make accumulator.localValue public, add tests

core/src/main/scala/spark/Accumulators.scala

@@ -35,7 +35,16 @@ class Accumulable[T,R] (
     else throw new UnsupportedOperationException("Can't use read value in task")
   }
 
-  private[spark] def localValue = value_
+  /**
+   * get the current value of this accumulator from within a task.
+   *
+   * This is NOT the global value of the accumulator.  To get the global value after a
+   * completed operation on the dataset, call `value`.
+   *
+   * The typical use of this method is to directly mutate the local value, eg., to add
+   * an element to a Set.
+   */
+  def localValue = value_
 
   def value_= (t: T) {
     if (!deserialized) value_ = t

core/src/test/scala/spark/AccumulatorSuite.scala

@@ -79,4 +79,18 @@ class AccumulatorSuite extends FunSuite with ShouldMatchers {
     }
   }
 
+  test ("localValue readable in tasks") {
+    import SetAccum._
+    val maxI = 1000
+    for (nThreads <- List(1, 10)) { //test single & multi-threaded
+    val sc = new SparkContext("local[" + nThreads + "]", "test")
+      val acc: Accumulable[mutable.Set[Any], Any] = sc.accumulable(new mutable.HashSet[Any]())
+      val d = sc.parallelize(1 to maxI)
+      d.foreach {
+        x => acc.localValue += x
+      }
+      acc.value should be ( (1 to maxI).toSet)
+    }
+  }
+
 }