folly: within - grab executor keepalive before moving

Summary: Previous within changes changed how long the executor keepalive existed for. This explicitly grabs the executor keepalive before calling within to ensure it still exists when we try to use it in `via`.

Reviewed By: LeeHowes

Differential Revision: D17054622

fbshipit-source-id: 7826c6ac0ad332a8f5f1331cd2d798619cf318c3

folly/futures/Future-inl.h

@@ -2013,11 +2013,10 @@ Future<T> Future<T>::within(Duration dur, E e, Timekeeper* tk) && {
     return std::move(*this);
   }
 
-  auto* exe = this->getExecutor();
-  return std::move(*this)
-      .semi()
-      .within(dur, e, tk)
-      .via(exe ? exe : &InlineExecutor::instance());
+  auto* ePtr = this->getExecutor();
+  auto exe =
+      folly::getKeepAliveToken(ePtr ? *ePtr : InlineExecutor::instance());
+  return std::move(*this).semi().within(dur, e, tk).via(std::move(exe));
 }
 
 template <class T>