[FLINK-20947][planner] Fix idle source doesn't work when pushing watermark into the source

[fsk119]

What is the purpose of the change

Fix idle source doesn't work when pushing watermark strategy into table source.

Brief change log

• Fix the bug.

Verifying this change

This change added tests and can be verified as follows:

• Idle source plan test.
• Idle source IT cases.

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• Dependencies (does it add or upgrade a dependency): (yes / no)
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• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)

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Last check on commit d21a2ca (Fri May 28 07:12:32 UTC 2021)

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[wuchong]

Thanks for the work.

I think we need an IT case for Kafka to verify the pushed idle watermark strategy can work well. You can add a test in KafkaTableITCase with:

• prepare 2 partitions, each with several messages. The last event time in the first partition is 2021-01-13 20:00:00, the last event time in the second partition is 2021-01-13 19:30:00.
• apply window query on the source with tumbling 1 hour
• the query should have output with window [19:00, 20:00) which means the watermark has reached 20:00 (before the fix, the query shouldn't have output)

If we have an IT case for Kafka, I think we don't need to adapt the TestValues source. The changes looks very complex and hard to maintain in the future.

[fsk119]

Thanks for your tips. I think I have already resolved your concerns.

[wuchong]

Great work! The pull request looks good to me now.

Could you also open a pull request for release-1.12 branch?