GAE Go用户服务登录网址包含％A（MISSING）

Question

So this one's been puzzling me for a little while!

Here's some code in the console:

 func GetLoginLinks(w *http.ResponseWriter, r *http.Request) (string, error) {
    c := appengine.NewContext(r)
    u := user.Current(c)
    if u == nil {
        url, err := user.LoginURL(c, r.URL.String())
        if err != nil {
            http.Error(*w, err.Error(), http.StatusInternalServerError)
            return "", err
        }

        c.Debugf("Return url: " + r.URL.String())
        c.Debugf("login url: " + url)
        c.Debugf("url type: %T", url)
        v := LoginItem(url, "Login")
        return v, nil
    }
 }

It gives the following output:

2013/06/17 21:48:28 DEBUG: Return url: /
2013/06/17 21:48:28 DEBUG: login url: /_ah/login?continue=http%A(MISSING)//localhost%A(MISSING)8080/
2013/06/17 21:48:28 DEBUG: url type: string

This also fails when uploaded to the app engine itself.

What I'm struggling with is the first 4 lines of the function comes directly from the developer guide.

Answer 1

package fmt

Format errors:

If an invalid argument is given for a verb, such as providing a string to %d, the generated string will contain a description of the problem, as in these examples:

Wrong type or unknown verb: %!verb(type=value)
  Printf("%d", hi):          %!d(string=hi)
Too many arguments: %!(EXTRA type=value)
  Printf("hi", "guys"):      hi%!(EXTRA string=guys)
Too few arguments: %!verb(MISSING)
  Printf("hi%d"):            hi %!d(MISSING)
Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC)
  Printf("%*s", 4.5, "hi"):  %!(BADWIDTH)hi
  Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi

All errors begin with the string "%!" followed sometimes by a single character (the verb) and end with a parenthesized description.

If an Error or String method triggers a panic when called by a print routine, the fmt package reformats the error message from the panic, decorating it with an indication that it came through the fmt package. For example, if a String method calls panic("bad"), the resulting formatted message will look like

%s(PANIC=bad)

The %s just shows the print verb in use when the failure occurred.

You have an invalid format string. You have an escaped URL query string in a format string: ":" is escaped to "%3A". The format arguments for the "%3A" verbs are missing. For safety, never use an arbitrary string as a format string. For example,

package main

import "fmt"

func main() {
    url := "/_ah/login?continue=http%3A//localhost%3A8080/"
    fmt.Printf("login url: " + url)
    fmt.Println()
    fmt.Printf("login url: %s", url)
    fmt.Println()
}

Output:

login url: /_ah/login?continue=http%A(MISSING)//localhost%A(MISSING)8080/
login url: /_ah/login?continue=http%3A//localhost%3A8080/

Write:

c.Debugf("login url: %s", url)