westonpace commented on a change in pull request #9095:
URL: https://github.com/apache/arrow/pull/9095#discussion_r564825292
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File path: cpp/src/arrow/util/future.h
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@@ -580,4 +615,74 @@ inline std::vector<int> WaitForAny(const
std::vector<Future<T>*>& futures,
return waiter->MoveFinishedFutures();
}
+template <typename T = detail::Empty>
+struct ControlFlow {
+ using BreakValueType = T;
+
+ bool IsBreak() const { return break_value_.has_value(); }
+
+ static Result<BreakValueType> MoveBreakValue(const ControlFlow& cf) {
+ return std::move(*cf.break_value_);
+ }
+
+ mutable util::optional<BreakValueType> break_value_;
+};
+
+struct Continue {
+ template <typename T>
+ operator ControlFlow<T>() && { // NOLINT explicit
+ return {};
+ }
+};
+
+template <typename T = detail::Empty>
+ControlFlow<T> Break(T break_value = {}) {
+ return ControlFlow<T>{std::move(break_value)};
+}
+
+template <typename Iterate,
+ typename Control = typename
detail::result_of_t<Iterate()>::ValueType,
+ typename BreakValueType = typename Control::BreakValueType>
+Future<BreakValueType> Loop(Iterate iterate) {
+ auto break_fut = Future<BreakValueType>::Make();
+
+ struct Callback {
+ bool CheckForTermination(const Result<Control>& maybe_control) {
+ if (!maybe_control.ok() || maybe_control->IsBreak()) {
+ Result<BreakValueType> maybe_break =
maybe_control.Map(Control::MoveBreakValue);
+ break_fut.MarkFinished(std::move(maybe_break));
+ return true;
+ }
+ return false;
+ }
+
+ void operator()(const Result<Control>& maybe_control) && {
+ if (CheckForTermination(maybe_control)) return;
+
+ auto control_fut = iterate();
+ while (control_fut.is_finished()) {
+ // There's no need to AddCallback on a finished future; we can
CheckForTermination
+ // now. This also avoids recursion and potential stack overflow.
Review comment:
There is a mutex inside of `AddCallback`. We could create a
`TryAddCallback` which returns false without running the callback if the future
is finished. This method could be guaranteed to never run the callback
synchronously. I will add this.
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