westonpace commented on a change in pull request #9095:
URL: https://github.com/apache/arrow/pull/9095#discussion_r564862995
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File path: cpp/src/arrow/util/task_group.cc
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@@ -135,6 +149,18 @@ class ThreadedTaskGroup : public TaskGroup {
// before cv.notify_one() has returned
std::unique_lock<std::mutex> lock(mutex_);
cv_.notify_one();
+ if (completion_future_.has_value()) {
+ // MarkFinished could be slow. We don't want to call it while we are
holding
+ // the lock.
+ // TODO: If optional is thread safe then we can skip this locking
entirely
+ auto future = *completion_future_;
+ auto finished = completion_future_->is_finished();
+ auto status = status_;
+ lock.unlock();
+ if (!finished) {
+ future.MarkFinished(status);
Review comment:
Indeed. I will make a test case to expose this.
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