client/web: fix serveAPIAuth in Login mode

In Login mode, must first run system auth. But once authorized,
should be able to reach rest of auth logic to check whether the
user can manage the node. This results in showing/hiding the
sign in button in the frontend login toggle.

Updates #10261

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/web.go

@@ -400,11 +400,22 @@ type viewerIdentity struct {
 // and returns an authResponse indicating the current auth state and any steps the user needs to take.
 func (s *Server) serveAPIAuth(w http.ResponseWriter, r *http.Request) {
 	var resp authResponse
+	session, whois, status, sErr := s.getSession(r)
 
-	session, whois, status, err := s.getSession(r)
-	switch {
-	case s.mode == LoginServerMode || errors.Is(err, errNotUsingTailscale):
-		// not using tailscale, so perform platform auth
+	if whois != nil {
+		resp.ViewerIdentity = &viewerIdentity{
+			LoginName:     whois.UserProfile.LoginName,
+			NodeName:      whois.Node.Name,
+			ProfilePicURL: whois.UserProfile.ProfilePicURL,
+		}
+		if addrs := whois.Node.Addresses; len(addrs) > 0 {
+			resp.ViewerIdentity.NodeIP = addrs[0].Addr().String()
+		}
+	}
+
+	// First verify platform auth.
+	// If platform auth is needed, this should happen first.
+	if s.mode == LoginServerMode {
 		switch distro.Get() {
 		case distro.Synology:
 			authorized, err := authorizeSynology(r)
@@ -414,6 +425,8 @@ func (s *Server) serveAPIAuth(w http.ResponseWriter, r *http.Request) {
 			}
 			if !authorized {
 				resp.AuthNeeded = synoAuth
+				writeJSON(w, resp)
+				return
 			}
 		case distro.QNAP:
 			if _, err := authorizeQNAP(r); err != nil {
@@ -423,21 +436,24 @@ func (s *Server) serveAPIAuth(w http.ResponseWriter, r *http.Request) {
 		default:
 			// no additional auth for this distro
 		}
-	case err != nil && errors.Is(err, errNotOwner):
+	}
+
+	switch {
+	case sErr != nil && errors.Is(sErr, errNotOwner):
 		// Restricted to the readonly view, no auth action to take.
 		s.lc.IncrementCounter(r.Context(), "web_client_viewing_not_owner", 1)
 		resp.AuthNeeded = ""
-	case err != nil && errors.Is(err, errTaggedLocalSource):
+	case sErr != nil && errors.Is(sErr, errTaggedLocalSource):
 		// Restricted to the readonly view, no auth action to take.
 		s.lc.IncrementCounter(r.Context(), "web_client_viewing_local_tag", 1)
 		resp.AuthNeeded = ""
-	case err != nil && errors.Is(err, errTaggedRemoteSource):
+	case sErr != nil && errors.Is(sErr, errTaggedRemoteSource):
 		// Restricted to the readonly view, no auth action to take.
 		s.lc.IncrementCounter(r.Context(), "web_client_viewing_remote_tag", 1)
 		resp.AuthNeeded = ""
-	case err != nil && !errors.Is(err, errNoSession):
+	case sErr != nil && !errors.Is(sErr, errNoSession):
 		// Any other error.
-		http.Error(w, err.Error(), http.StatusInternalServerError)
+		http.Error(w, sErr.Error(), http.StatusInternalServerError)
 		return
 	case session.isAuthorized(s.timeNow()):
 		if whois.Node.StableID == status.Self.ID {
@@ -451,16 +467,6 @@ func (s *Server) serveAPIAuth(w http.ResponseWriter, r *http.Request) {
 		resp.AuthNeeded = tailscaleAuth
 	}
 
-	if whois != nil {
-		resp.ViewerIdentity = &viewerIdentity{
-			LoginName:     whois.UserProfile.LoginName,
-			NodeName:      whois.Node.Name,
-			ProfilePicURL: whois.UserProfile.ProfilePicURL,
-		}
-		if addrs := whois.Node.Addresses; len(addrs) > 0 {
-			resp.ViewerIdentity.NodeIP = addrs[0].Addr().String()
-		}
-	}
 	writeJSON(w, resp)
 }