[FLINK-19441][network] Avoid loading of ResultPartition wrapper class for consumable notifications when possible.

[StephanEwen]

What is the purpose of the change

A small code optimization attempting to reduce the virtual method call overhead on the per-record methods in ResultPartition.

It does that by making the class wrapping the ResultPartition (formerly ConsumableNotifyingResultPartitionWriterDecorator) a different class than the one with the factory method. That way, if the factory method never needs a wrapper class (under normal configuration, it never needs one) the wrapping class is never loaded, and the JIT can find out (through CHA) that only one implementation for the per-record methods on ResultPartition exists.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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): yes
• 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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[AHeise]

Interesting find and idea. The final code looks a bit confusing.

I was wondering if we should move the factory method into Task to make it easier again to understand. I think having deployment descriptors leaked into PartitionWriter is a sign that the method is misplaced to being with.

[AHeise]

After offline discussion, I realized that ConsumableNotifyingResultPartitionWriterDecorator is now a utility class, so LGTM.

[StephanEwen]

Adjusted the code to make the role of the utility class more clear.