### What changes were proposed in this pull request?
This is an improvement, we mount all the user specific configuration files(except the templates and spark properties files) from `SPARK_CONF_DIR` at the point of spark-submit, to both executor and driver pods. Currently, only `spark.properties` is mounted, only on driver.
### Why are the changes needed?
`SPARK_CONF_DIR` hosts several configuration files, for example,
1) `spark-defaults.conf` - containing all the spark properties.
2) `log4j.properties` - Logger configuration.
3) `core-site.xml` - Hadoop related configuration.
4) `fairscheduler.xml` - Spark's fair scheduling policy at the job level.
5) `metrics.properties` - Spark metrics.
6) Any user specific - library or framework specific configuration file.
At the moment, we can cannot propagate these files to the driver and executor configuration directory.
There is a design doc, with more details, and this patch is currently providing a reference implementation. Please take a look at the doc and comment, how we can improve. [google docs link to the doc](https://bit.ly/spark-30985)
### Further scope
Support user defined configMaps.
### Does this PR introduce any user-facing change?
Yes, previously the user configuration files(e.g. hdfs-site.xml, log4j.properties etc...) were not propagated by default, now after this patch it is propagated to driver and executor pods' `SPARK_CONF_DIR`.
### How was this patch tested?
Added tests.
Also manually tested, by deploying it to a minikube cluster and observing the additional configuration files were present, and taking effect. For example, changes to log4j.properties was properly applied to executors.
Closes#27735 from ScrapCodes/SPARK-30985/spark-conf-k8s-propagate.
Authored-by: Prashant Sharma <prashsh1@in.ibm.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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