There isn't a good way to write a SparkListener that receives all SparkListener events and which will be future-compatible (e.g. it will receive events introduced in newer versions of Spark without having to override new methods to process those events).
To address this, this patch adds `SparkFirehoseListener`, a SparkListener implementation that receives all events and dispatches them to a single `onEvent` method (which can be overridden by users).
Author: Josh Rosen <joshrosen@databricks.com>
Closes#4210 from JoshRosen/firehose-listener and squashes the following commits:
223f579 [Josh Rosen] Expand comment to explain rationale for this being a Java class.
ecdfaed [Josh Rosen] Add SparkFirehoseListener class for consuming all SparkListener events.
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