[SPARK-10351] [SQL] Fixes UTF8String.fromAddress to handle off-heap memory

CC rxin marmbrus

Author: Feynman Liang <fliang@databricks.com>

Closes #8523 from feynmanliang/SPARK-10351.

sql/core/src/test/scala/org/apache/spark/sql/UnsafeRowSuite.scala

@@ -43,12 +43,12 @@ class UnsafeRowSuite extends SparkFunSuite {
     val arrayBackedUnsafeRow: UnsafeRow =
       UnsafeProjection.create(Array[DataType](StringType, StringType, IntegerType)).apply(row)
     assert(arrayBackedUnsafeRow.getBaseObject.isInstanceOf[Array[Byte]])
-    val bytesFromArrayBackedRow: Array[Byte] = {
+    val (bytesFromArrayBackedRow, field0StringFromArrayBackedRow): (Array[Byte], String) = {
       val baos = new ByteArrayOutputStream()
       arrayBackedUnsafeRow.writeToStream(baos, null)
-      baos.toByteArray
+      (baos.toByteArray, arrayBackedUnsafeRow.getString(0))
     }
-    val bytesFromOffheapRow: Array[Byte] = {
+    val (bytesFromOffheapRow, field0StringFromOffheapRow): (Array[Byte], String) = {
       val offheapRowPage = MemoryAllocator.UNSAFE.allocate(arrayBackedUnsafeRow.getSizeInBytes)
       try {
         Platform.copyMemory(
@@ -69,13 +69,14 @@ class UnsafeRowSuite extends SparkFunSuite {
         val baos = new ByteArrayOutputStream()
         val writeBuffer = new Array[Byte](1024)
         offheapUnsafeRow.writeToStream(baos, writeBuffer)
-        baos.toByteArray
+        (baos.toByteArray, offheapUnsafeRow.getString(0))
       } finally {
         MemoryAllocator.UNSAFE.free(offheapRowPage)
       }
     }
 
     assert(bytesFromArrayBackedRow === bytesFromOffheapRow)
+    assert(field0StringFromArrayBackedRow === field0StringFromOffheapRow)
   }
 
   test("calling getDouble() and getFloat() on null columns") {

unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -90,11 +90,7 @@ public final class UTF8String implements Comparable<UTF8String>, Externalizable
    * Creates an UTF8String from given address (base and offset) and length.
    */
   public static UTF8String fromAddress(Object base, long offset, int numBytes) {
-    if (base != null) {
-      return new UTF8String(base, offset, numBytes);
-    } else {
-      return null;
-    }
+    return new UTF8String(base, offset, numBytes);
   }
 
   /**