[FLINK-22085][tests] Update NetworkFailureHandler to prevent closeOnFlush() from being called recursively

[lindong28]

Contribution Checklist

This PR updated NetworkFailureHandler to prevent closeOnFlush() from being called recursively. The test might hang indefinitely without this change.

Brief change log

This PR updated NetworkFailureHandler to prevent closeOnFlush() from being called recursively.

Verifying this change

Verified that KafkaSourceLegacyITCase::testBrokerFailure could hang before this PR. And the same test could consistently pass across 200 runs after this PR.

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: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

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Automated Checks

Last check on commit 70809b0 (Fri May 28 09:10:37 UTC 2021)

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• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
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• ❓ 5. Overall code [quality] is good.

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[lindong28]

Ping @dawidwys @becketqin for review.

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CI report:

• 70809b0 Azure: FAILURE

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[dawidwys]

Thanks for your findings and a fix @lindong28 !

Could you @pnowojski take a look at the PR if it makes sense to you? Especially the explanation in the corresponding JIRA ticket?

[dawidwys]

Looks good from my side. Still I'd wait for a review from @pnowojski

[pnowojski]

I don't really remember the code, but from a quick look this change makes sense to me as well.

[lindong28]

Thank you @dawidwys for the helpful comments. I have updated the PR as suggested.

Could you take another look?

[dawidwys]

Thank you @lindong28 for your contribution!

The failure is unrelated. Merging...