Let Futures Barrier::ControlBlock ctor fully construct

Summary:
[Folly] Let Futures `Barrier::ControlBlock` ctor fully construct.

Invoke the constructor when allocating the control block, rather than try to initialize each field afterward.

Differential Revision: D8605292

fbshipit-source-id: d51648a1267ed7e3e13d9a9e8d8e5d1c18e24a98

folly/futures/Barrier.cpp

@@ -39,11 +39,11 @@ Barrier::~Barrier() {
 }
 
 auto Barrier::allocateControlBlock() -> ControlBlock* {
-  auto block = static_cast<ControlBlock*>(malloc(controlBlockSize(size_)));
-  if (!block) {
+  auto storage = malloc(controlBlockSize(size_));
+  if (!storage) {
     throw_exception<std::bad_alloc>();
   }
-  std::atomic_init(&block->valueAndReaderCount, uint64_t(0));
+  auto block = ::new (storage) ControlBlock();
 
   auto p = promises(block);
   uint32_t i = 0;

folly/futures/Barrier.h

@@ -70,7 +70,7 @@ class Barrier {
   struct ControlBlock {
     // Reader count in most significant 32 bits
     // Value in least significant 32 bits
-    std::atomic<uint64_t> valueAndReaderCount;
+    std::atomic<uint64_t> valueAndReaderCount{0};
   };
 
   struct ControlBlockAndPromise {