[FLINK-17817] Fix type serializer duplication in CollectSinkFunction

[tsreaper]

What is the purpose of the change

This is a hot fix for CollectSinkFunction. TypeSerializers are not thread safe but currently CollectSinkFunction reuses them among two threads. This PR fixes this problem.

Actually FLINK-17774 also solves this problem, but we should add a quick fix now to not shade other failures in tests.

Brief change log

• Fix serializer thread safe problem in CollectSinkFunction

Verifying this change

This change is already covered by existing tests.

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 ed5e8c9 (Fri Oct 16 10:52:44 UTC 2020)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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Review Progress

• ❓ 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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[JingsongLi]

Consider breaking master now, I will merge it after test passing.

[flinkbot]

CI report:

• ed5e8c9 Azure: FAILURE

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[tsreaper]

Azure passed in https://dev.azure.com/tsreaper96/Flink/_build/results?buildId=32&view=results

[JingsongLi]

+1