[FLINK-18403][checkpointing] Ensure that unaligned checkpointing is only activated for EXACTLY_ONCE (1.11).

[AHeise]

(Unchanged backport of #12739)

What is the purpose of the change

Currently, unaligned checkpoint config option can be used together with AT_LEAST_ONCE (or disabled checkpointing), which may lead to unexpected behavior.
This PR ensures that unaligned checkpointing is only activated for EXACTLY_ONCE.

Brief change log

• Deactivates unaligned checkpointing if checkpointing mode is not EXACTLY_ONCE and gives warning.

Verifying this change

Added two unit tests.

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)

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[uce]

LGTM.

nit: the Javadoc for enableUnalignedCheckpoints(boolean) contains a typo: reduce should be reduces.

Did you also consider to throw instead of only logging a warning? I can see the argument go both ways (and I'm fine with both), but configuring unaligned checkpoints could also be read as a strong indication that I actually want exactly once rather. In that case it could be surprising to see checkpointing be executed in at least once mode.

[pnowojski]

Yes, we considered it @uce. We were actually swinging both directions and the deciding factor was that there are quite a bit of individual tests that are manually setting at least once mode. With hard crash it would add quite a bit of effort to modify all of those places to manually disable unaligned checkpoints (we are enabling unaligned checkpoints by default in our tests).

We could also choose to refine this behaviour in the future.

[pnowojski]

(I've fixed the typo as separate hotfix commit)