Modify Future::getTryVia to be r-value qualified and destructive

Summary: For consistency with get() make getTryVia r-value qualified and move the result out. This has the benefit of removing surprise copies in code that already operated on a temporary future but ended up with an l-value reference to copy from as a result of this operation.

Reviewed By: yfeldblum

Differential Revision: D23489967

fbshipit-source-id: db11c9c5946edd402103b3aa4140c0a1db5aae5e

folly/futures/Future-inl.h

@@ -2393,17 +2393,17 @@ T Future<T>::getVia(TimedDrivableExecutor* e, HighResDuration dur) {
 }
 
 template <class T>
-Try<T>& Future<T>::getTryVia(DrivableExecutor* e) {
-  return waitVia(e).result();
+Try<T> Future<T>::getTryVia(DrivableExecutor* e) && {
+  return std::move(waitVia(e).result());
 }
 
 template <class T>
-Try<T>& Future<T>::getTryVia(TimedDrivableExecutor* e, HighResDuration dur) {
+Try<T> Future<T>::getTryVia(TimedDrivableExecutor* e, HighResDuration dur) && {
   waitVia(e, dur);
   if (!this->isReady()) {
     throw_exception<FutureTimeout>();
   }
-  return result();
+  return std::move(result());
 }
 
 namespace futures {

folly/futures/Future.h

@@ -1143,11 +1143,11 @@ class Future : private futures::detail::FutureBase<T> {
   /// Call e->drive() repeatedly until the future is fulfilled. Examples
   /// of DrivableExecutor include EventBase and ManualExecutor. Returns a
   /// reference to the Try of the value.
-  Try<T>& getTryVia(DrivableExecutor* e);
+  Try<T> getTryVia(DrivableExecutor* e) &&;
 
   /// getTryVia but will wait only until `dur` elapses. Returns the
   /// Try of the value (moved-out) or may throw a FutureTimeout exception.
-  Try<T>& getTryVia(TimedDrivableExecutor* e, HighResDuration dur);
+  Try<T> getTryVia(TimedDrivableExecutor* e, HighResDuration dur) &&;
 
   /// Unwraps the case of a Future<Future<T>> instance, and returns a simple
   /// Future<T> instance.

folly/futures/test/ViaTest.cpp

@@ -465,7 +465,7 @@ TEST(Via, getTryVia) {
     ManualExecutor x;
     auto f = via(&x).thenValue([](auto&&) { return 23; });
     EXPECT_FALSE(f.isReady());
-    EXPECT_EQ(23, f.getTryVia(&x).value());
+    EXPECT_EQ(23, std::move(f).getTryVia(&x).value());
   }
 
   {
@@ -473,14 +473,14 @@ TEST(Via, getTryVia) {
     ManualExecutor x;
     auto f = via(&x).then();
     EXPECT_FALSE(f.isReady());
-    auto t = f.getTryVia(&x);
+    auto t = std::move(f).getTryVia(&x);
     EXPECT_TRUE(t.hasValue());
   }
 
   {
     DummyDrivableExecutor x;
     auto f = makeFuture(23);
-    EXPECT_EQ(23, f.getTryVia(&x).value());
+    EXPECT_EQ(23, std::move(f).getTryVia(&x).value());
     EXPECT_FALSE(x.ran);
   }
 }
@@ -489,7 +489,8 @@ TEST(Via, SimpleTimedGetTryVia) {
   TimedDrivableExecutor e2;
   Promise<folly::Unit> p;
   auto f = p.getFuture();
-  EXPECT_THROW(f.getTryVia(&e2, std::chrono::seconds(1)), FutureTimeout);
+  EXPECT_THROW(
+      std::move(f).getTryVia(&e2, std::chrono::seconds(1)), FutureTimeout);
 }
 
 TEST(Via, waitVia) {