[FLINK-20267][runtime] The JaasModule didn't support symbolic links.

[XComp]

What is the purpose of the change

Directory creation was introduced to JaasModule in FLINK-19252 in case the folder does not exist, yet. Unfortunately, the used logic was not able to deal with symbolic links which caused exception in such cases.

The behavior could be reproduced through a JUnit test and was fixed in this PR. One goal of this change was to have the symbolic link resolution not being exposed outside generateDefaultConfigFile(String) to have the passed workingDir parameter match the directory path of the returned File instance.

Brief change log

• Utilization of Path.toRealPath() logic to resolve symbolic links.
• The change makes sure that the symbolic link resolution happens within the method only. The returned File instance uses the path containing symbolic link again.
• A unit test was added to cover this behavior.

Verifying this change

This change added tests and can be verified as follows:

• A new test JaasModuleTest.testJaasModuleFilePathIfWorkingDirIsSymLink was added.

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? not applicable

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Last check on commit 3617fa7 (Mon Nov 23 11:50:05 UTC 2020)

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CI report:

• dd6b808 Azure: SUCCESS

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[KarmaGYZ]

Thanks for the quick patch @XComp ! It LGTM. Only left a minor thought regarding the test coverage.

[XComp]

Thanks @KarmaGYZ I addressed your changes in dd6b808.

[tillrohrmann]

Thanks for creating this PR @XComp. The change itself looks good to me. I had a question for my understanding.

[tillrohrmann]

Why did you do this change here?

[tillrohrmann]

Is it in order to maintain the symlinks in the path?

[XComp]

Yes, exactly. I wanted to avoid having a different path than the one the user specified in the configuration. The old implementation would have lead to the resolved path being exposed.

[tillrohrmann]

Thanks for the clarification @XComp. LGTM. Merging this PR once AZP has passed.