class method self.lock

# File actionmailer/lib/action_mailer/vendor/tmail-1.2.3/tmail/mailbox.rb, line 222
    def UNIXMbox.lock( fname )
      begin
        f = File.open(fname, 'r+')
        f.flock File::LOCK_EX
        yield f
      ensure
        f.flock File::LOCK_UN
        f.close if f and not f.closed?
      end
    end