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fix: Fix a leak in PrefixAndTail operator. #1623

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fix: Fix a leak in PrefixAndTail operator. #1623

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4 changes: 3 additions & 1 deletion stream/src/main/scala/org/apache/pekko/stream/impl/fusing/StreamOfStreams.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -237,7 +237,9 @@ import pekko.util.ccompat.JavaConverters._
override def onUpstreamFinish(): Unit = {
if (!prefixComplete) {
// This handles the unpulled out case as well
emit(out, (builder.result(), Source.empty), () => completeStage())
val prefix = builder.result();
builder = null // free for GC
emit(out, (prefix, Source.empty), () => completeStage())
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it will leak before.

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looks good - if @queimadus can try this change too that would be very useful

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I actually can't seem to reproduce the issue with the example I mentioned with flatMapPrefix with prefixAndTail.

E.g. running the following runs fine with and without these changes

  val s = Source
    .repeat(())
    .map(_ => ByteString('a' * 4000000))
    .take(1000000)
    .prefixAndTail(50000)
    .flatMapConcat { case (prefix, tail) => tail }

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

What doesn't run fine is the the following snippet, but it may be a different problem.

 def myLogic(prefix: Seq[ByteString]): Flow[ByteString, ByteString, NotUsed] =
    Flow[ByteString]

  val s = Source
    .repeat(())
    .map(_ => ByteString('a' * 400000))
    .take(1000000)
    .prefixAndTail(50000)
    .flatMapConcat { case (prefix, tail) =>
      Source(prefix).concatLazy(tail).via(myLogic(prefix))
    }

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

That said, these changes still seem to make sense.

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@He-Pin He-Pin Dec 22, 2024 •
edited
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This problem is a kind of AsyncCallback leak, #408
where many instances consume memories.

To totally fix this, we will lose some support for debugging I think, eg the org.apache.pekko.stream.impl.fusing.GraphInterpreter#toSnapshot

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@queimadus Yes, different problem, the new problem is the interpreter keeps many things in memory

} else {
if (!tailSource.isClosed) tailSource.complete()
completeStage()
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