Github user aljoscha commented on the pull request:
https://github.com/apache/flink/pull/1542#issuecomment-190650790
Hi @wenlonglwl,
you are right about this. We are thinking about how we can change the
system so that finite sources can emit a final Long.MAX_VALUE watermark when
they are finished. When writing a custom finite source you can always emit a
Long.MAX_VALUE watermark yourself, once you know that the source is finished.
Sorry for the inconvenience this has caused, we first wanted to make this
right for infinite sources but I see that you also have a valid use case.
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