Spark on YARN won't ever ask for more executors than there were containers at time of context creation

Looking at YarnSchedulerBackend, it appears that totalExpectedExecutors is only ever set at startup.

Based on my experience of running on EMR (and a quick browse through the code supports this), Spark will never ask for more executors than what this was set to.

This means that if I add more nodes to my YARN cluster, Spark will never pick them up. This is bad. The whole point of using Spark on EMR/YARN is to be able to add or remove nodes to the cluster and have Spark "Do The Right Thing".