Remove Promise::getFuture from Future-inl.h 2/4 - waitImpl

Summary: Replace calls to Promise::getFuture in waitImpl with getSemiFuture. There is a slight behavioural change here in that we now set the executor on the returned future, but this appears to be the correct behaviour.

Reviewed By: yfeldblum

Differential Revision: D7104627

fbshipit-source-id: fd19c3f6884b560f7d5ea1276f3c80c082dfc53d

folly/futures/Future-inl.h

@@ -1543,6 +1543,19 @@ void waitImpl(FutureType& f) {
   assert(f.isReady());
 }
 
+template <class T>
+void convertFuture(SemiFuture<T>&& sf, Future<T>& f) {
+  // Carry executor from f, inserting an inline executor if it did not have one
+  auto* currentExecutor = f.getExecutor();
+  f = std::move(sf).via(
+      currentExecutor ? currentExecutor : &folly::InlineExecutor::instance());
+}
+
+template <class T>
+void convertFuture(SemiFuture<T>&& sf, SemiFuture<T>& f) {
+  f = std::move(sf);
+}
+
 template <class FutureType, typename T = typename FutureType::value_type>
 void waitImpl(FutureType& f, Duration dur) {
   // short-circuit if there's nothing to do
@@ -1551,13 +1564,13 @@ void waitImpl(FutureType& f, Duration dur) {
   }
 
   Promise<T> promise;
-  auto ret = promise.getFuture();
+  auto ret = promise.getSemiFuture();
   auto baton = std::make_shared<FutureBatonType>();
   f.setCallback_([baton, promise = std::move(promise)](Try<T>&& t) mutable {
     promise.setTry(std::move(t));
     baton->post();
   });
-  f = std::move(ret);
+  convertFuture(std::move(ret), f);
   if (baton->try_wait_for(dur)) {
     assert(f.isReady());
   }

folly/futures/Future.h

@@ -193,6 +193,8 @@ class FutureBase {
   typename std::enable_if<R::ReturnsFuture::value, typename R::Return>::type
   thenImplementation(F&& func, futures::detail::argResult<isTry, F, Args...>);
 };
+template <class T>
+void convertFuture(SemiFuture<T>&& sf, Future<T>& f);
 } // namespace detail
 } // namespace futures
 
@@ -814,6 +816,11 @@ class Future : private futures::detail::FutureBase<T> {
   /// predicate behaves like std::function<bool(void)>
   template <class P, class F>
   friend Future<Unit> whileDo(P&& predicate, F&& thunk);
+
+  template <class FT>
+  friend void futures::detail::convertFuture(
+      SemiFuture<FT>&& sf,
+      Future<FT>& f);
 };
 
 } // namespace folly