HADOOP-18530. ChecksumFileSystem::readVectored might return byte buffers not positioned at 0 (#5168)

Contributed by Harshit Gupta

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/VectoredReadUtils.java

@@ -307,9 +307,16 @@ public final class VectoredReadUtils {
                                    FileRange request) {
     int offsetChange = (int) (request.getOffset() - readOffset);
     int requestLength = request.getLength();
+    // Create a new buffer that is backed by the original contents
+    // The buffer will have position 0 and the same limit as the original one
     readData = readData.slice();
+    // Change the offset and the limit of the buffer as the reader wants to see
+    // only relevant data
     readData.position(offsetChange);
     readData.limit(offsetChange + requestLength);
+    // Create a new buffer after the limit change so that only that portion of the data is
+    // returned to the reader.
+    readData = readData.slice();
     return readData;
   }
 

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/fs/TestVectoredReadUtils.java

@@ -61,6 +61,9 @@ public class TestVectoredReadUtils extends HadoopTestBase {
             .describedAs("Slicing on the same offset shouldn't " +
                     "create a new buffer")
             .isEqualTo(slice);
+    Assertions.assertThat(slice.position())
+        .describedAs("Slicing should return buffers starting from position 0")
+        .isEqualTo(0);
 
     // try slicing a range
     final int offset = 100;
@@ -77,6 +80,9 @@ public class TestVectoredReadUtils extends HadoopTestBase {
             .describedAs("Slicing should use the same underlying " +
                     "data")
             .isEqualTo(slice.array());
+    Assertions.assertThat(slice.position())
+        .describedAs("Slicing should return buffers starting from position 0")
+        .isEqualTo(0);
     // test the contents of the slice
     intBuffer = slice.asIntBuffer();
     for(int i=0; i < sliceLength / Integer.BYTES; ++i) {