[GitHub] [spark] HyukjinKwon opened a new pull request #25321: [SPARK-28153][PYTHON][BRANCH-2.4] Use AtomicReference at InputFileBlockHolder (to support input_file_name with Python UDF)

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HyukjinKwon opened a new pull request #25321: [SPARK-28153][PYTHON][BRANCH-2.4] Use AtomicReference at InputFileBlockHolder (to support input_file_name with Python UDF)
URL: https://github.com/apache/spark/pull/25321
 
 
   ## What changes were proposed in this pull request?
   
   This PR backports https://github.com/apache/spark/pull/24958 to branch-2.4.
   
   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.
   
   

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