[FLINK-19251][connectors] Avoid confusing queue handling in "SplitReader.handleSplitsChanges()"

[StephanEwen]

What is the purpose of the change

Currently, the method SplitReader.handleSplitsChanges() gets passed a queue of split changes to handle. The method may decide to handle all of them, or only a subset of them. It it handles a subset of them, the method is later invoked with the remaining changes.

In practice, this ends up being confusing and problematic:

• It is important to remove the elements from the queue. If you just iterate over the splits, or the splits will get handles multiple time.
• If the queue is not left empty, the task to handle the changes is immediately re-enqueued and the method immediately re-invoked. No other operation can happen before all split changes from the queue are handled. That means you have to handle all splits immediately, just spread it out over multiple method invocations.

A simpler contract would be to simply pass a the split changes (list of splits) directly only once.
The fetcher would pick those changes up and can internally stash them if it wants to process them later.
For all source implementations we did so far, this was sufficient and easier.

Brief change log

• Change signature of void handleSplitsChanges(Queue<SplitsChange<SplitT>> splitsChanges)
  to void handleSplitsChanges(SplitsChange<SplitT> splitsChanges);
• Adjust invoking and testing code

Verifying this change

This change is a simple rework 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 6161ceb (Wed Sep 16 08:30:29 UTC 2020)

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CI report:

• 6161ceb UNKNOWN
• 3cc7614 Azure: PENDING

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[becketqin]

@StephanEwen Thanks for the improvement. LGTM.

[StephanEwen]

Thanks for double checking this!

Will merge...