Fix null handling for Arrow data (#6227)

include/LightGBM/arrow.tpp

@@ -144,7 +144,7 @@ struct ArrayIndexAccessor {
     //  - The structure of validity bitmasks is taken from here:
     //    https://arrow.apache.org/docs/format/Columnar.html#validity-bitmaps
     //  - If the bitmask is NULL, all indices are valid
-    if (validity == nullptr || !(validity[buffer_idx / 8] & (1 << (buffer_idx % 8)))) {
+    if (validity == nullptr || (validity[buffer_idx / 8] & (1 << (buffer_idx % 8)))) {
       // In case the index is valid, we take it from the data buffer
       auto data = static_cast<const T*>(array->buffers[1]);
       return static_cast<double>(data[buffer_idx]);

tests/cpp_tests/test_arrow.cpp

@@ -41,10 +41,12 @@ class ArrowChunkedArrayTest : public testing::Test {
     // 1) Create validity bitmap
     char* validity = nullptr;
     if (!null_indices.empty()) {
-      validity = static_cast<char*>(calloc(values.size() + sizeof(char) - 1, sizeof(char)));
+      auto num_bytes = (values.size() + 7) / 8;
+      validity = static_cast<char*>(calloc(num_bytes, sizeof(char)));
+      memset(validity, 0xff, num_bytes * sizeof(char));
       for (size_t i = 0; i < values.size(); ++i) {
         if (std::find(null_indices.begin(), null_indices.end(), i) != null_indices.end()) {
-          validity[i / 8] |= (1 << (i % 8));
+          validity[i / 8] &= ~(1 << (i % 8));
         }
       }
     }

tests/python_package_test/test_arrow.py

@@ -46,6 +46,16 @@ def generate_simple_arrow_table() -> pa.Table:
     return pa.Table.from_arrays(columns, names=[f"col_{i}" for i in range(len(columns))])
 
 
+def generate_nullable_arrow_table() -> pa.Table:
+    columns = [
+        pa.chunked_array([[1, None, 3, 4, 5]], type=pa.float32()),
+        pa.chunked_array([[None, 2, 3, 4, 5]], type=pa.float32()),
+        pa.chunked_array([[1, 2, 3, 4, None]], type=pa.float32()),
+        pa.chunked_array([[None, None, None, None, None]], type=pa.float32()),
+    ]
+    return pa.Table.from_arrays(columns, names=[f"col_{i}" for i in range(len(columns))])
+
+
 def generate_dummy_arrow_table() -> pa.Table:
     col1 = pa.chunked_array([[1, 2, 3], [4, 5]], type=pa.uint8())
     col2 = pa.chunked_array([[0.5, 0.6], [0.1, 0.8, 1.5]], type=pa.float32())
@@ -95,6 +105,7 @@ def dummy_dataset_params() -> Dict[str, Any]:
     [  # Use lambda functions here to minimize memory consumption
         (lambda: generate_simple_arrow_table(), dummy_dataset_params()),
         (lambda: generate_dummy_arrow_table(), dummy_dataset_params()),
+        (lambda: generate_nullable_arrow_table(), dummy_dataset_params()),
         (lambda: generate_random_arrow_table(3, 1000, 42), {}),
         (lambda: generate_random_arrow_table(100, 10000, 43), {}),
     ],