pitrou commented on code in PR #45372:
URL: https://github.com/apache/arrow/pull/45372#discussion_r1941468163
##########
cpp/src/arrow/record_batch.cc:
##########
@@ -102,8 +103,10 @@ class SimpleRecordBatch : public RecordBatch {
std::shared_ptr<Array> column(int i) const override {
std::shared_ptr<Array> result = std::atomic_load(&boxed_columns_[i]);
if (!result) {
- result = MakeArray(columns_[i]);
- std::atomic_store(&boxed_columns_[i], result);
+ auto new_array = MakeArray(columns_[i]);
+ if (std::atomic_compare_exchange_strong(&boxed_columns_[i], &result,
new_array)) {
Review Comment:
```suggestion
// Be careful not to overwrite existing entry if another thread has
been calling
// `column(i)` at the same time, since the `boxed_columns_` contents
are exposed
// by `columns()` (see GH-45371).
if (std::atomic_compare_exchange_strong(&boxed_columns_[i], &result,
new_array)) {
```
##########
cpp/src/arrow/record_batch_test.cc:
##########
@@ -393,6 +399,27 @@ TEST_F(TestRecordBatch, RemoveColumnEmpty) {
AssertBatchesEqual(*added, *batch1);
}
+TEST_F(TestRecordBatch, ColumnsThreadSafety) {
+ const int length = 10;
+
+ random::RandomArrayGenerator gen(42);
+ std::shared_ptr<ArrayData> array_data = gen.ArrayOf(utf8(), length)->data();
+ auto schema = ::arrow::schema({field("f1", utf8())});
+ auto record_batch = RecordBatch::Make(schema, length, {array_data});
+ std::atomic_bool start_flag{false};
+ std::thread t([record_batch, &start_flag]() {
+ start_flag.store(true);
+ auto columns = record_batch->columns();
+ ASSERT_EQ(columns.size(), 1);
Review Comment:
> This test will either produce a TSAN warning or not, but there is no
assertion that will fail consistently under the data race.
Actually, one could check that the returned `columns` store identical
`Array*` pointers?
Something like:
```c++
constexpr int kNumThreads = 10;
random::RandomArrayGenerator gen(42);
std::shared_ptr<ArrayData> array_data = gen.ArrayOf(utf8(), 1)->data();
auto schema = ::arrow::schema({field("f1", utf8())});
auto record_batch = RecordBatch::Make(schema, length, {array_data});
std::mutex mutex;
std::vector<std::shared_ptr<Array>> all_columns;
for (auto& thread : threads) {
thread = std::thread([&]() {
const auto& columns = record_batch->columns();
all_columns.push_back(columns[0].get());
});
}
for (auto& thread : threads) {
thread.join();
}
for (const auto& col : all_columns) {
ASSERT_EQ(col.get(), all_columns[0].get());
}
```
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