proftpd-mod-counter - 0.6.1-2 main

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.
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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.