[SPARK-16620][CORE] Add back the tokenization process in `RDD.pipe(command: String)`

## What changes were proposed in this pull request?

Currently `RDD.pipe(command: String)`:
- works only when the command is specified without any options, such as `RDD.pipe("wc")`
- does NOT work when the command is specified with some options, such as `RDD.pipe("wc -l")`

This is a regression from Spark 1.6.

This patch adds back the tokenization process in `RDD.pipe(command: String)` to fix this regression.

## How was this patch tested?
Added a test which:
- would pass in `1.6`
- _[prior to this patch]_ would fail in `master`
- _[after this patch]_ would pass in `master`

Author: Liwei Lin <lwlin7@gmail.com>

Closes #14256 from lw-lin/rdd-pipe.

core/src/main/scala/org/apache/spark/rdd/RDD.scala

@@ -699,14 +699,18 @@ abstract class RDD[T: ClassTag](
    * Return an RDD created by piping elements to a forked external process.
    */
   def pipe(command: String): RDD[String] = withScope {
-    pipe(command)
+    // Similar to Runtime.exec(), if we are given a single string, split it into words
+    // using a standard StringTokenizer (i.e. by spaces)
+    pipe(PipedRDD.tokenize(command))
   }
 
   /**
    * Return an RDD created by piping elements to a forked external process.
    */
   def pipe(command: String, env: Map[String, String]): RDD[String] = withScope {
-    pipe(command, env)
+    // Similar to Runtime.exec(), if we are given a single string, split it into words
+    // using a standard StringTokenizer (i.e. by spaces)
+    pipe(PipedRDD.tokenize(command), env)
   }
 
   /**

core/src/test/scala/org/apache/spark/rdd/PipedRDDSuite.scala

@@ -51,6 +51,22 @@ class PipedRDDSuite extends SparkFunSuite with SharedSparkContext {
     }
   }
 
+  test("basic pipe with tokenization") {
+    if (testCommandAvailable("wc")) {
+      val nums = sc.makeRDD(Array(1, 2, 3, 4), 2)
+
+      // verify that both RDD.pipe(command: String) and RDD.pipe(command: String, env) work good
+      for (piped <- Seq(nums.pipe("wc -l"), nums.pipe("wc -l", Map[String, String]()))) {
+        val c = piped.collect()
+        assert(c.size === 2)
+        assert(c(0).trim === "2")
+        assert(c(1).trim === "2")
+      }
+    } else {
+      assert(true)
+    }
+  }
+
   test("failure in iterating over pipe input") {
     if (testCommandAvailable("cat")) {
       val nums =