pitrou commented on code in PR #45954:
URL: https://github.com/apache/arrow/pull/45954#discussion_r2021141798
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cpp/src/arrow/util/async_generator.h:
##########
@@ -772,20 +782,30 @@ class ReadaheadGenerator {
Future<T> operator()() {
if (state_->readahead_queue.empty()) {
// This is the first request, let's pump the underlying queue
- state_->num_running.store(state_->max_readahead);
+ {
+ auto guard = state_->mutex.Lock();
+ state_->num_running = state_->max_readahead;
+ }
for (int i = 0; i < state_->max_readahead; i++) {
auto next = state_->source_generator();
auto next_after_check = AddMarkFinishedContinuation(std::move(next));
state_->readahead_queue.push(std::move(next_after_check));
}
}
// Pop one and add one
- auto result = state_->readahead_queue.front();
+ auto result = std::move(state_->readahead_queue.front());
state_->readahead_queue.pop();
- if (state_->finished.load()) {
+ bool is_finished = false;
+ {
+ auto guard = state_->mutex.Lock();
+ is_finished = state_->finished;
+ if (!is_finished) {
+ ++state_->num_running;
+ }
Review Comment:
Suggestion: add a comment to explain this
```suggestion
if (!is_finished) {
// We're going to push to the queue below, but we need
// to update `num_running` while we're holding the lock.
++state_->num_running;
}
```
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