Memory leak when using flatMapPrefix/prefixAndTail with Concat

[queimadus]

When using flatMapPrefix or prefixAndTail (or any other flow that closes over data) followed by concat the accumulated portion doesn't seem to be GC'd.

I reduced a problem I found into this snippet:

val s = Source
  .repeat(())
  .map(_ => ByteString('a' * 40000))
  .take(1000000)
  .flatMapPrefix(50000) { prefix => Flow[ByteString] } 

Source.empty
  .concatAllLazy(List.tabulate(30000)(_ => s): _*)
  .runWith(Sink.ignore).onComplete(println(_))  

If we don't use concat, it runs fine:

  Source
    .repeat(s)
    .take(30000)
    .flatMapConcat(x => x)
    .runWith(Sink.ignore)
    .onComplete(println(_))