ProFTPD module mod_counter

 The mod_counter module is designed to allow a sort of "locking" to be
 enforced when the same file is being uploaded or downloaded by multiple
 clients at the same time.
 .
 The mod_counter works by creating a SysV semaphore for a file being
 read/written, and placing a number of reader or writer "counters" in that
 semaphore. When the configured maximum number of counters is reached, the
 FTP command which seeks to add another reader/writer counter will be denied.
 This allows site to configure the maximum number of clients which can be
 reading/writing any file at one time.