Golang invalid character 'b' looking for beginning of value

Question

I am trying to post json with a xml message inside it. However it returns:

invalid character 'b' looking for beginning of value

I think the possible reason is that I am trying to marshal the returns body which is not in the json format.

func (s *BackendConfiguration) Do(req *http.Request, v interface{}) error {
    log.Printf("Requesting %v %v%v\n", req.Method, req.URL.Host, req.URL.Path)
    start := time.Now()

    res, err := s.HTTPClient.Do(req)

    if debug {
        log.Printf("Completed in %v\n", time.Since(start))
    }

    if err != nil {
        log.Printf("Request to sakura failed: %v\n", err)
        return err
    }
    defer res.Body.Close()

    resBody, err := ioutil.ReadAll(res.Body)
    if err != nil {
        log.Printf("Cannot parse sakura response: %v\n", err)
        return err
    }

    if debug {
        log.Printf("sakura response: %q\n", resBody)
    }

    if v != nil {
        return json.Unmarshal(resBody, v)
    }

    return nil
}

The error happened at this line:

return json.Unmarshal(resBody, v)

Answer 1

The error indicates that the server did not return a valid JSON response. I suggest adding the following code to debug the issue:

err := json.Unmarshal(resBody, v)
if err != nil {
    log.Printf("error decoding sakura response: %v", err)
    if e, ok := err.(*json.SyntaxError); ok {
        log.Printf("syntax error at byte offset %d", e.Offset)
    }
    log.Printf("sakura response: %q", resBody)
    return err
}

Answer 2

Reason for this eerie error message is :

// When unmarshaling quoted strings, invalid UTF-8 or
// invalid UTF-16 surrogate pairs are not treated as an error.
// Instead, they are replaced by the Unicode replacement
// character U+FFFD.

https://golang.org/src/encoding/json/decode.go

In my case I saved my json as string then parsed it by : stringData = JSON.parse(myJsonString)

I also had the same error another time using gin-context-ShouldBind() (https://godoc.org/github.com/gin-gonic/gin#Context.ShouldBind) and mapping my json to go object: error was because it needs a json as string, so I used : JSON.stringify(jsonObject) when sending my request from front-end part.

Answer 3

Please Check the request header first. Check the Content-Type. In my cases, it's not an application/JSON. It's application/x-www-form-urlencoded.