Future<typename>::getTryVia

Summary:
[Folly] `Future<typename>::getTryVia`.

We have `waitVia` and `getVia` and `getTry`, but `getTryVia` was missing.

Reviewed By: fugalh

Differential Revision: D4254972

fbshipit-source-id: 403e1a3496ad5dfc93c4c55ab75a83bcc89e6c64

folly/futures/Future-inl.h

@@ -415,6 +415,11 @@ Try<T>& Future<T>::getTry() {
   return core_->getTry();
 }
 
+template <class T>
+Try<T>& Future<T>::getTryVia(DrivableExecutor* e) {
+  return waitVia(e).getTry();
+}
+
 template <class T>
 Optional<Try<T>> Future<T>::poll() {
   Optional<Try<T>> o;

folly/futures/Future.h

@@ -129,6 +129,11 @@ class Future {
   /** A reference to the Try of the value */
   Try<T>& getTry();
 
+  /// 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);
+
   /// If the promise has been fulfilled, return an Optional with the Try<T>.
   /// Otherwise return an empty Optional.
   /// Note that this moves the Try<T> out.

folly/futures/test/ViaTest.cpp

@@ -400,6 +400,32 @@ TEST(Via, getVia) {
   }
 }
 
+TEST(Via, getTryVia) {
+  {
+    // non-void
+    ManualExecutor x;
+    auto f = via(&x).then([] { return 23; });
+    EXPECT_FALSE(f.isReady());
+    EXPECT_EQ(23, f.getTryVia(&x).value());
+  }
+
+  {
+    // void
+    ManualExecutor x;
+    auto f = via(&x).then();
+    EXPECT_FALSE(f.isReady());
+    auto t = f.getTryVia(&x);
+    EXPECT_TRUE(t.hasValue());
+  }
+
+  {
+    DummyDrivableExecutor x;
+    auto f = makeFuture(23);
+    EXPECT_EQ(23, f.getTryVia(&x).value());
+    EXPECT_FALSE(x.ran);
+  }
+}
+
 TEST(Via, waitVia) {
   {
     ManualExecutor x;