes.

Strangely, when I take out the go keyword, I get a deadlock. This is rather counterintuitive to me. What exactly is the go keyword doing here?

Answer 1

The key point here is range when coupled with a channel.

When you range over a channel (in this case, ch), it will wait for items to be sent on the channel before iterating through the loop. This is a safe, "blocking" action, that will not deadlock while it waits for the channel to receive an item.

The deadlock occurs when not using a goroutine because your channel isn't buffered. If you don't use a goroutine, then the method call is synchronous, Walk puts something on the channel.. and it blocks until that is popped off. It never gets popped off.. because the method call was synchronous.

I was under the impression that the go keyword basically spawns a new thread

..that is incorrect. There are many more important implementation details required to understand what goes on there. You should separate your thought process of a goroutine from a thread.. and just think of a goroutine as a concurrently executing piece of code, without a "thread".