Github user aarondav commented on the pull request:
https://github.com/apache/spark/pull/1332#issuecomment-48376758
This type of concurrency problem can be a correctness issue, albeit with
low probability. For instance, we could at any point read a
not-fully-instantiated "_cachedColumnBuffers", which would not be null but
still be in an invalid state, leading to a very tricky exception. This is a
problem even without the byte buffer reuse -- use of the same InMemoryRelation
in two different threads could now cause this issue.
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