错误包装/展开&&使用errors.Is()进行类型检查

Question

我正在检查 Go v1.13 Go v1.14 中的错误跟踪。为什么看起来只能找到不带参数或带值接收器的错误实现errors.Is()？这意味着能够包装的错误实现必须有一个值接收器，以便能够通过 找到errors.Is()。

package main

import (
    "fmt"
    "errors"
)

type someAtomicError struct {}
func (e *someAtomicError) Error() string { return "Hi!" }
func checkAtomicError() {
    e := &someAtomicError{}
    e2 := fmt.Errorf("whoa!: %w", e)
    e2IsE := errors.Is(e2, &someAtomicError{})
    fmt.Println("atomic error trace ---\t\t", e2, "\t\t--- is traceable: ", e2IsE)
}


type someWrapperError struct {
    Msg string
    Err error
}
func (e someWrapperError) Error() string { return fmt.Sprintf("%s: %v", e.Msg, e.Err) }
func (e someWrapperError) Unwrap() error { return e.Err }
func checkWrapperError() {
    e := someWrapperError{"Hi!", nil}
    e2 := fmt.Errorf("whoa!: %w", e)
    e2IsE := errors.Is(e2, someWrapperError{"Hi!", nil})
    fmt.Println("wrapper error trace ---\t\t", e2, "\t--- is traceable: ", e2IsE)
}


type somePointerWrapperError struct {
    Msg string
    Err error
}
func (e *somePointerWrapperError) Error() string { return fmt.Sprintf("%s: %v", e.Msg, e.Err) }
func (e *somePointerWrapperError) Unwrap() error { return e.Err }
func checkPointerWrapperError() {
    e := &somePointerWrapperError{"Hi!", nil}
    e2 := fmt.Errorf("whoa!: %w", e)
    e2IsE := errors.Is(e2, &somePointerWrapperError{"Hi!", nil})
    fmt.Println("pointer wrapper error trace ---\t", e2, "\t--- is traceable: ", e2IsE)
}


func main() {
    checkAtomicError()
    checkWrapperError() 
    checkPointerWrapperError()
}

//atomic error trace ---         whoa!: Hi!         --- is traceable:  true
//wrapper error trace ---        whoa!: Hi!: <nil>  --- is traceable:  true
//pointer wrapper error trace ---    whoa!: Hi!: <nil>  --- is traceable:  false

https://play.golang.org/p/-hSukZ-gii2

似乎参数中的任何差异（包括包装的错误参数 ）Err都会导致无法使用 找到该类型errors.Is()。

Answer 1

false您在尝试在另一个过孔中查找一个错误时遇到的原因errors.Is是，虽然这两个错误可能具有相同的字段值，但它们是两个不同的内存指针：

e  := &somePointerWrapperError{"Hi!", nil}
e2 := &somePointerWrapperError{"Hi!", nil} // e2 != e

ew := fmt.Errorf("whoa!: %w", e)

errors.Is(ew, e)  // true
errors.Is(ew, e2) // false - because `ew` wraps `e` not `e2`

那么如何检测这种“类型”的错误并获取其值：errors.As改为使用：

e := &somePointerWrapperError{"Hi!", nil}
e2 := fmt.Errorf("whoa!: %w", e)

var ev *somePointerWrapperError

if errors.As(e2, &ev) {
    fmt.Printf("%#v\n", ev) // &somePointerWrapperError{Msg:"Hi!", Err:error(nil)}
}

https://play.golang.org/p/CttKThLasXD

Answer 2

远程相关，但也许它对某人有帮助：我花了一些时间才意识到errors.As(...)实际上需要一个指向目标的双指针，而errors.Is(...)没有：

var _ error = (*CustomError)(nil) // ensure CustomError implements error

type CustomError struct {
    msg string
}

func (e CustomError) Error() string {
    return e.msg
}

func main() {
    err := &CustomError{"Hello, world!"} // Methods return pointers to errors, allowing them to be nil
    
    var eval *CustomError

    as := errors.As(err, &eval) // yes, that's **CustomError
    asFaulty := errors.As(err, eval) // no compile error, so it wrongly seems okay
    is := errors.Is(err, eval) // that's just *CustomError

    fmt.Printf("as: %t, asFaulty: %t, is: %t", as, asFaulty, is) // as: true, asFaulty: false, is: true
}