[FLINK-13765][task] Introduce TwoInputSelectionHandler for selecting input in StreamTwoInputSelectableProcessor

[zhijiangW]

What is the purpose of the change

In StreamTwoInputSelectableProcessor there are three fields {InputSelectable, InputSelection, availableInputsMask} to be used together for the function of selecting next available input index.

From design aspect, these fields can be abstracted into a separate component called TwoInputSelectionHandler, which is passed into the constructor of StreamTwoInputSelectableProcessor as a final field. So the internal implementation details of TwoInputSelectionHandler is hidden from processor view which only needs to interact with exposed methods from selection handler.

Another tiny benefit is that we make the StreamTwoInputSelectableProcessor a bit because two methods are removed from it.

Brief change log

• Introduce the TwoInputSelectionHandler for handling the logic of next available input index
• Refactor the related process in StreamTwoInputSelectableProcessor based on InputSelectionHandler

Verifying this change

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

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• Dependencies (does it add or upgrade a dependency): (yes / no)
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• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, 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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CI report:

• 71e7fae : SUCCESS Build
• 6dd3cd3 : SUCCESS Build
• 7d75f94 : CANCELED Build
• e21b3e5 : SUCCESS Build

[pnowojski]

I'm not sure how big value has this PR. Yes, you have encapsulated 3 fields, that's a plus, but very little logic was extracted. For example you replaced inputSelection.isALLMaskOf2() with inputSelectionHandler.isALLInputsSelected(). Or inputSelection.isInputSelected(1) with inputSelectionHandler.isFirstInputSelected(). However if you prefer it this way +1 from my side.

Feel free to merge this once you checked my comments

[zhijiangW]

Thanks for this review @pnowojski !

Actually this PR would not be a blocker for the following work as I misleading before. If just from the design aspect, the benefits might not be very obvious as you mentioned. My previous thought:

• We could make StreamTwoInputSelectableProcessor a bit thin after refactoring, because we remove two non-final vars and two methods from the processor.

• In previous way, StreamTwoInputSelectableProcessor would involve in three fields {InputSelectable, InputSelection, availableInputsMask} to expose too many details. After refactoring, the processor only interacts with one component and some details are hidden behind.

I could understand your concern and it would be better if we could further reduce some complex interactive logics provided by TwoInputSelectionHandler.
So I think this pr can be pending now, and after we find something very confirmable to be done, we can merge then.

[pnowojski]

As I wrote before, please feel free to merge this. Keeping it open for "rethinking" in the future it's probably a bad idea.

[zhijiangW]

Thanks for the confirmation and I would merge it after travis green. @pnowojski

[zhijiangW]

@flinkbot run travis