[FLINK-8931] TASK_KILLING is not covered by match in TaskMonitor#whenUnhandled

[yanghua]

What is the purpose of the change

This pull request makes TASK_KILLING covered by match in TaskMonitor#whenUnhandled

Brief change log

• *Added TASK_KILLING in last case *

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): (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, 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)

[yanghua]

cc @tillrohrmann

[tillrohrmann]

Thanks for the contribution @yanghua. I fear that the TASK_KILLING states have been left out deliberately. When a task is being killed, then you don't stop. Thus, the issue is actually a non issue. What we could do is to make the match exhaustive to add a TASK_KILLING => noOp.

[yanghua]

hi @tillrohrmann you mean change to stay()? I have changed to this style. It seems that in scala FSM we could not do nothing, it should return task monitor's state.

[tillrohrmann]

Changes look good to me. Thanks for your contribution @yanghua. Merging this PR.