[FLINK-18612][fs][local] Fix the issue when creating file under the current directory with relative path

[gaoyunhaii]

What is the purpose of the change

Currently when we want to create a file under the current directory with relative path (e.g., new Path("file")), the LocalFileSystem fails when trying to create the parent directory. This is due to that the parent path is stored as an empty string and it is transformed to new File("") and new File("").isDirectory() returns false, which indicates the caller that we fail to create the parent directory.

To address this issue, we made the empty path transformed to new File("."), with which isDirectory() returns the expected true.

Brief change log

(for example:)

• ab340bf changes the method pathToFile(Path path) to process the empty path specially and transform it to new File(".").

Verifying this change

This change added tests and can be verified as follows:

• Added UT to verify we could successfully create the file under current directory with relative path. Since Java could not change the current directory, we will have to try creating file under the current directory instead of an TemporaryFolder (A another option might be start a new Process to use the TemporaryFolder but it seems to be too heavy). We tried best to remove the target file after the test.

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 ab340bf (Fri Jul 17 06:38:23 UTC 2020)

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CI report:

• a292a39 Azure: SUCCESS

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[zhijiangW]

Thanks for the fix @gaoyunhaii ! Will merge after azure pass.