Fix inconsistent handling / encoding of record lengths.

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeInMemorySorter.java

@@ -156,6 +156,7 @@ public boolean hasNext() {
 
     @Override
     public void loadNext() {
+      // This pointer points to a 4-byte record length, followed by the record's bytes
       final long recordPointer = sortBuffer[position];
       baseObject = memoryManager.getPage(recordPointer);
       baseOffset = memoryManager.getOffsetInPage(recordPointer) + 4;  // Skip over record length

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeSorterSpillWriter.java

@@ -91,6 +91,14 @@ private void writeIntToBuffer(int v, int offset) throws IOException {
     writeBuffer[offset + 3] = (byte)(v >>>  0);
   }
 
+  /**
+   * Write a record to a spill file.
+   *
+   * @param baseObject the base object / memory page containing the record
+   * @param baseOffset the base offset which points directly to the record data.
+   * @param recordLength the length of the record.
+   * @param keyPrefix a sort key prefix
+   */
   public void write(
       Object baseObject,
       long baseOffset,
@@ -105,8 +113,8 @@ public void write(
     writeIntToBuffer(recordLength, 0);
     writeLongToBuffer(keyPrefix, 4);
     int dataRemaining = recordLength;
-    int freeSpaceInWriteBuffer = DISK_WRITE_BUFFER_SIZE - 4 - 8;
-    long recordReadPosition = baseOffset + 4; // skip over record length
+    int freeSpaceInWriteBuffer = DISK_WRITE_BUFFER_SIZE - 4 - 8; // space used by prefix + len
+    long recordReadPosition = baseOffset;
     while (dataRemaining > 0) {
       final int toTransfer = Math.min(freeSpaceInWriteBuffer, dataRemaining);
       PlatformDependent.copyMemory(

core/src/test/java/org/apache/spark/util/collection/unsafe/sort/UnsafeInMemorySorterSuite.java

@@ -34,14 +34,13 @@
 
 public class UnsafeInMemorySorterSuite {
 
-  private static String getStringFromDataPage(Object baseObject, long baseOffset) {
-    final int strLength = PlatformDependent.UNSAFE.getInt(baseObject, baseOffset);
-    final byte[] strBytes = new byte[strLength];
+  private static String getStringFromDataPage(Object baseObject, long baseOffset, int length) {
+    final byte[] strBytes = new byte[length];
     PlatformDependent.copyMemory(
       baseObject,
-      baseOffset + 4,
+      baseOffset,
       strBytes,
-      PlatformDependent.BYTE_ARRAY_OFFSET, strLength);
+      PlatformDependent.BYTE_ARRAY_OFFSET, length);
     return new String(strBytes);
   }
 
@@ -116,7 +115,7 @@ public int compare(long prefix1, long prefix2) {
       // position now points to the start of a record (which holds its length).
       final int recordLength = PlatformDependent.UNSAFE.getInt(baseObject, position);
       final long address = memoryManager.encodePageNumberAndOffset(dataPage, position);
-      final String str = getStringFromDataPage(baseObject, position);
+      final String str = getStringFromDataPage(baseObject, position + 4, recordLength);
       final int partitionId = hashPartitioner.getPartition(str);
       sorter.insertRecord(address, partitionId);
       position += 4 + recordLength;
@@ -127,9 +126,8 @@ public int compare(long prefix1, long prefix2) {
     Arrays.sort(dataToSort);
     while (iter.hasNext()) {
       iter.loadNext();
-      // TODO: the logic for how we manipulate record length offsets here is confusing; clean
-      // this up and clarify it in comments.
-      final String str = getStringFromDataPage(iter.getBaseObject(), iter.getBaseOffset() - 4);
+      final String str =
+        getStringFromDataPage(iter.getBaseObject(), iter.getBaseOffset(), iter.getRecordLength());
       final long keyPrefix = iter.getKeyPrefix();
       assertThat(str, isIn(Arrays.asList(dataToSort)));
       assertThat(keyPrefix, greaterThanOrEqualTo(prevPrefix));