[SPARK-3121] Wrong implementation of implicit bytesWritableConverter

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -21,6 +21,7 @@ import scala.language.implicitConversions
 
 import java.io._
 import java.net.URI
+import java.util.Arrays
 import java.util.concurrent.atomic.AtomicInteger
 import java.util.{Properties, UUID}
 import java.util.UUID.randomUUID
@@ -1409,7 +1410,10 @@ object SparkContext extends Logging {
     simpleWritableConverter[Boolean, BooleanWritable](_.get)
 
   implicit def bytesWritableConverter(): WritableConverter[Array[Byte]] = {
-    simpleWritableConverter[Array[Byte], BytesWritable](_.getBytes)
+    simpleWritableConverter[Array[Byte], BytesWritable](bw =>
+      // getBytes method returns array which is longer then data to be returned
+      Arrays.copyOfRange(bw.getBytes, 0, bw.getLength)
+    )
   }
 
   implicit def stringWritableConverter(): WritableConverter[String] =

core/src/test/scala/org/apache/spark/SparkContextSuite.scala

@@ -0,0 +1,40 @@
+/*
+ * 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
+
+import org.scalatest.FunSuite
+
+import org.apache.hadoop.io.BytesWritable
+
+class SparkContextSuite extends FunSuite {
+  //Regression test for SPARK-3121
+  test("BytesWritable implicit conversion is correct") {
+    val bytesWritable = new BytesWritable()
+    val inputArray = (1 to 10).map(_.toByte).toArray
+    bytesWritable.set(inputArray, 0, 10)
+    bytesWritable.set(inputArray, 0, 5)
+
+    val converter = SparkContext.bytesWritableConverter()
+    val byteArray = converter.convert(bytesWritable)
+    assert(byteArray.length === 5)
+
+    bytesWritable.set(inputArray, 0, 0)
+    val byteArray2 = converter.convert(bytesWritable)
+    assert(byteArray2.length === 0)
+  }
+}