[FLINK-16443][checkpointing] Make sure that CheckpointException are also serialized in DeclineCheckpoint.

[AHeise]

What is the purpose of the change

The problem of having exceptions that are only in the user code classloader was fixed by proactively serializing them inside the CheckpointException. That means all consumers of CheckpointException now need to be aware of that and unwrap the serializable exception.

I believe the right way to fix this would have been to use a SerializedException in the DeclineCheckpoint message instead, which would have localized the change to the actual problem: RPC transport.

Brief change log

• Use a SerializedException in the DeclineCheckpoint message
• Revert [FLINK-14076] Ensure CheckpointException can always be deserialized on JobManager #9742

Verifying this change

It's a rather minor change, where proper testing would require a rather complicated setup.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• 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)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Last check on commit 8118763 (Mon Nov 23 14:09:53 UTC 2020)

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[AHeise]

Converted to draft, so that we merge only after 1.12 branch is forked.

[tillrohrmann]

Thanks for creating this PR @AHeise. I had a couple of comments. Please take a look.

[tillrohrmann]

Shouldn't we change the type if we know that it is always serialized?

[tillrohrmann]

The same actually applies to the return type of getReason() method.

[tillrohrmann]

I guess that we can now also adapt CheckpointCoordinator.getCheckpointException.

[AHeise]

I did the first two things, but didn't know how to proceed with the third: the method is also called from onTriggerFailure where we just get a Throwable from the future.

[tillrohrmann]

I overlooked the other usages of this method. I guess it should be fine then.

[tillrohrmann]

Thanks for updating this PR @AHeise. LGTM. +1 for merging.