Fix addNullsForUnselectedRows (facebookincubator#9279)

Summary:
Pull Request resolved: facebookincubator#9279

addNullsForUnselectedRows used memcpy to copy bits from SelectivityVector into
the nulls buffer. Memcpy copies full bytes, hence, it copies a few extra bits
at the tail when rows.end() is not a multiple of 8. This causes lambda
functions that use addNullsForUnselectedRows to generate invalid vectors and
results in crashes.

See facebookincubator#5990 for additional context.

Reviewed By: bikramSingh91

Differential Revision: D55455300

fbshipit-source-id: e98b515b1f4f1876bacb7a483a305fba098522cd

velox/functions/lib/LambdaFunctionUtil.cpp

@@ -159,10 +159,16 @@ BufferPtr addNullsForUnselectedRows(
     const SelectivityVector& rows) {
   // Set nulls for rows not present in 'rows'.
   BufferPtr nulls = allocateNulls(vector->size(), vector->pool(), bits::kNull);
-  memcpy(
+
+  // bits::kNull is 0. Hence, bits::orBits() simply copies the bits from the
+  // selectivity vector into the nulls buffer. We cannot use memcpy because it
+  // will copy extra bits at the tail if rows.end() is not a multiple of 8.
+  bits::orBits(
       nulls->asMutable<uint64_t>(),
       rows.asRange().bits(),
-      bits::nbytes(rows.end()));
+      rows.begin(),
+      rows.end());
+
   if (vector->nulls() != nullptr) {
     // Transfer original nulls
     bits::andBits(

velox/functions/lib/tests/CMakeLists.txt

@@ -19,6 +19,7 @@ add_executable(
   IsNullTest.cpp
   IsNotNullTest.cpp
   KllSketchTest.cpp
+  LambdaFunctionUtilTest.cpp
   MapConcatTest.cpp
   Re2FunctionsTest.cpp
   RepeatTest.cpp

velox/functions/lib/tests/LambdaFunctionUtilTest.cpp

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "velox/functions/lib/LambdaFunctionUtil.h"
+#include "velox/vector/tests/utils/VectorTestBase.h"
+
+namespace facebook::velox::functions {
+namespace {
+
+class LambdaFunctionUtilTest : public testing::Test,
+                               public test::VectorTestBase {
+ protected:
+  static void SetUpTestCase() {
+    memory::MemoryManager::testingSetInstance({});
+  }
+};
+
+TEST_F(LambdaFunctionUtilTest, addNullsForUnselectedRows) {
+  auto vector = makeFlatVector<int64_t>({1, 2, 3, 4, 5});
+
+  SelectivityVector rows(3);
+  auto nulls = addNullsForUnselectedRows(vector, rows);
+
+  auto* rawNulls = nulls->as<uint64_t>();
+
+  EXPECT_FALSE(bits::isBitNull(rawNulls, 0));
+  EXPECT_FALSE(bits::isBitNull(rawNulls, 1));
+  EXPECT_FALSE(bits::isBitNull(rawNulls, 2));
+  EXPECT_TRUE(bits::isBitNull(rawNulls, 3));
+  EXPECT_TRUE(bits::isBitNull(rawNulls, 4));
+}
+
+} // namespace
+} // namespace facebook::velox::functions