Improve stack-tracing for tasks launched by the merge() internals

Summary:
The stack-traces for child worker tasks launched to process each
incoming AsyncGenerator now have the same common consumer-task
as the parent frame, which is then a detached-task, rather than
each of the child workers being their own detached tasks.

Reviewed By: andriigrynenko

Differential Revision: D24480161

fbshipit-source-id: c02887abf676bde9b297ee6d8f0fcabe959616e3

folly/experimental/coro/Merge-inl.h

@@ -25,6 +25,7 @@
 #include <folly/experimental/coro/WithCancellation.h>
 #include <folly/experimental/coro/detail/Barrier.h>
 #include <folly/experimental/coro/detail/BarrierTask.h>
+#include <folly/experimental/coro/detail/CurrentAsyncFrame.h>
 #include <folly/experimental/coro/detail/Helpers.h>
 #include <exception>
 #include <memory>
@@ -111,13 +112,15 @@ AsyncGenerator<Reference, Value> merge(
 
     detail::Barrier barrier{1};
 
+    auto& asyncFrame = co_await detail::co_current_async_stack_frame;
+
     exception_wrapper ex;
     try {
       while (auto item = co_await sources_.next()) {
         if (state->cancelSource.isCancellationRequested()) {
           break;
         }
-        makeWorkerTask(state, *std::move(item)).start(&barrier);
+        makeWorkerTask(state, *std::move(item)).start(&barrier, asyncFrame);
       }
     } catch (const std::exception& e) {
       ex = exception_wrapper{std::current_exception(), e};
@@ -135,7 +138,7 @@ AsyncGenerator<Reference, Value> merge(
             CallbackRecord<Reference>{callback_record_error, std::move(ex)};
         state->recordPublished.post();
       }
-    };
+    }
 
     // Wait for all worker tasks to finish consuming the entirety of their
     // input streams.

folly/experimental/coro/detail/BarrierTask.h

@@ -197,6 +197,15 @@ class DetachedBarrierTask {
     std::move(*this).start(barrier, FOLLY_ASYNC_STACK_RETURN_ADDRESS());
   }
 
+  FOLLY_NOINLINE void start(
+      Barrier* barrier,
+      folly::AsyncStackFrame& parentFrame) &&
+      noexcept {
+    assert(coro_);
+    coro_.promise().getAsyncFrame().setParentFrame(parentFrame);
+    std::move(*this).start(barrier, FOLLY_ASYNC_STACK_RETURN_ADDRESS());
+  }
+
   void start(Barrier* barrier, void* returnAddress) && noexcept {
     assert(coro_);
     assert(barrier != nullptr);