Github user tillrohrmann commented on the issue:
https://github.com/apache/flink/pull/2629
Ah, I think now I understand. Upon checkpointing you disable the output
emission. The emission can only be reactivated when adding a `StreamRecord`, a
`Watermark` or a `LatencyMarker` to the `AsyncCollectorBuffer`. And this cannot
happen as long as the checkpointing isn't completed, because it holds the
checkpoint lock.
But isn't this a problem in the following case: What if we have some
elements pending in the `queue` when doing the checkpoint. After the checkpoint
has completed, there won't be any other `StreamRecords`, `Watermarks` (because
we haven't activated them) and `LatencyMarkers` coming on the input. This would
imply that the pending elements in the `AsyncCollectorBuffer` will never be
emitted, won't they?
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