[FLINK-14261] Add PermanentlyFencedRpcEndpoint

[tillrohrmann]

What is the purpose of the change

This PR is based on #9789.

This PR adds PermanentlyFencedRpcEndpoint which requires a non-null fencing token at startup and does not allow to change it.

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): (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, 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 87ad063 (Wed Oct 16 08:34:06 UTC 2019)

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Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
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• ❓ 5. Overall code [quality] is good.

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CI report:

• f1bcc25 : FAILURE Build
• 87ad063 : SUCCESS Build

[tisonkun]

Thanks for opening this pull request. The specific commit f1bcc25 corresponding to FLINK-14261 looks good to me.

It seems this commit doesn't have to be based on other commits in the commits list so that we can even treat it individually.

Only one comment that we might mark fencingToken parameter of PermanentlyFencedRpcEndpoint ctor as @Nonnull but Andery once said Flink's convention is all variable is non-null by default so it is no more than a suggestion.

[tillrohrmann]

You are right that the commit can be independent of the others. It just happened to be in a chain of commits which I need for FLINK-11843 and I wanted to keep the chain in order which makes it easier for me to keep track where we are in the merge process.

Concerning @Nonnull I think the current community agreement was to not use it. Hence, I would start to stick to it so that we might eventually converge towards a consistent code base. This might still take a bit of time ;-)