Simplify auth.Challenge interface to SetHeaders

This removes the erroneous http.Handler interface in favor a simple SetHeaders
method that only operattes on the response. Several unnecessary uses of pointer
types were also fixed up.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/auth/silly/access.go

@@ -75,7 +75,10 @@ type challenge struct {
 	scope   string
 }
 
-func (ch *challenge) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+var _ auth.Challenge = challenge{}
+
+// SetHeaders sets a simple bearer challenge on the response.
+func (ch challenge) SetHeaders(w http.ResponseWriter) {
 	header := fmt.Sprintf("Bearer realm=%q,service=%q", ch.realm, ch.service)
 
 	if ch.scope != "" {
@@ -85,7 +88,7 @@ func (ch *challenge) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("WWW-Authenticate", header)
 }
 
-func (ch *challenge) Error() string {
+func (ch challenge) Error() string {
 	return fmt.Sprintf("silly authentication challenge: %#v", ch)
 }
 

docs/auth/silly/access_test.go

@@ -21,7 +21,7 @@ func TestSillyAccessController(t *testing.T) {
 		if err != nil {
 			switch err := err.(type) {
 			case auth.Challenge:
-				err.ServeHTTP(w, r)
+				err.SetHeaders(w)
 				w.WriteHeader(http.StatusUnauthorized)
 				return
 			default: