[SPARK-8019] [SPARKR] Support SparkR spawning worker R processes with a command other then Rscript

This is a simple change to add a new environment variable
"spark.sparkr.r.command" that specifies the command that SparkR will
use when creating an R engine process.  If this is not specified,
"Rscript" will be used by default.

I did not add any documentation, since I couldn't find any place where
environment variables (such as "spark.sparkr.use.daemon") are
documented.

I also did not add a unit test.  The only test that would work
generally would be one starting SparkR with
sparkR.init(sparkEnvir=list(spark.sparkr.r.command="Rscript")), just
using the default value.  I think that this is a low-risk change.

Likely committers: shivaram

Author: Michael Sannella x268 <msannell@tibco.com>

Closes #6557 from msannell/altR and squashes the following commits:

7eac142 [Michael Sannella x268] add spark.sparkr.r.command config parameter

1 files changed, 1 insertions, 1 deletions

diff --git a/core/src/main/scala/org/apache/spark/api/r/RRDD.scala b/core/src/main/scala/org/apache/spark/api/r/RRDD.scala
index 4dfa732593..524676544d 100644
--- a/core/src/main/scala/org/apache/spark/api/r/RRDD.scala
+++ b/core/src/main/scala/org/apache/spark/api/r/RRDD.scala
@@ -391,7 +391,7 @@ private[r] object RRDD {
   }
 
   private def createRProcess(rLibDir: String, port: Int, script: String): BufferedStreamThread = {
-    val rCommand = "Rscript"
+    val rCommand = SparkEnv.get.conf.get("spark.sparkr.r.command", "Rscript")
     val rOptions = "--vanilla"
     val rExecScript = rLibDir + "/SparkR/worker/" + script
     val pb = new ProcessBuilder(List(rCommand, rOptions, rExecScript))