Future API for CacheClientCommon

Summary:
Allow mutable functors in Promise::fulfil.

Facebook:
Works both with EventBase and without it (thanks to wangle::Future thread-safety). CacheClientImpl lifetime is guaranteed using shared_ptr.

Test Plan: unit test

Reviewed By: stepan@fb.com

FB internal diff: D1291024

folly/wangle/Promise-inl.h

@@ -124,8 +124,8 @@ void Promise<T>::setValue() {
 
 template <class T>
 template <class F>
-void Promise<T>::fulfil(const F& func) {
-  fulfilHelper(func);
+void Promise<T>::fulfil(F&& func) {
+  fulfilHelper(std::forward<F>(func));
 }
 
 template <class T>
@@ -133,7 +133,7 @@ template <class F>
 typename std::enable_if<
   std::is_convertible<typename std::result_of<F()>::type, T>::value &&
   !std::is_same<T, void>::value>::type
-inline Promise<T>::fulfilHelper(const F& func) {
+inline Promise<T>::fulfilHelper(F&& func) {
   throwIfFulfilled();
   try {
     setValue(func());
@@ -147,7 +147,7 @@ template <class F>
 typename std::enable_if<
   std::is_same<typename std::result_of<F()>::type, void>::value &&
   std::is_same<T, void>::value>::type
-inline Promise<T>::fulfilHelper(const F& func) {
+inline Promise<T>::fulfilHelper(F&& func) {
   throwIfFulfilled();
   try {
     func();

folly/wangle/Promise.h

@@ -73,7 +73,7 @@ public:
     p.fulfil([] { do something that may throw; return a T; });
   */
   template <class F>
-  void fulfil(const F& func);
+  void fulfil(F&& func);
 
 private:
   typedef typename Future<T>::objPtr objPtr;
@@ -91,13 +91,13 @@ private:
   typename std::enable_if<
     std::is_convertible<typename std::result_of<F()>::type, T>::value &&
     !std::is_same<T, void>::value>::type
-  fulfilHelper(const F& func);
+  fulfilHelper(F&& func);
 
   template <class F>
   typename std::enable_if<
     std::is_same<typename std::result_of<F()>::type, void>::value &&
     std::is_same<T, void>::value>::type
-  fulfilHelper(const F& func);
+  fulfilHelper(F&& func);
 };
 
 }}