[SPARK-5064][GraphX] Add numEdges upperbound validation for R-MAT graph...

[SPARK-5064][GraphX] Add numEdges upperbound validation for R-MAT graph generator to prevent infinite loop

I looked into GraphGenerators#chooseCell, and found that chooseCell can't generate more edges than pow(2, (2 * (log2(numVertices)-1))) to make a Power-law graph. (Ex. numVertices:4 upperbound:4, numVertices:8 upperbound:16, numVertices:16 upperbound:64)
If we request more edges over the upperbound, rmatGraph fall into infinite loop. So, how about adding an argument validation?

Author: Kenji Kikushima <kikushima.kenji@lab.ntt.co.jp>

Closes #3950 from kj-ki/SPARK-5064 and squashes the following commits:

4ee18c7 [Ankur Dave] Reword error message and add unit test
d760bc7 [Kenji Kikushima] Add numEdges upperbound validation for R-MAT graph generator to prevent infinite loop.

graphx/src/main/scala/org/apache/spark/graphx/util/GraphGenerators.scala

@@ -133,6 +133,12 @@ object GraphGenerators {
     // This ensures that the 4 quadrants are the same size at all recursion levels
     val numVertices = math.round(
       math.pow(2.0, math.ceil(math.log(requestedNumVertices) / math.log(2.0)))).toInt
+    val numEdgesUpperBound =
+      math.pow(2.0, 2 * ((math.log(numVertices) / math.log(2.0)) - 1)).toInt
+    if (numEdgesUpperBound < numEdges) {
+      throw new IllegalArgumentException(
+        s"numEdges must be <= $numEdgesUpperBound but was $numEdges")
+    }
     var edges: Set[Edge[Int]] = Set()
     while (edges.size < numEdges) {
       if (edges.size % 100 == 0) {

graphx/src/test/scala/org/apache/spark/graphx/util/GraphGeneratorsSuite.scala

@@ -110,4 +110,14 @@ class GraphGeneratorsSuite extends FunSuite with LocalSparkContext {
     }
   }
 
+  test("SPARK-5064 GraphGenerators.rmatGraph numEdges upper bound") {
+    withSpark { sc =>
+      val g1 = GraphGenerators.rmatGraph(sc, 4, 4)
+      assert(g1.edges.count() === 4)
+      intercept[IllegalArgumentException] {
+        val g2 = GraphGenerators.rmatGraph(sc, 4, 8)
+      }
+    }
+  }
+
 }