MINOR: Fix IOOBE in ListVector/LargeListVector.getFieldBuffers() for never-allocated offset buffer

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

@@ -277,8 +277,23 @@ public List<ArrowBuf> getFieldBuffers() {
     List<ArrowBuf> result = new ArrayList<>(2);
     setReaderAndWriterIndex();
     result.add(validityBuffer);
+    if (valueCount == 0 && offsetBuffer.capacity() == 0 && offsetBuffer.writerIndex() > 0) {
+      long writerIdx = offsetBuffer.writerIndex();
+      ArrowBuf oldOffsetBuffer = offsetBuffer;
+      if (validityBuffer == oldOffsetBuffer) {
+        validityBuffer.readerIndex(0);
+        validityBuffer.writerIndex(0);
+      } else {
+        oldOffsetBuffer.getReferenceManager().release();
+      }
+      long allocSize =
+          Math.max((long) (valueCount + 1) * OFFSET_WIDTH, INITIAL_VALUE_ALLOCATION * OFFSET_WIDTH);
+      offsetBuffer = allocator.buffer(allocSize);
+      offsetBuffer.setZero(0, offsetBuffer.capacity());
+      offsetBuffer.readerIndex(0);
+      offsetBuffer.writerIndex(writerIdx);
+    }
     result.add(offsetBuffer);
-
     return result;
   }
 

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

@@ -235,8 +235,23 @@ public List<ArrowBuf> getFieldBuffers() {
     List<ArrowBuf> result = new ArrayList<>(2);
     setReaderAndWriterIndex();
     result.add(validityBuffer);
+    if (valueCount == 0 && offsetBuffer.capacity() == 0 && offsetBuffer.writerIndex() > 0) {
+      long writerIdx = offsetBuffer.writerIndex();
+      ArrowBuf oldOffsetBuffer = offsetBuffer;
+      if (validityBuffer == oldOffsetBuffer) {
+        validityBuffer.readerIndex(0);
+        validityBuffer.writerIndex(0);
+      } else {
+        oldOffsetBuffer.getReferenceManager().release();
+      }
+      long allocSize =
+          Math.max((long) (valueCount + 1) * OFFSET_WIDTH, INITIAL_VALUE_ALLOCATION * OFFSET_WIDTH);
+      offsetBuffer = allocator.buffer(allocSize);
+      offsetBuffer.setZero(0, offsetBuffer.capacity());
+      offsetBuffer.readerIndex(0);
+      offsetBuffer.writerIndex(writerIdx);
+    }
     result.add(offsetBuffer);
-
     return result;
   }
 

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

@@ -1120,6 +1120,32 @@ public void testEmptyLargeListOffsetBuffer() {
     }
   }
 
+  @Test
+  public void testEmptyLargeListOffsetBufferWithoutAllocate() {
+    // Regression test for the Arrow 19 IOOBE: a never-allocated LargeListVector must produce a
+    // valid offset buffer from getFieldBuffers() even when allocateNew() was never called.
+    try (LargeListVector list = LargeListVector.empty("list", allocator)) {
+      list.addOrGetVector(FieldType.nullable(MinorType.INT.getType()));
+      list.setValueCount(0);
+
+      List<ArrowBuf> buffers = list.getFieldBuffers();
+      ArrowBuf validityBuf = buffers.get(0);
+      assertEquals(0, validityBuf.readerIndex());
+      assertEquals(0, validityBuf.writerIndex());
+      assertEquals(0, validityBuf.readableBytes());
+
+      ArrowBuf offsetBuf = buffers.get(1);
+      assertTrue(
+          offsetBuf.capacity() >= LargeListVector.OFFSET_WIDTH,
+          "Returned offset buffer should have capacity >= " + LargeListVector.OFFSET_WIDTH);
+      assertTrue(
+          offsetBuf.readableBytes() >= LargeListVector.OFFSET_WIDTH,
+          "Returned offset buffer should have readableBytes >= " + LargeListVector.OFFSET_WIDTH);
+      assertEquals(0L, offsetBuf.getLong(0));
+      // Vector owns the buffer — no manual close needed
+    }
+  }
+
   private void writeIntValues(UnionLargeListWriter writer, int[] values) {
     writer.startList();
     for (int v : values) {

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

@@ -1399,6 +1399,35 @@ public void testEmptyListOffsetBuffer() {
     }
   }
 
+  @Test
+  public void testEmptyListOffsetBufferWithoutAllocate() {
+    // Regression test for the Arrow 19 IOOBE: a never-allocated ListVector must produce a valid
+    // offset buffer from getFieldBuffers() even when allocateNew() was never called.
+    // getFieldBuffers() allocates a real offset buffer for an empty vector whose own offset buffer
+    // has capacity 0 but writerIndex > 0 (the inconsistent state from Arrow 19).
+    try (ListVector list = ListVector.empty("list", allocator)) {
+      list.addOrGetVector(FieldType.nullable(MinorType.INT.getType()));
+      list.setValueCount(0);
+
+      List<ArrowBuf> buffers = list.getFieldBuffers();
+      ArrowBuf validityBuf = buffers.get(0);
+      assertEquals(0, validityBuf.readerIndex());
+      assertEquals(0, validityBuf.writerIndex());
+      assertEquals(0, validityBuf.readableBytes());
+
+      ArrowBuf offsetBuf = buffers.get(1);
+      assertTrue(
+          offsetBuf.capacity() >= BaseRepeatedValueVector.OFFSET_WIDTH,
+          "Returned offset buffer should have capacity >= " + BaseRepeatedValueVector.OFFSET_WIDTH);
+      assertTrue(
+          offsetBuf.readableBytes() >= BaseRepeatedValueVector.OFFSET_WIDTH,
+          "Returned offset buffer should have readableBytes >= "
+              + BaseRepeatedValueVector.OFFSET_WIDTH);
+      assertEquals(0, offsetBuf.getInt(0));
+      // Vector owns the buffer — no manual close needed
+    }
+  }
+
   private void writeIntValues(UnionListWriter writer, int[] values) {
     writer.startList();
     for (int v : values) {