[SPARK-8452] [SPARKR] expose jobGroup API in SparkR

This pull request adds following methods to SparkR:

```R
setJobGroup()
cancelJobGroup()
clearJobGroup()
```
For each method, the spark context is passed as the first argument. There does not seem to be a good way to test these in R.

cc shivaram and davies

Author: Hossein <hossein@databricks.com>

Closes #6889 from falaki/SPARK-8452 and squashes the following commits:

9ce9f1e [Hossein] Added basic tests to verify methods can be called and won't throw errors
c706af9 [Hossein] Added examples
a2c19af [Hossein] taking spark context as first argument
343ca77 [Hossein] Added setJobGroup, cancelJobGroup and clearJobGroup to SparkR

R/pkg/NAMESPACE

@@ -10,6 +10,11 @@ export("sparkR.init")
 export("sparkR.stop")
 export("print.jobj")
 
+# Job group lifecycle management methods
+export("setJobGroup",
+       "clearJobGroup",
+       "cancelJobGroup")
+
 exportClasses("DataFrame")
 
 exportMethods("arrange",

R/pkg/R/sparkR.R

@@ -278,3 +278,47 @@ sparkRHive.init <- function(jsc = NULL) {
   assign(".sparkRHivesc", hiveCtx, envir = .sparkREnv)
   hiveCtx
 }
+
+#' Assigns a group ID to all the jobs started by this thread until the group ID is set to a
+#' different value or cleared.
+#'
+#' @param sc existing spark context
+#' @param groupid the ID to be assigned to job groups
+#' @param description description for the the job group ID
+#' @param interruptOnCancel flag to indicate if the job is interrupted on job cancellation
+#' @examples
+#'\dontrun{
+#' sc <- sparkR.init()
+#' setJobGroup(sc, "myJobGroup", "My job group description", TRUE)
+#'}
+
+setJobGroup <- function(sc, groupId, description, interruptOnCancel) {
+  callJMethod(sc, "setJobGroup", groupId, description, interruptOnCancel)
+}
+
+#' Clear current job group ID and its description
+#'
+#' @param sc existing spark context
+#' @examples
+#'\dontrun{
+#' sc <- sparkR.init()
+#' clearJobGroup(sc)
+#'}
+
+clearJobGroup <- function(sc) {
+  callJMethod(sc, "clearJobGroup")
+}
+
+#' Cancel active jobs for the specified group
+#'
+#' @param sc existing spark context
+#' @param groupId the ID of job group to be cancelled
+#' @examples
+#'\dontrun{
+#' sc <- sparkR.init()
+#' cancelJobGroup(sc, "myJobGroup")
+#'}
+
+cancelJobGroup <- function(sc, groupId) {
+  callJMethod(sc, "cancelJobGroup", groupId)
+}

R/pkg/inst/tests/test_context.R

@@ -48,3 +48,10 @@ test_that("rdd GC across sparkR.stop", {
   count(rdd3)
   count(rdd4)
 })
+
+test_that("job group functions can be called", {
+  sc <- sparkR.init()
+  setJobGroup(sc, "groupId", "job description", TRUE)
+  cancelJobGroup(sc, "groupId")
+  clearJobGroup(sc)
+})