Github user ueshin commented on the issue:
https://github.com/apache/spark/pull/20404
@HyukjinKwon I'm afraid that's not enough because we need to synchronize at
all the places we touch the `defaultSession`, otherwise another thread can set
the value between `defaultSession.get() eq null` and
`defaultSession.set(session)`.
Fortunately `defaultSession` is an `AtomicReference`, so maybe we can use
`defaultSession.compareAndSet(null, session)` without the synchronization in
the method `setDefaultSessionIfUnset()` if we only need to set the default
session is `null`.
I'm not sure whether we want to replace the default session which is
already stopped, but in that case, we may also have to send the original
default session to compare.
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