Discourse is an open-source discussion platform. Prior to versions "3.3.4" on the "stable" branch and "3.4.0.beta5" on the "beta" branch, someone who is about to reach the limit of users in a group DM may send requests to add new users in parallel. The requests might all go through ignoring the limit due to a race condition. The patch in versions "3.3.4" and "3.4.0.beta5" uses the "lock" step in service to wrap part of the "add_users_to_channel" service inside a distributed lock/mutex in order to avoid the race condition.