如何对errors.Wrap创建的错误使用errors.Is？

Question

我喜欢这样做：

var ErrMyCustomError = errors.New("something went wrong")

func doAThing(input string) error {
    if input == "bad input" {
        return ErrMyCustomError
    }
    return nil
}

然后在我的测试中：

func TestFailCase() {
    err := doAThing("bad input")
    require.True(errors.Is(err, ErrMyCustomError)
}

我的问题是，我如何做到这一切，而不是让return ErrMyCustomError在ErrMyCustomError中包装我从其他地方获得的错误(以便errors.Is仍然有效)。

func doAThing(input string) error {
    err := doSomething(input)
    if err != nil {
        return errors.Wrap(err, "some context message") // <-- this line here needs to return an ErrMyCustomError somehow but also wrap err
    }
}

// Imagine this is in some third party lib and I can't alter it.
func doSomething(input string) error {
    if input == "bad input" {
        return errors.New("some error I will later wrap")
    }
    return nil
}

func TestFailCase() {
    err := doAThing("bad input")
    require.True(errors.Is(err, ErrMyCustomError) // <-- this is no longer true but I want it to be
}

Answer 1

这是我能想到的最好的办法了。我更喜欢只使用var MyCustomErr = errors.New("something went wrong")，但是这样我就没有办法把它放入错误链中。我更喜欢使用errors.Is，而不是声明一个接收器指针来检查errors.As，但是到目前为止，我还没有找到这样做的方法。

package main

import (
    "errors"
    "fmt"
)

type MyCustomError struct {
    Err   error
}

func (e MyCustomError) Error() string {
    return "something went wrong: " + e.Err.Error()
}

func main() {
    err := myWrapper()
    ptr := &MyCustomError{}
    print(errors.As(err, ptr)) // prints "true"
}

func myWrapper() error {
    err := thirdPartyFunc()
    if err != nil {
        return fmt.Errorf("some additional context: %w", MyCustomError{Err: err})
    }
    return nil
}

func thirdPartyFunc() error {
    return errors.New("this is the error I want to wrap")
}