Don't leak private group existence by redirecting from namespace controller to group controller.

CHANGELOG

@@ -31,6 +31,7 @@ v 7.10.0 (unreleased)
   - Replace commits calendar with faster contribution calendar that includes issues and merge requests
   - Add inifinite scroll to user page activity
   - Don't show commit comment button when user is not signed in.
+  - Don't leak private group existence by redirecting from namespace controller to group controller.
 v 7.9.0
   - Send EmailsOnPush email when branch or tag is created or deleted.

app/controllers/namespaces_controller.rb

@@ -4,14 +4,22 @@ class NamespacesController < ApplicationController
   def show
     namespace = Namespace.find_by(path: params[:id])
-    unless namespace
-      return render_404
+    if namespace
+      if namespace.is_a?(Group)
+        group = namespace
+      else
+        user = namespace.owner
+      end
     end
-    if namespace.type == "Group"
-      redirect_to group_path(namespace)
+    if user
+      redirect_to user_path(user)
+    elsif group && can?(current_user, :read_group, group)
+      redirect_to group_path(group)
+    elsif current_user.nil?
+      authenticate_user!
     else
-      redirect_to user_path(namespace.owner)
+      render_404
     end
   end
 end

app/models/concerns/mentionable.rb

@@ -52,7 +52,7 @@ module Mentionable
       if identifier == "all"
         users.push(*project.team.members.flatten)
       elsif namespace = Namespace.find_by(path: identifier)
-        if namespace.type == "Group"
+        if namespace.is_a?(Group)
           users.push(*namespace.users)
         else
           users << namespace.owner

lib/gitlab/markdown.rb

@@ -221,7 +221,7 @@ module Gitlab
         link_to("@all", namespace_project_url(project.namespace, project), options)
       elsif namespace = Namespace.find_by(path: identifier)
         url =
-          if namespace.type == "Group"
+          if namespace.is_a?(Group)
             group_url(identifier)
           else 
             user_url(identifier)