Github user zhijiangW commented on the issue:
https://github.com/apache/flink/pull/4559
@NicoK, I have submitted two `[hotfix]` commits for the above issues.
One is for `Nullable` annotation and tests of backlog statistics. And the
other is for updating backlog in thread-safe.
For updating backlog, I think it should be done in `PipelinedSubpartition`
and `SpillableSubpartition` separately in order to under synchronized region,
although it seems somewhat redundant. But I notice that the `isReleased()`
method in `ResultSubpartition` is also implemented in this way.
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