There is a narrow race in this code that is caused when the code being
run in assertSpilled / assertNotSpilled runs more than a single job.
SpillListener assumed that only a single job was run, and so would only
block waiting for that single job to finish when `numSpilledStages` was
called. But some tests (like SQL tests that call `checkAnswer`) run more
than one job, and so that wait was basically a no-op.
This could cause the next test to install a listener to receive events
from the previous job. Which could cause test failures in certain cases.
The change fixes that race, and also uninstalls listeners after the
test runs, so they don't accumulate when the SparkContext is shared
among multiple tests.
Author: Marcelo Vanzin <vanzin@cloudera.com>
Closes#21639 from vanzin/SPARK-24653.
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