[FLINK-25937] Restore the environment parallelism before transforming in SinkExpander#expand.

[pltbkd]

Parallelism of a transformation with default parallelism(-1) is set when transforming, using the default parallelism set in the environment. However, in SinkExpander#expand, the environment parallelism is set to -1 at the entrance, to verify if the parallelism of a expanded transformation is set. The environment parallelism will be restored when exiting the method, but at present the transform is called within this scope. If the parallelism of a sink is not set, the parallelism of the sink transformation and all transformations expanded from it will not be handled, so the JobGraph generated will have vertices with -1 parallelism, causing the assertion failure in AdaptiveScheduler.

This pr fixes the bug by putting the restoring of the environment parallelism ahead of transforming the sink transformations.

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CI report:

• 5b0d16d Azure: FAILURE

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[dmvk]

Thanks for the PR @pltbkd 👍 Would it be possible to add a failing unit test verifying the change and preventing future regression?

[pltbkd]

@dmvk

Sure. I have added a test to verify this. The case will fail if I revert the change of this PR.

But I am not sure if it's at the right place. Since no translator has a test, I add it in the StreamGraphGeneratorTest. Do you think it's ok?

[dmvk]

Sounds like a right place to me 👍

[dmvk]

Thanks @pltbkd for adding the test case 👍 This looks really good. Great job 🎉

[zentol]

@pltbkd Could you rebase the PR?

Actually, I'll do that. I need to run CI manually anyway before the merge.

[pltbkd]

Noted.
Thanks @dmvk and @zentol for the review.