UDF bug fix caused by ThreadStatic BroadcastVariablesRegistry

[Niharikadutta]

This PR fixes a bug found when using UDFs.

The class JvmBroadcastRegistry has a variable defined that stores the list of all active Broadcast variables' jvmObjects that are to be shipped to the workers. This variable was marked as ThreadStatic to maintain a thread-specific copy of the object. The way ThreadStatic works is it only initializes the object for the very first thread that uses it, and subsequent threads are not initialized. This caused a Null reference exception to be thrown when using UDFs that used a different thread.

Solution: This was fixed by declaring it to be a ThreadLocal<> variable instead, that let's you provide an initialization function, which would define a new instance every time before it's accessed in a new thread.

[imback82]

Is this still WIP?

[imback82]

Can you also update the description please (make it more general instead of limiting to interactive session)?

[Niharikadutta]

@imback82 Updated the description and opened the PR for review. Thanks!

[imback82]

Can you add a test? Any bug fix should come with a test that validates the fix. I think different threads just defining UDFs should suffice.

[suhsteve]

Can you update the title to say it's related to Broadcast?

[suhsteve]

LGTM.

[imback82]

LGTM, thanks @Niharikadutta!