[FLINK-18677] Added handling of suspended or lost connections

[XComp]

I added handling of suspended or lost connections within the ZooKeeperLeaderRetrievalService.

The listener needs to be notified in case of a connection loss so that it is able to initiate necessary actions on its side.

What is the purpose of the change

• The ZooKeeperLeaderRetrievalService needs to inform its listener in case of connection loss since it might miss a leader change. The old implementation didn't trigger the listener in such a case.
• This behavior was caused, for instance, when the TaskManager was lost its connection to the ZooKeeper and, therefore, wasn't informed about a leader change. Its tasks continued to run even though they got already recovered by another TaskManager. Hence, the same data got processed.

Brief change log

• A method call was added when handling lost or suspended connections notifying the listener about the lost connection.
• Two test cases were added to check the behavior.

Verifying this change

This change added tests and can be verified as follows:

• Added unit test for checking whether the listener is NOT informed when there was not leader information retrieved, yet.
• Added unit test for checking whether the listener is notified when the connection was lost but there was some previous leader information present already.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: yes
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? -

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Last check on commit f5df4b1 (Sat Aug 28 11:16:01 UTC 2021)

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[XComp]

@tillrohrmann the PR is ready to be reviewed.

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CI report:

• 6acb8da UNKNOWN
• f5df4b1 Azure: SUCCESS

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[tillrohrmann]

Thanks for creating this PR @XComp. The changes look good. I had a couple of comments concerning the thread safety of the addition and some smaller suggestions. Please take a look at my comments.

[tillrohrmann]

Thanks for addressing my comments. I had a few minor comments left.