SPARK-1240: handle the case of empty RDD when takeSample

https://spark-project.atlassian.net/browse/SPARK-1240

It seems that the current implementation does not handle the empty RDD case when run takeSample

In this patch, before calling sample() inside takeSample API, I add a checker for this case and returns an empty Array when it's a empty RDD; also in sample(), I add a checker for the invalid fraction value

In the test case, I also add several lines for this case

Author: CodingCat <zhunansjtu@gmail.com>

Closes apache#135 from CodingCat/SPARK-1240 and squashes the following commits:

fef57d4 [CodingCat] fix the same problem in PySpark
36db06b [CodingCat] create new test cases for takeSample from an empty red
810948d [CodingCat] further fix
a40e8fb [CodingCat] replace if with require
ad483fd [CodingCat] handle the case with empty RDD when take sample

Conflicts:
	core/src/main/scala/org/apache/spark/rdd/RDD.scala
	core/src/test/scala/org/apache/spark/rdd/RDDSuite.scala

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

@@ -319,8 +319,10 @@ abstract class RDD[T: ClassTag](
   /**
    * Return a sampled subset of this RDD.
    */
-  def sample(withReplacement: Boolean, fraction: Double, seed: Int): RDD[T] =
+  def sample(withReplacement: Boolean, fraction: Double, seed: Int): RDD[T] = {
+    require(fraction >= 0.0, "Invalid fraction value: " + fraction)
     new SampledRDD(this, withReplacement, fraction, seed)
+  }
 
   def takeSample(withReplacement: Boolean, num: Int, seed: Int): Array[T] = {
     var fraction = 0.0
@@ -333,6 +335,10 @@ abstract class RDD[T: ClassTag](
       throw new IllegalArgumentException("Negative number of elements requested")
     }
 
+    if (initialCount == 0) {
+      return new Array[T](0)
+    }
+
     if (initialCount > Integer.MAX_VALUE - 1) {
       maxSelected = Integer.MAX_VALUE - 1
     } else {
@@ -351,7 +357,7 @@ abstract class RDD[T: ClassTag](
     var samples = this.sample(withReplacement, fraction, rand.nextInt()).collect()
 
     // If the first sample didn't turn out large enough, keep trying to take samples;
-    // this shouldn't happen often because we use a big multiplier for thei initial size
+    // this shouldn't happen often because we use a big multiplier for the initial size
     while (samples.length < total) {
       samples = this.sample(withReplacement, fraction, rand.nextInt()).collect()
     }

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

@@ -455,6 +455,7 @@ class RDDSuite extends FunSuite with SharedSparkContext {
 
   test("takeSample") {
     val data = sc.parallelize(1 to 100, 2)
+
     for (seed <- 1 to 5) {
       val sample = data.takeSample(withReplacement=false, 20, seed)
       assert(sample.size === 20)        // Got exactly 20 elements
@@ -486,6 +487,12 @@ class RDDSuite extends FunSuite with SharedSparkContext {
     }
   }
 
+  test("takeSample from an empty rdd") {
+    val emptySet = sc.parallelize(Seq.empty[Int], 2)
+    val sample = emptySet.takeSample(false, 20, 1)
+    assert(sample.length === 0)
+  }
+
   test("runJob on an invalid partition") {
     intercept[IllegalArgumentException] {
       sc.runJob(sc.parallelize(1 to 10, 2), {iter: Iterator[Int] => iter.size}, Seq(0, 1, 2), false)

python/pyspark/rdd.py

@@ -252,6 +252,7 @@ def sample(self, withReplacement, fraction, seed):
         >>> sc.parallelize(range(0, 100)).sample(False, 0.1, 2).collect() #doctest: +SKIP
         [2, 3, 20, 21, 24, 41, 42, 66, 67, 89, 90, 98]
         """
+        assert fraction >= 0.0, "Invalid fraction value: %s" % fraction
         return self.mapPartitionsWithIndex(RDDSampler(withReplacement, fraction, seed).func, True)
 
     # this is ported from scala/spark/RDD.scala
@@ -272,6 +273,9 @@ def takeSample(self, withReplacement, num, seed):
         if (num < 0):
             raise ValueError
 
+        if (initialCount == 0):
+            return list()
+
         if initialCount > sys.maxint - 1:
             maxSelected = sys.maxint - 1
         else: