[FLINK-4877] About SourceFunction extends Serializable

[vim-wj]

SourceFunction extends Function and Serializable, but Function has been extends Serializable, so SourceFunction needn't extends Serializable again.

[StephanEwen]

Thanks for opening this pull request.

I think it is correct that SourceFunction is not required to implement Serializable. But I think it does not cause any problems, as far as I know, it has no implication at all on the runtime or behavior.

It does serve as a "reminder" that sources need to be serializable, so it could actually be helpful to keep it.

Given that it makes no difference, should we keep it?

[greghogan]

@vim-wj if you are okay with Stephan's suggestion could you close this pull request?

Also, a small note: FLINK-4877 references a Jira ticket rather than a pull request (use [hotfix] for simple issues not requiring a ticket).

[vim-wj]

@greghogan , @StephanEwen ,sorry fo my late reply.I will close this pull request.