[WIP][BEAM-12776, fixes #21095] Limit parallel closes from the prior element to the next element.

[lukecwik]

This will reduce OOMs in the case where we are adding new writes faster then we are able to clean-up older ones across elements.

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[lukecwik]

@scwhittle What do you think about this solution for limiting OOMs?

[lukecwik]

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[kellen]

@lukecwik We (spotify) are seeing ConcurrentModificationException errors due to this PR.

The issue starts in WriteFiles.java where you construct a new future, then clear the existing closeFutures.

I think the underlying cause is in MoreFutures.java where you close over the input futures inside thenApply. If you instead used the array result of futuresToCompletableFutures I suspect the issue could be avoided.

Something like this?

      CompletableFuture<? extends T>[] f = futuresToCompletableFutures(futures);
      CompletionStage<Void> blockAndDiscard = CompletableFuture.allOf(f);

      return blockAndDiscard.thenApply(
          nothing ->
              Arrays.stream(f)
                  .map(CompletableFuture::join)
                  .collect(Collectors.toList()));

Similarly for allAsListWithExceptions.

Stack trace for the error (from a scio test):

[info]   org.apache.beam.sdk.Pipeline$PipelineExecutionException: java.util.concurrent.ExecutionException: java.util.ConcurrentModificationException
[info]   at org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:374)
[info]   at org.apache.beam.runners.direct.DirectRunner$DirectPipelineResult.waitUntilFinish(DirectRunner.java:342)
[info]   at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:218)
[info]   at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:67)
[info]   at org.apache.beam.sdk.Pipeline.run(Pipeline.java:323)
[info]   at org.apache.beam.sdk.Pipeline.run(Pipeline.java:309)
[info]   at com.spotify.scio.ScioContext.execute(ScioContext.scala:586)
[info]   at com.spotify.scio.ScioContext$$anonfun$run$1.apply(ScioContext.scala:573)
[info]   at com.spotify.scio.ScioContext$$anonfun$run$1.apply(ScioContext.scala:561)
[info]   at com.spotify.scio.ScioContext.requireNotClosed(ScioContext.scala:652)
[info]   ...
[info]   Cause: java.util.concurrent.ExecutionException: java.util.ConcurrentModificationException
[info]   at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
[info]   at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1908)
[info]   at org.apache.beam.sdk.util.MoreFutures.get(MoreFutures.java:60)
[info]   at org.apache.beam.sdk.io.WriteFiles$WriteShardsIntoTempFilesFn.processElement(WriteFiles.java:984)
[info]   ...
[info]   Cause: java.util.ConcurrentModificationException:
[info]   at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1390)
[info]   at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
[info]   at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
[info]   at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
[info]   at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
[info]   at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)
[info]   at org.apache.beam.sdk.util.MoreFutures.lambda$allAsList$5(MoreFutures.java:174)
[info]   at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:616)
[info]   at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:591)
[info]   at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:488)

[lukecwik]

@kellen Filed https://github.com/apache/beam/issues/23809 with a fix in https://github.com/apache/beam/pull/23811 based upon your supplied patch

[lukecwik]

@kellen If you could review the PR then we could have it merged. Comment LGTM on PR if it looks good to you.