GH-35521: [C++] Hash null bitmap only if null count is 0 (#35522)

micah-white · pitrou · web-flow · commit e7a885d89989 · 2023-05-11T19:52:09.000+02:00
### Are these changes tested? I am new to contributing and am having a hard time creating a good test for this. The steps to reproduce this bug originally are too complicated for a simple test. I included my attempt at making a good test in the PR, but some help would be nice. --> ### Are there any user-facing changes? No. **This PR contains a "Critical Fix".** * Closes: #35521 Lead-authored-by: micah-white <micahwhitecs@gmail.com> Co-authored-by: Antoine Pitrou <antoine@python.org> Signed-off-by: Antoine Pitrou <antoine@python.org>
diff --git a/cpp/src/arrow/scalar.cc b/cpp/src/arrow/scalar.cc
@@ -153,9 +153,10 @@ struct ScalarHashImpl {
 
   Status ArrayHash(const ArrayData& a) {
     RETURN_NOT_OK(StdHash(a.length) & StdHash(a.GetNullCount()));
-    if (a.buffers[0] != nullptr) {
+    if (a.GetNullCount() != 0 && a.buffers[0] != nullptr) {
       // We can't visit values without unboxing the whole array, so only hash
-      // the null bitmap for now.
+      // the null bitmap for now. Only hash the null bitmap if the null count
+      // is not 0 to ensure hash consistency.
       RETURN_NOT_OK(BufferHash(*a.buffers[0]));
     }
     for (const auto& child : a.child_data) {
diff --git a/cpp/src/arrow/scalar_test.cc b/cpp/src/arrow/scalar_test.cc
@@ -1064,7 +1064,6 @@ class TestListScalar : public ::testing::Test {
   using ScalarType = typename TypeTraits<T>::ScalarType;
 
   void SetUp() {
-    //     type_ = std::make_shared<T>(int16());
     type_ = MakeListType<T>(int16(), 3);
     value_ = ArrayFromJSON(int16(), "[1, 2, null]");
   }
@@ -1106,6 +1105,24 @@ class TestListScalar : public ::testing::Test {
     ASSERT_RAISES(Invalid, scalar.ValidateFull());
   }
 
+  void TestHashing() {
+    // GH-35521: the hash value of a non-null list scalar should not
+    // depend on the presence or absence of a null bitmap in the underlying
+    // list values.
+    ScalarType empty_bitmap_scalar(ArrayFromJSON(int16(), "[1, 2, 3]"));
+    ASSERT_OK(empty_bitmap_scalar.ValidateFull());
+    // Underlying list array doesn't have a null bitmap
+    ASSERT_EQ(empty_bitmap_scalar.value->data()->buffers[0], nullptr);
+
+    auto list_array = ArrayFromJSON(type_, "[[1, 2, 3], [4, 5, null]]");
+    ASSERT_OK_AND_ASSIGN(auto set_bitmap_scalar_uncasted, list_array->GetScalar(0));
+    auto set_bitmap_scalar = checked_pointer_cast<ScalarType>(set_bitmap_scalar_uncasted);
+    // Underlying list array has a null bitmap
+    ASSERT_NE(set_bitmap_scalar->value->data()->buffers[0], nullptr);
+    // ... yet it's hashing equal to the other scalar
+    ASSERT_EQ(empty_bitmap_scalar.hash(), set_bitmap_scalar->hash());
+  }
+
  protected:
   std::shared_ptr<DataType> type_;
   std::shared_ptr<Array> value_;
@@ -1119,6 +1136,8 @@ TYPED_TEST(TestListScalar, Basics) { this->TestBasics(); }
 
 TYPED_TEST(TestListScalar, ValidateErrors) { this->TestValidateErrors(); }
 
+TYPED_TEST(TestListScalar, TestHashing) { this->TestHashing(); }
+
 TEST(TestFixedSizeListScalar, ValidateErrors) {
   const auto ty = fixed_size_list(int16(), 3);
   FixedSizeListScalar scalar(ArrayFromJSON(int16(), "[1, 2, 5]"), ty);
