[GitHub] [spark] viirya opened a new pull request #24521: [SPARK-27629][PySpark] Prevent Unpickler from intervening each unpickling

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viirya opened a new pull request #24521: [SPARK-27629][PySpark] Prevent Unpickler from intervening each unpickling
URL: https://github.com/apache/spark/pull/24521
 
 
   ## What changes were proposed in this pull request?
   
   In SPARK-27612, one correctness issue was reported. When protocol 4 is used to pickle Python objects, we found that unpickled objects were wrong. A temporary fix was proposed by not using highest protocol.
   
   It was found that Opcodes.MEMOIZE was appeared in the opcodes in protocol 4. It is suspect to this issue.
   
   A deeper dive found that Opcodes.MEMOIZE stores objects into internal map of Unpickler object. We use single Unpickler object to unpickle serialized Python bytes. Stored objects intervenes next round of unpickling, if the map is not cleared.
   
   We has two options:
   
   1. Continues to reuse Unpickler, but calls its close after each unpickling.
   2. Not to reuse Unpickler and create new Unpickler object in each unpickling.
   
   This patch takes option 1.
   
   ## How was this patch tested?
   
   Passing the test added in SPARK-27612.
   

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