SPARK-8336 Fix NullPointerException with functions.rand()

This PR fixes the problem reported by Justin Yip in the thread 'NullPointerException with functions.rand()'

Tested using spark-shell and verified that the following works:
sqlContext.createDataFrame(Seq((1,2), (3, 100))).withColumn("index", rand(30)).show()

Author: tedyu <yuzhihong@gmail.com>

Closes #6793 from tedyu/master and squashes the following commits:

62fd97b [tedyu] Create RandomSuite
750f92c [tedyu] Add test for Rand() with seed
a1d66c5 [tedyu] Fix NullPointerException with functions.rand()

2 files changed, 38 insertions, 1 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/random.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/random.scala
index cc34467391..45588bacd2 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/random.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/random.scala
@@ -37,7 +37,11 @@ abstract class RDG(seed: Long) extends LeafExpression with Serializable {
    * Record ID within each partition. By being transient, the Random Number Generator is
    * reset every time we serialize and deserialize it.
    */
-  @transient protected lazy val rng = new XORShiftRandom(seed + TaskContext.get().partitionId())
+  @transient protected lazy val partitionId = TaskContext.get() match {
+    case null => 0
+    case _ => TaskContext.get().partitionId()
+  }
+  @transient protected lazy val rng = new XORShiftRandom(seed + partitionId)
 
   override def deterministic: Boolean = false
 
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/RandomSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/RandomSuite.scala
new file mode 100644
index 0000000000..9be2b23a53
--- /dev/null
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/RandomSuite.scala
@@ -0,0 +1,33 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.expressions
+
+import org.scalatest.Matchers._
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.catalyst.dsl.expressions._
+import org.apache.spark.sql.types.{DoubleType, IntegerType}
+
+
+class RandomSuite extends SparkFunSuite with ExpressionEvalHelper {
+
+  test("random") {
+    val row = create_row(1.1, 2.0, 3.1, null)
+    checkDoubleEvaluation(Rand(30), (0.7363714192755834 +- 0.001), row)
+  }
+}