mastodon/app/services/process_mentions_service.rb

class ProcessMentionsService < BaseService
  # Scan status for mentions and fetch remote mentioned users, create
  # local mention pointers, send Salmon notifications to mentioned
  # remote users
  # @param [Status] status
  def call(status)
    return unless status.local?

    status.text.scan(Account::MENTION_RE).each do |match|
      username, domain  = match.first.split('@')
      mentioned_account = Account.find_remote(username, domain)

      if mentioned_account.nil? && !domain.nil?
        mentioned_account = follow_remote_account_service.("#{match.first}")
        next if mentioned_account.nil?
      end

      mentioned_account.mentions.where(status: status).first_or_create(status: status)
    end

    status.mentions.each do |mention|
      mentioned_account = mention.account

      if mentioned_account.local?
        NotificationMailer.mention(mentioned_account, status).deliver_later
      else
        NotificationWorker.perform_async(status.stream_entry.id, mentioned_account.id)
      end
    end
  end

  private

  def follow_remote_account_service
    @follow_remote_account_service ||= FollowRemoteAccountService.new
  end
end