Decode a JSON response while also adding the JSON response body to the unmarshalling error.

decode_json_response.go

// DecodeJSONResp decodes the body of the given response into the given target.
// If an error occurs, instead of only returning the unmarshaling error, the error will also contain the first 100 chars
// of the response JSON. This allows for better debugging.
func DecodeJSONResp(resp *http.Response, target interface{}) error {
	pr, pw := io.Pipe()
	tr := io.TeeReader(resp.Body, pw)
	respCh := make(chan string)
	errCh := make(chan error)

	go func() {
		firstChars := make([]byte, 100)
		defer func() {
			_ = pr.Close()
			respCh <- string(firstChars)
			close(errCh)
		}()

		_, err := io.ReadFull(pr, firstChars)
		if err == io.EOF || err == io.ErrUnexpectedEOF {
			return
		} else if err != nil {
			errCh <- err
			return
		}

		// Discard the rest; we are only interested in the first x chars.
		_, err = io.Copy(io.Discard, pr)
		if err != nil {
			errCh <- err
			return
		}
	}()

	var err error
	decoder := json.NewDecoder(tr)
	for {
		err = decoder.Decode(target)
		if err != nil {
			break
		}
	}
	_ = pw.Close()
	firstChars := <-respCh
	if err != io.EOF {
		err2 := <-errCh
		if err2 != nil {
			return errors.Errorf("url '%s': %v (response reading error: %v, response body starting with '%s')", resp.Request.URL.String(), err, err2, firstChars)
		}
		return errors.Errorf("url '%s': %v (response body starting with '%s')", resp.Request.URL.String(), err, firstChars)
	}

	if resp.StatusCode >= 300 {
		return errors.Errorf("url '%s': response status was %d; body was %s", resp.Request.URL.String(), resp.StatusCode, firstChars)
	}

	return nil
}