### What changes were proposed in this pull request?
If a Spark distribution has built-in hadoop runtime, Spark will not populate the hadoop classpath from `yarn.application.classpath` and `mapreduce.application.classpath` when a job is submitted to Yarn. Users can override this behavior by setting `spark.yarn.populateHadoopClasspath` to `true`.
### Why are the changes needed?
Without this, Spark will populate the hadoop classpath from `yarn.application.classpath` and `mapreduce.application.classpath` even Spark distribution has built-in hadoop. This results jar conflict and many unexpected behaviors in runtime.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Manually test with two builds, with-hadoop and no-hadoop builds.
Closes#28788 from dbtsai/yarn-classpath.
Authored-by: DB Tsai <d_tsai@apple.com>
Signed-off-by: DB Tsai <d_tsai@apple.com>
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