解释Golang错误代码

Question

So most examples of go error handling I see just pass any errors back up the stack. At some point these need interpreting and this is what I am trying to do. Here's a snippet of my attempt:

    resp, err := http.Get(string(url))
    defer out_count.Dec()

    if err != nil {

           switch err {
            case http.ErrBodyReadAfterClose:
                    fmt.Println("Read after close error")
            case http.ErrMissingFile:
                    fmt.Println("Missing File")
            {some more cases here}
            case io.EOF:
                    fmt.Println("EOF error found")
            default:
                    fmt.Printf("Error type is %T
", err)
                    panic(err)
            }
            return

This isn't working though for my current case(edited to remove url}:

ERROR: Failed to crawl "http://{removed URL}"
Error type is *url.Error
panic: Get http://{removed url}: EOF

goroutine 658 [running]:
runtime.panic(0x201868, 0x106352c0)
        /usr/lib/go/src/pkg/runtime/panic.c:279 +0x1a0
github.com/cbehopkins/grab/grab.crawl(0x10606210, 0x27, 0x105184b0, 0x105184e0, 0x10500460)

I can't figure out a way to get the switch statement to catch this error since the text of the error changes every time and has no explicit value I can catch against. (as the URL changes all the time). Now maybe I could do some sort of regex match in the case statement or sub-slice the error string, but that feels like a very bad way to solve this problem.

Any suggestions? There must be an idiomatic way to catch errors such as this surely?

Answer 1

The simplest way would be to have package level error values in your code:

var URLFetchError = errors.New("Cannot fetch URL")

url := "http://www.google.com"
res, err := http.Get(url)
if err != nil {
    return URLFetchError
}

The switch then becomes:

switch err {
case http.ErrBodyReadAfterClose:
    fmt.Println("Read after close error")
case URLFetchError:
    fmt.Println("Error fetching URL")

If you want to pass more information with the error, you can create your own custom errors:

type MyError struct {
    URL string
}

func (e MyError) Error() string {
    return fmt.Sprintf("Error getting: %v", e.URL)
}

Then, you can create this error whenever required. For example:

url := "http://www.google.com"
res, err := http.Get(url)
if err != nil {
    return MyError{url}
}

Finally, in your error checking method, you can use type switches instead of simple switches to get the error:

switch err.(type) {
case MyError:
    fmt.Println("Error:", err)
default:
    fmt.Println("No Error")
}

In your case, since you have a mix of regular error, you can include this check in a nested switch:

switch err {
case http.ErrBodyReadAfterClose:
    fmt.Println("Read after close error")
case http.ErrMissingFile:
    fmt.Println("Missing File")
case io.EOF:
    fmt.Println("EOF error found")
default: // check for custom errors
    switch err.(type) {
    case MyError:
        fmt.Println("custom error:", err)
    default:
        panic(err)
    }
}