fix: offset buffer with empty array

Author: Yicong Huang

vector/src/main/java/org/apache/arrow/vector/complex/LargeListVector.java

@@ -275,6 +275,14 @@ public void loadFieldBuffers(ArrowFieldNode fieldNode, List<ArrowBuf> ownBuffers
   @Override
   public List<ArrowBuf> getFieldBuffers() {
     List<ArrowBuf> result = new ArrayList<>(2);
+
+    // Ensure offset buffer has at least one entry for offset[0].
+    // According to Arrow specification, offset buffer must have N+1 entries,
+    // even when N=0, it should contain [0].
+    if (offsetBuffer.capacity() == 0) {
+      offsetBuffer = allocateOffsetBuffer(OFFSET_WIDTH);
+    }
+
     setReaderAndWriterIndex();
     result.add(validityBuffer);
     result.add(offsetBuffer);
@@ -309,7 +317,8 @@ private void setReaderAndWriterIndex() {
     offsetBuffer.readerIndex(0);
     if (valueCount == 0) {
       validityBuffer.writerIndex(0);
-      offsetBuffer.writerIndex(0);
+      // Even when valueCount is 0, offset buffer should have offset[0] per Arrow spec
+      offsetBuffer.writerIndex(Math.min(OFFSET_WIDTH, offsetBuffer.capacity()));
     } else {
       validityBuffer.writerIndex(BitVectorHelper.getValidityBufferSizeFromCount(valueCount));
       offsetBuffer.writerIndex((valueCount + 1) * OFFSET_WIDTH);

vector/src/main/java/org/apache/arrow/vector/complex/ListVector.java

@@ -233,6 +233,14 @@ public void loadFieldBuffers(ArrowFieldNode fieldNode, List<ArrowBuf> ownBuffers
   @Override
   public List<ArrowBuf> getFieldBuffers() {
     List<ArrowBuf> result = new ArrayList<>(2);
+
+    // Ensure offset buffer has at least one entry for offset[0].
+    // According to Arrow specification, offset buffer must have N+1 entries,
+    // even when N=0, it should contain [0].
+    if (offsetBuffer.capacity() == 0) {
+      offsetBuffer = allocateOffsetBuffer(OFFSET_WIDTH);
+    }
+
     setReaderAndWriterIndex();
     result.add(validityBuffer);
     result.add(offsetBuffer);
@@ -267,7 +275,8 @@ private void setReaderAndWriterIndex() {
     offsetBuffer.readerIndex(0);
     if (valueCount == 0) {
       validityBuffer.writerIndex(0);
-      offsetBuffer.writerIndex(0);
+      // Even when valueCount is 0, offset buffer should have offset[0] per Arrow spec
+      offsetBuffer.writerIndex(Math.min(OFFSET_WIDTH, offsetBuffer.capacity()));
     } else {
       validityBuffer.writerIndex(BitVectorHelper.getValidityBufferSizeFromCount(valueCount));
       offsetBuffer.writerIndex((valueCount + 1) * OFFSET_WIDTH);

vector/src/test/java/org/apache/arrow/vector/TestLargeListVector.java

@@ -1100,6 +1100,36 @@ public void testCopyValueSafeForExtensionType() throws Exception {
     }
   }
 
+  @Test
+  public void testNestedEmptyLargeListOffsetBuffer() {
+    // Test that nested LargeListVector properly allocates offset buffer
+    // even when nested writers are never invoked. According to Arrow spec,
+    // offset buffer must have N+1 entries. Even when N=0, it should contain [0].
+    try (LargeListVector outerList = LargeListVector.empty("outer", allocator)) {
+      // Setup LargeList<LargeList<Int>>
+      outerList.addOrGetVector(FieldType.nullable(MinorType.LARGELIST.getType()));
+      LargeListVector innerList = (LargeListVector) outerList.getDataVector();
+      innerList.addOrGetVector(FieldType.nullable(MinorType.INT.getType()));
+
+      // Allocate outer only - simulates case where inner is never written to
+      outerList.allocateNew();
+      outerList.setValueCount(0);
+
+      // Get field buffers - this is what IPC serialization uses
+      List<ArrowBuf> innerBuffers = innerList.getFieldBuffers();
+
+      // Verify inner list offset buffer has at least OFFSET_WIDTH (8) bytes
+      assertTrue(
+          innerBuffers.get(1).readableBytes() >= LargeListVector.OFFSET_WIDTH,
+          "Inner LargeList offset buffer should have at least "
+              + LargeListVector.OFFSET_WIDTH
+              + " bytes for offset[0]");
+
+      // Verify offset[0] = 0
+      assertEquals(0L, innerList.getOffsetBuffer().getLong(0));
+    }
+  }
+
   private void writeIntValues(UnionLargeListWriter writer, int[] values) {
     writer.startList();
     for (int v : values) {

vector/src/test/java/org/apache/arrow/vector/TestListVector.java

@@ -1379,6 +1379,45 @@ public void testCopyValueSafeForExtensionType() throws Exception {
     }
   }
 
+  @Test
+  public void testNestedEmptyListOffsetBuffer() {
+    // Test that 3-level nested ListVector properly allocates offset buffers
+    // even when nested writers are never invoked. According to Arrow spec,
+    // offset buffer must have N+1 entries. Even when N=0, it should contain [0].
+    try (ListVector level0 = ListVector.empty("level0", allocator)) {
+      // Setup List<List<List<Int>>> - 3 levels
+      level0.addOrGetVector(FieldType.nullable(MinorType.LIST.getType()));
+      ListVector level1 = (ListVector) level0.getDataVector();
+      level1.addOrGetVector(FieldType.nullable(MinorType.LIST.getType()));
+      ListVector level2 = (ListVector) level1.getDataVector();
+      level2.addOrGetVector(FieldType.nullable(MinorType.INT.getType()));
+
+      // Only allocate level0 - simulates case where all nested levels are empty
+      level0.allocateNew();
+      level0.setValueCount(0);
+
+      // Verify all levels have properly allocated offset buffers
+      List<ArrowBuf> level1Buffers = level1.getFieldBuffers();
+      List<ArrowBuf> level2Buffers = level2.getFieldBuffers();
+
+      assertTrue(
+          level1Buffers.get(1).readableBytes() >= BaseRepeatedValueVector.OFFSET_WIDTH,
+          "Level1 offset buffer should have at least "
+              + BaseRepeatedValueVector.OFFSET_WIDTH
+              + " bytes for offset[0]");
+
+      assertTrue(
+          level2Buffers.get(1).readableBytes() >= BaseRepeatedValueVector.OFFSET_WIDTH,
+          "Level2 offset buffer should have at least "
+              + BaseRepeatedValueVector.OFFSET_WIDTH
+              + " bytes for offset[0]");
+
+      // Verify offset[0] = 0 for all levels
+      assertEquals(0, level1.getOffsetBuffer().getInt(0));
+      assertEquals(0, level2.getOffsetBuffer().getInt(0));
+    }
+  }
+
   private void writeIntValues(UnionListWriter writer, int[] values) {
     writer.startList();
     for (int v : values) {