From 85256aff75f29c739c198ea150f391a6e3fd35d9 Mon Sep 17 00:00:00 2001
From: Nayana Naik <nayana@labs.digiscrypt.com>
Date: Tue, 27 Oct 2026 13:55:25 +0530
Subject: [PATCH] Check for empty index batch in dictionary DecodeArrow

---
 cpp/src/parquet/decoder.cc       |  3 +++
 cpp/src/parquet/encoding_test.cc | 16 ++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/cpp/src/parquet/decoder.cc b/cpp/src/parquet/decoder.cc
index f90b56a7321..de628ce94b1 100644
--- a/cpp/src/parquet/decoder.cc
+++ b/cpp/src/parquet/decoder.cc
@@ -1392,6 +1392,9 @@ class DictByteArrayDecoderImpl : public DictDecoderImpl<ByteArrayType> {
         int32_t batch_size =
             std::min<int32_t>(kBufferSize, num_values - num_appended - null_count);
         int num_indices = idx_decoder_.GetBatch(indices, batch_size);
+        if (ARROW_PREDICT_FALSE(num_indices < 1)) {
+          return Status::Invalid("Invalid number of indices: ", num_indices);
+        }
 
         int i = 0;
         while (true) {
diff --git a/cpp/src/parquet/encoding_test.cc b/cpp/src/parquet/encoding_test.cc
index 142dcbbf106..eb0a2e0c0ba 100644
--- a/cpp/src/parquet/encoding_test.cc
+++ b/cpp/src/parquet/encoding_test.cc
@@ -1373,6 +1373,22 @@ TEST_F(DictEncoding, CheckDecodeArrowNonNullDictBuilder) {
   this->CheckDecodeArrowNonNullUsingDictBuilder();
 }
 
+TEST_F(DictEncoding, CheckDecodeArrowDictBuilderTruncatedIndices) {
+  // The index stream ends before num_values - null_count indices have been
+  // decoded, so a refill of the index buffer returns no indices at all
+  InitTestCase(::arrow::binary(), /*null_probability=*/0.5, /*create_dict=*/true);
+  dict_decoder_->SetData(num_values_, buffer_->data(),
+                         static_cast<int>(buffer_->size()) / 2);
+  auto builder = CreateDictBuilder();
+  EXPECT_THROW_THAT(
+      [&]() {
+        decoder_->DecodeArrow(num_values_, null_count_, valid_bits_, 0, builder.get());
+      },
+      ParquetException,
+      ::testing::Property(&ParquetException::what,
+                          ::testing::HasSubstr("Invalid number of indices")));
+}
+
 TEST_F(DictEncoding, CheckDecodeIndicesSpaced) {
   for (auto np : null_probabilities_) {
     InitTestCase(::arrow::binary(), np, /*create_dict=*/true);