Update AsyncScope so that the first frame's return-address is the .add() callsite

Summary:
Modify the AsyncScope::add() method so that it records the call-site of .add()
as the first frame's return address instead of recording .add() itself as the
first frame.

This should hopefully give a bit more insight into where the detached task was
launched.

Reviewed By: andriigrynenko

Differential Revision: D24478808

fbshipit-source-id: 0723891e77af5e52c7f8c13e218e4fb9120867e3

folly/experimental/coro/AsyncScope.h

@@ -149,12 +149,13 @@ inline std::size_t AsyncScope::remaining() const noexcept {
 }
 
 template <typename Awaitable>
-inline void AsyncScope::add(Awaitable&& awaitable) {
+FOLLY_NOINLINE inline void AsyncScope::add(Awaitable&& awaitable) {
   assert(
       !joined_ &&
       "It is invalid to add() more work after work has been joined");
   anyTasksStarted_.store(true, std::memory_order_relaxed);
-  addImpl((Awaitable &&) awaitable).start(&barrier_);
+  addImpl((Awaitable &&) awaitable)
+      .start(&barrier_, FOLLY_ASYNC_STACK_RETURN_ADDRESS());
 }
 
 inline Task<void> AsyncScope::joinAsync() noexcept {

folly/experimental/coro/detail/BarrierTask.h

@@ -194,12 +194,16 @@ class DetachedBarrierTask {
   }
 
   FOLLY_NOINLINE void start(Barrier* barrier) && noexcept {
+    std::move(*this).start(barrier, FOLLY_ASYNC_STACK_RETURN_ADDRESS());
+  }
+
+  void start(Barrier* barrier, void* returnAddress) && noexcept {
     assert(coro_);
     assert(barrier != nullptr);
     barrier->add(1);
     auto coro = std::exchange(coro_, {});
     coro.promise().setBarrier(barrier);
-    coro.promise().getAsyncFrame().setReturnAddress();
+    coro.promise().getAsyncFrame().setReturnAddress(returnAddress);
     folly::resumeCoroutineWithNewAsyncStackRoot(coro);
   }