[FLINK-18372][runtime] Fix NPE problem when a slot is offered before JM is connected to a RM

[zhuzhurk]

What is the purpose of the change

This PR is to fix the NPE problem which can happen when a slot is offered before JM is connected to a RM.

Brief change log

See the commit.

Verifying this change

• Added unit test for the case that a slot is offered before SlotPool is connected to a RM

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)

[zhuzhurk]

@azagrebin @tillrohrmann would you take a look at this fix when convenient?

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Automated Checks

Last check on commit 8e28cd7 (Sun Jun 21 09:00:30 UTC 2020)

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CI report:

• 6f03311 UNKNOWN
• 922cafd Azure: PENDING
• 16edfbe Azure: SUCCESS

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[tillrohrmann]

Thanks for creating this PR @zhuzhurk. The changes look good to me. I had a comment concerning querying pendingRequests which might have resulted from wrong assumptions about the internal state of the component.

[tillrohrmann]

I think you could directly ask pendingRequest.getAllocationId(). That way you would not need to query pendingRequests again.

[tillrohrmann]

Moreover, there is no invariant which states that pendingRequests contains the mapping. Hence, it would also make sense to directly ask pendingRequest here.

[zhuzhurk]

Agreed. done.