[FLINK-19385] Request partitions for each InputGate independently

[rkhachatryan]

What is the purpose of the change

Please see FLINK-19385 for motivation.

Verifying this change

The issue is covered by StreamTaskSelectiveReadingITCase: #13351 fails without this change.

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? no

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[AHeise]

LGTM, I wonder why we didn't do it before.

I was wondering if we can add a test for it? The ticket description looks like we could easily transfer it to this PR.

[zhijiangW]

also +1 for the changes.

In theory every input gate can recovery state and then request partitions independently without precision concern. The formal way somehow considered all the buffer resources used in recovery for loading states firstly before loading new data from upstreams. Then we can guarantee the recovery process can end ASAP to avoid over loading them again once failure happens while mix processing new data with state data. But indeed it is not a strong requirement. Just for clarifying the concerns from @AHeise .

I agree to add somehow UT if possible. Also it will be better to give some descriptions in commit message to understand the background why we need this change. It is for resolving the potential problem of channel selection priority in multiple gates, although we have not supported it in unaligned checkpoint yet.

[rkhachatryan]

Thanks for reviewing!
I'll try to pull some changes from #13351 to this PR.

[rkhachatryan]

Subsumed by #13351