BEAM-6018: memory leak in thread pool instantiation

[Udi Meiri]

HI,
I've identified a memory leak when GcsUtil.java instantiates a
ThreadPoolExecutor (https://issues.apache.org/jira/browse/BEAM-6018).
The code uses the getExitingExecutorService
<https://github.com/apache/beam/blob/279a05604b83a54e8e5a79e13d8761f94841f326/sdks/java/extensions/google-cloud-platform-core/src/main/java/org/apache/beam/sdk/util/GcsUtil.java#L551>
wrapper,
which leaks memory. The question is, why is that wrapper necessary
if executor.shutdown(); is later unconditionally called?

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