Implement review comments for !12445 from @godfat and @rymai.

- Use `GlobalPolicy` to authorize the users that a non-authenticated user can
  fetch from `/api/v4/users`. We allow access if the `Gitlab::VisibilityLevel::PUBLIC`
  visibility level is not restricted.

- Further, as before, `/api/v4/users` is only accessible to unauthenticated users if
  the `username` parameter is passed.

- Turn off `authenticate!` for the `/api/v4/users` endpoint by matching on the actual
  route + method, rather than the description.

- Change the type of `current_user` check in `UsersFinder` to be more
  compatible with EE.

2 files changed, 13 insertions, 17 deletions

diff --git a/lib/api/helpers.rb b/lib/api/helpers.rb
index 1322afaa64f..a3aec8889d7 100644
--- a/lib/api/helpers.rb
+++ b/lib/api/helpers.rb
@@ -410,8 +410,8 @@ module API
 
     # Does the current route match the route identified by
     # `description`?
-    def route_matches_description?(description)
-      options.dig(:route_options, :description) == description
+    def request_matches_route?(method, route)
+      request.request_method == method && request.path == route
     end
   end
 end
diff --git a/lib/api/users.rb b/lib/api/users.rb
index 34619c90d8b..18ce58299e7 100644
--- a/lib/api/users.rb
+++ b/lib/api/users.rb
@@ -4,7 +4,7 @@ module API
 
     before do
       allow_access_with_scope :read_user if request.get?
-      authenticate! unless route_matches_description?("Get the list of users")
+      authenticate! unless request_matches_route?('GET', '/api/v4/users')
     end
 
     resource :users, requirements: { uid: /[0-9]*/, id: /[0-9]*/ } do
@@ -55,22 +55,18 @@ module API
 
         users = UsersFinder.new(current_user, params).execute
 
-        authorized =
-          if current_user
-            can?(current_user, :read_users_list)
-          else
-            # When `current_user` is not present, require that the `username`
-            # parameter is passed, to prevent an unauthenticated user from accessing
-            # a list of all the users on the GitLab instance. `UsersFinder` performs
-            # an exact match on the `username` parameter, so we are guaranteed to
-            # get either 0 or 1 `users` here.
-            params[:username].present? &&
-              users.all? { |user| can?(current_user, :read_user, user) }
-          end
+        authorized = can?(current_user, :read_users_list)
+
+        # When `current_user` is not present, require that the `username`
+        # parameter is passed, to prevent an unauthenticated user from accessing
+        # a list of all the users on the GitLab instance. `UsersFinder` performs
+        # an exact match on the `username` parameter, so we are guaranteed to
+        # get either 0 or 1 `users` here.
+        authorized &&= params[:username].present? if current_user.blank?
 
-        render_api_error!("Not authorized.", 403) unless authorized
+        forbidden!("Not authorized to access /api/v4/users") unless authorized
 
-        entity = current_user.try(:admin?) ? Entities::UserWithAdmin : Entities::UserBasic
+        entity = current_user&.admin? ? Entities::UserWithAdmin : Entities::UserBasic
         present paginate(users), with: entity
       end