westonpace commented on a change in pull request #9892:
URL: https://github.com/apache/arrow/pull/9892#discussion_r609655564
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File path: cpp/src/arrow/util/thread_pool.h
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@@ -233,15 +233,7 @@ class ARROW_EXPORT SerialExecutor : public Executor {
if (final_fut.is_finished()) {
return final_fut.result();
}
- final_fut = final_fut.Then(
- [this](const T& res) {
- MarkFinished();
- return res;
- },
- [this](const Status& st) -> Result<T> {
- MarkFinished();
- return st;
- });
+ final_fut.AddCallback([this](const Result<T>&) { MarkFinished(); });
Review comment:
There are some differences between `AddCallback` and `Then` here.
Callbacks are run after the future waiters are notified. This means that
`final_fut.result()` would return before `MarkFinished()` has run. It
"shouldn't" matter since it would probably be bad form to mark the final future
complete while there are other tasks lying around on the executor. However, I
wasn't sure it would always be safe. For example, whomever called
`MarkFinished` is going to destruct themselves and it might be nice to ensure
that happens before we return from `result()`.
On the other hand, by using `Then` we ensure that `MarkFinished()` has
completed executing before `final_fut.result()` returns.
So I'm not certain this change is a problem but I feel it does add risk that
we don't need.
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