## What changes were proposed in this pull request?
This PR proposes to use `AtomicReference` so that parent and child threads can access to the same file block holder.
Python UDF expressions are turned to a plan and then it launches a separate thread to consume the input iterator. In the separate child thread, the iterator sets `InputFileBlockHolder.set` before the parent does which the parent thread is unable to read later.
1. In this separate child thread, if it happens to call `InputFileBlockHolder.set` first without initialization of the parent's thread local (which is done when the `ThreadLocal.get()` is first called), the child thread seems calling its own `initialValue` to initialize.
2. After that, the parent calls its own `initialValue` to initializes at the first call of `ThreadLocal.get()`.
3. Both now have two different references. Updating at child isn't reflected to parent.
This PR fixes it via initializing parent's thread local with `AtomicReference` for file status so that they can be used in each task, and children thread's update is reflected.
I also tried to explain this a bit more at https://github.com/apache/spark/pull/24958#discussion_r297203041.
## How was this patch tested?
Manually tested and unittest was added.
Closes#24958 from HyukjinKwon/SPARK-28153.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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