refactor auth interface to return error when verify failure (#22119)

This PR changed the Auth interface signature from 
`Verify(http *http.Request, w http.ResponseWriter, store DataStore, sess
SessionStore) *user_model.User`
to 
`Verify(http *http.Request, w http.ResponseWriter, store DataStore, sess
SessionStore) (*user_model.User, error)`.

There is a new return argument `error` which means the verification
condition matched but verify process failed, we should stop the auth
process.

Before this PR, when return a `nil` user, we don't know the reason why
it returned `nil`. If the match condition is not satisfied or it
verified failure? For these two different results, we should have
different handler. If the match condition is not satisfied, we should
try next auth method and if there is no more auth method, it's an
anonymous user. If the condition matched but verify failed, the auth
process should be stop and return immediately.

This will fix #20563

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: Jason Song <i@wolfogre.com>

services/auth/reverseproxy.go

@@ -51,10 +51,10 @@ func (r *ReverseProxy) Name() string {
 // If a username is available in the "setting.ReverseProxyAuthUser" header an existing
 // user object is returned (populated with username or email found in header).
 // Returns nil if header is empty.
-func (r *ReverseProxy) getUserFromAuthUser(req *http.Request) *user_model.User {
+func (r *ReverseProxy) getUserFromAuthUser(req *http.Request) (*user_model.User, error) {
 	username := r.getUserName(req)
 	if len(username) == 0 {
-		return nil
+		return nil, nil
 	}
 	log.Trace("ReverseProxy Authorization: Found username: %s", username)
 
@@ -62,11 +62,11 @@ func (r *ReverseProxy) getUserFromAuthUser(req *http.Request) *user_model.User {
 	if err != nil {
 		if !user_model.IsErrUserNotExist(err) || !r.isAutoRegisterAllowed() {
 			log.Error("GetUserByName: %v", err)
-			return nil
+			return nil, err
 		}
 		user = r.newUser(req)
 	}
-	return user
+	return user, nil
 }
 
 // getEmail extracts the email from the "setting.ReverseProxyAuthEmail" header
@@ -106,12 +106,15 @@ func (r *ReverseProxy) getUserFromAuthEmail(req *http.Request) *user_model.User
 // First it will attempt to load it based on the username (see docs for getUserFromAuthUser),
 // and failing that it will attempt to load it based on the email (see docs for getUserFromAuthEmail).
 // Returns nil if the headers are empty or the user is not found.
-func (r *ReverseProxy) Verify(req *http.Request, w http.ResponseWriter, store DataStore, sess SessionStore) *user_model.User {
-	user := r.getUserFromAuthUser(req)
+func (r *ReverseProxy) Verify(req *http.Request, w http.ResponseWriter, store DataStore, sess SessionStore) (*user_model.User, error) {
+	user, err := r.getUserFromAuthUser(req)
+	if err != nil {
+		return nil, err
+	}
 	if user == nil {
 		user = r.getUserFromAuthEmail(req)
 		if user == nil {
-			return nil
+			return nil, nil
 		}
 	}
 
@@ -124,7 +127,7 @@ func (r *ReverseProxy) Verify(req *http.Request, w http.ResponseWriter, store Da
 	store.GetData()["IsReverseProxy"] = true
 
 	log.Trace("ReverseProxy Authorization: Logged in user %-v", user)
-	return user
+	return user, nil
 }
 
 // isAutoRegisterAllowed checks if EnableReverseProxyAutoRegister setting is true