Make a SharedPromise from a Future

Summary: Makes it easy to split a Future into multiple Futures

Reviewed By: rongrong

Differential Revision: D3885897

fbshipit-source-id: 6ac9fb22444dd828fbdebb44b06bf3d93d0f7583

folly/futures/SharedPromise-inl.h

@@ -47,6 +47,11 @@ SharedPromise<T>& SharedPromise<T>::operator=(
   return *this;
 }
 
+template <class T>
+SharedPromise<T>::SharedPromise(Future<T> future) {
+  future.then(&SharedPromise<T>::setTry, this);
+}
+
 template <class T>
 size_t SharedPromise<T>::size() {
   std::lock_guard<std::mutex> g(mutex_);

folly/futures/SharedPromise.h

@@ -46,6 +46,13 @@ public:
   SharedPromise(SharedPromise<T>&&) noexcept;
   SharedPromise& operator=(SharedPromise<T>&&) noexcept;
 
+  /**
+   * Provide a way to split a Future<T>. Note that while the Futures from
+   * `getFuture()' depend on the completion of the parameter Future they do not
+   * inherit any other properties such as Executor's passed to `via' etc.
+   */
+  explicit SharedPromise(Future<T>);
+
   /**
    * Return a Future tied to the shared core state. Unlike Promise::getFuture,
    * this can be called an unlimited number of times per SharedPromise.

folly/futures/test/SharedPromiseTest.cpp

@@ -125,3 +125,33 @@ TEST(SharedPromise, interruptHandler) {
   f.cancel();
   EXPECT_TRUE(flag);
 }
+
+TEST(SharedPromise, splitFutureSuccess) {
+  Promise<int> p;
+  SharedPromise<int> sp(p.getFuture());
+  auto f1 = sp.getFuture();
+  EXPECT_FALSE(f1.isReady());
+  p.setValue(1);
+  EXPECT_TRUE(f1.isReady());
+  EXPECT_TRUE(f1.hasValue());
+  auto f2 = sp.getFuture();
+  EXPECT_TRUE(f2.isReady());
+  EXPECT_TRUE(f2.hasValue());
+}
+
+TEST(SharedPromise, splitFutureFailure) {
+  Promise<int> p;
+  SharedPromise<int> sp(p.getFuture());
+  auto f1 = sp.getFuture();
+  EXPECT_FALSE(f1.isReady());
+  try {
+    throw std::runtime_error("Oops");
+  } catch (...) {
+    p.setException(exception_wrapper(std::current_exception()));
+  }
+  EXPECT_TRUE(f1.isReady());
+  EXPECT_TRUE(f1.hasException());
+  auto f2 = sp.getFuture();
+  EXPECT_TRUE(f2.isReady());
+  EXPECT_TRUE(f2.hasException());
+}