Fix darwin and hphp build failures for PrimaryPtr.

Summary:
On darwin with Xcode, deleter for a shared_ptr is passed by copying the whole lambda, causing issues when building PrimaryPtr.

In this diff we try to work around this problem by putting the last promise value on the heap while sharing the control block with the shared_ptr exposed to user.

Reviewed By: yfeldblum

Differential Revision: D27354087

fbshipit-source-id: 9cb23912c9f1058505f3c20384eb998233eba2da

folly/experimental/PrimaryPtr.h

@@ -203,10 +203,7 @@ class PrimaryPtr {
           return std::move(this->unreferenced_);
         })
         // start cleanup tasks
-        .deferValue([this](folly::Unit) {
-          auto cleanup = getCleanup(innerPtr_.get());
-          return std::move(cleanup);
-        })
+        .deferValue([this](folly::Unit) { return getCleanup(innerPtr_.get()); })
         .defer([this](folly::Try<folly::Unit> r) {
           if (r.hasException()) {
             LOG(FATAL) << "Cleanup actions must be noexcept.";
@@ -235,10 +232,26 @@ class PrimaryPtr {
     auto referencesContract = folly::makePromiseContract<folly::Unit>();
     unreferenced_ = std::move(std::get<1>(referencesContract));
 
+    // The deleter object needs to be copyable in std::shared_ptr on some
+    // platform. To work around this limitation we can slightly tweak the
+    // semantics of deleter copy constructor and check we always use this
+    // object at most once.
+    class LastReference {
+     public:
+      LastReference(Promise<Unit>&& p) : p_(std::move(p)) {}
+      LastReference(LastReference&&) = default;
+      LastReference(LastReference& other) : LastReference(std::move(other)) {}
+      void operator()(T*) {
+        DCHECK(!p_.isFulfilled());
+        p_.setValue();
+      }
+
+     private:
+      Promise<Unit> p_;
+    };
     auto innerPtrShared = std::shared_ptr<T>(
         innerPtr_.get(),
-        [lastReference = std::move(std::get<0>(referencesContract))](
-            T*) mutable { lastReference.setValue(folly::Unit{}); });
+        LastReference{std::move(std::get<0>(referencesContract))});
 
     outerPtrWeak_ = outerPtrShared_ =
         std::make_shared<std::shared_ptr<T>>(innerPtrShared);