Go Quiz: 从Go面试题看函数命名返回值的注意事项(超过80%的人都回答错了)

题目

Redhat的首席工程师、Prometheus开源项目Maintainer Bartłomiej Płotka 在Twitter上出了一道Go编程题，结果超过80%的人都回答错了。

题目如下所示，回答下面这段程序的输出结果。

// named_return.go
package main

import "fmt"

func aaa() (done func(), err error) {
    return func() { print("aaa: done") }, nil
}

func bbb() (done func(), _ error) {
    done, err := aaa()
    return func() { print("bbb: surprise!"); done() }, err
}

func main() {
    done, _ := bbb()
    done()
}

• A: bbb: surprise!
• B: bbb: surprise!aaa: done
• C: 编译报错
• D: 递归栈溢出

大家可以先思考下这段代码的输出结果是什么。

解析

在函数bbb最后执行return语句，会对返回值变量done进行赋值，

done := func() { print("bbb: surprise!"); done() }

注意：闭包func() { print("bbb: surprise!"); done() }里的done并不会被替换成done, err := aaa()里的done的值。

因此函数bbb执行完之后，返回值之一的done实际上成为了一个递归函数，先是打印"bbb: surprise!"，然后再调用自己，这样就会陷入无限递归，直到栈溢出。因此本题的答案是D。

那为什么函数bbb最后return的闭包func() { print("bbb: surprise!"); done() }里的done并不会被替换成done, err := aaa()里的done的值呢？如果替换了，那本题的答案就是B了。

这个时候就要搬出一句老话了：

This is a feature, not a bug

我们可以看下面这个更为简单的例子，来帮助我们理解：

// named_return1.go
package main

import "fmt"

func test() (done func()) {
    return func() { fmt.Println("test"); done() }
}

func main() {
    done := test()
    // 下面的函数调用会进入死循环，不断打印test
    done()
}

正如上面代码里的注释说明，这段程序同样会进入无限递归直到栈溢出。

如果函数test最后return的闭包func() { fmt.Println("test"); done() }里的done是被提前解析了的话，因为done是一个函数类型，done的零值是nil，那闭包里的done的值就会是nil，执行nil函数是会引发panic的。

但实际上Go设计是允许上面的代码正常执行的，因此函数test最后return的闭包里的done的值并不会提前解析，test函数执行完之后，实际上产生了下面的效果，返回的是一个递归函数，和本文开始的题目一样。

done := func() { fmt.Println("test"); done() }

因此也会进入无限递归，直到栈溢出。

总结

这个题目其实很tricky，在实际编程中，要避免对命名返回值采用这种写法，非常容易出错。

想了解国外Go开发者对这个题目的讨论详情可以参考Go Named Return Parameters Discussion。

另外题目作者也给了如下所示的解释，原文地址可以参考详细解释：

package main

func aaa() (done func(), err error) {
    return func() { print("aaa: done") }, nil
}

func bbb() (done func(), _ error) {
    // NOTE(bwplotka): Here is the problem. We already defined special "return argument" variable called "done".
    // By using `:=` and not `=` we define a totally new variable with the same name in
    // new, local function scope.
    done, err := aaa()

    // NOTE(bwplotka): In this closure (anonymous function), we might think we use `done` from the local scope,
    // but we don't! This is because Go "return" as a side effect ASSIGNS returned values to
    // our special "return arguments". If they are named, this means that after return we can refer
    // to those values with those names during any execution after the main body of function finishes
    // (e.g in defer or closures we created).
    //
    // What is happening here is that no matter what we do in the local "done" variable, the special "return named"
    // variable `done` will get assigned with whatever was returned. Which in bbb case is this closure with
    // "bbb:surprise" print. This means that anyone who runs this closure AFTER `return` did the assignment
    // will start infinite recursive execution.
    //
    // Note that it's a feature, not a bug. We use this often to capture
    // errors (e.g https://github.com/efficientgo/tools/blob/main/core/pkg/errcapture/doc.go)
    //
    // Go compiler actually detects that `done` variable defined above is NOT USED. But we also have `err`
    // variable which is actually used. This makes compiler to satisfy that unused variable check,
    // which is wrong in this context..
    return func() { print("bbb: surprise!"); done() }, err
}

func main() {
    done, _ := bbb()
    done()
}

不过这个解释是有瑕疵的，主要是这句描述：

By using := and not = we define a totally new variable with the same name in
new, local function scope.

对于done, err := aaa()，返回变量done并不是一个新的变量，而是和函数bbb的返回变量done是同一个变量。

这里有一个小插曲：本人把这个瑕疵反馈给了原作者，原作者同意了我的意见，删除了这块解释。

最新版的英文解释如下，原文地址可以参考修正版解释。

package main

func aaa() (done func()) {
    return func() { print("aaa: done") }
}

func bbb() (done func()) {
    done = aaa()

    // NOTE(bwplotka): In this closure (anonymous function), we might think we use `done` value assigned to aaa(),
    // but we don't! This is because Go "return" as a side effect ASSIGNS returned values to
    // our special "return arguments". If they are named, this means that after return we can refer
    // to those values with those names during any execution after the main body of function finishes
    // (e.g in defer or closures we created).
    //
    // What is happening here is that no matter what we do with our "done" variable, the special "return named"
    // variable `done` will get assigned with whatever was returned when the function ends.
    // Which in bbb case is this closure with "bbb:surprise" print. This means that anyone who runs
    // this closure AFTER `return` did the assignment, will start infinite recursive execution.
    //
    // Note that it's a feature, not a bug. We use this often to capture
    // errors (e.g https://github.com/efficientgo/tools/blob/main/core/pkg/errcapture/doc.go)
    return func() { print("bbb: surprise!"); done() }
}

func main() {
    done := bbb()
    done()
}

思考题

下面这段代码同样使用了命名返回值，大家可以看看这个道题的输出结果是什么。可以给微信公众号发送消息nrv获取答案。

package main

func bar() (r int) {
    defer func() {
        r += 4
        if recover() != nil {
            r += 8
        }
    }()
    
    var f func()
    defer f()
    f = func() {
        r += 2
    }

    return 1
}

func main() {
    println(bar())
}

开源地址

文章和示例代码开源在GitHub: Go语言初级、中级和高级教程。

公众号：coding进阶。关注公众号可以获取最新Go面试题和技术栈。

个人网站：Jincheng's Blog。

知乎：无忌。

References

• https://twitter.com/bwplotka/...
• https://go.dev/play/p/ELPEi2A...
• https://go.dev/play/p/9J5a3Zt...