Extracted from #2968 (comment):

@gleb-cloudius wrote:
backpresure mechanism in sink_impl<Serializer, Out...>::operator() does not work as expected. It uses semaphore _sem to limit the amount of work on the other shard but units are released before foreign_ptr is freed, so another shard may accumulate a lot of them.
The solution as I see it is to make semaphore_units lifetime to be the same as foreign_ptr (by holding it in the snd_buf for instance).