Github user markhamstra commented on the issue:
https://github.com/apache/spark/pull/16291
> You are right, that's why the executor service is single threaded which
guarantees ordered processing of the events per listener.
But this is still substantially different from the ordering guarantees in
the current implementation, and I'm not sure that this PR wouldn't invalidate
some assumptions made by current listeners. Events will end up in the same
order in each `ListenerEventExecutor` (at least by my understanding, which
someone else should double-check), but there is no synchronization between
`ListenerEventExecutor`s of the processing of their `eventQueue`s; so it is
entirely possible. for example, for one `ListenerEventExecutor` to process a
task end event for a particular task before another `ListenerEventExecutor` has
worked sufficiently through its `eventQueue` to have even seen the
corresponding task start event. That is quite a bit different than the prior
ordering guarantee implied by the comment "`onPostEvent` is guaranteed to be
called in the same thread for all listeners."
Whether this actually constitutes a problem or not, I'm not certain; but
serializing event processing within each `ListenerEventExecutor` isn't enough
if `Listener`s on different `ListenerEventExecutor` threads expect to share
mutable state.
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