Simplify logic for popping the current AsyncStackFrame

Summary:
Rather than conditionally calling either popAsyncStackFrameCallee()
or deactivateAsyncStackFrame() depending on whether there is a
parent frame, we now extend popAsyncStackFrameCallee() to handle
the null parent case.

Reviewed By: andriigrynenko

Differential Revision: D24554471

fbshipit-source-id: 7f7ce64da63db1fa726d9fb23669fe49d03c2d6c

folly/tracing/AsyncStack-inl.h

@@ -56,11 +56,12 @@ inline void pushAsyncStackFrameCallerCallee(
 inline void popAsyncStackFrameCallee(
     folly::AsyncStackFrame& calleeFrame) noexcept {
   checkAsyncStackFrameIsActive(calleeFrame);
-  assert(calleeFrame.parentFrame != nullptr);
-  auto& callerFrame = *calleeFrame.parentFrame;
-  auto& stackRoot = *calleeFrame.stackRoot;
-  callerFrame.stackRoot = &stackRoot;
-  stackRoot.topFrame.store(&callerFrame, std::memory_order_release);
+  auto* callerFrame = calleeFrame.parentFrame;
+  auto* stackRoot = calleeFrame.stackRoot;
+  if (callerFrame != nullptr) {
+    callerFrame->stackRoot = stackRoot;
+  }
+  stackRoot->topFrame.store(callerFrame, std::memory_order_release);
 
   // Clearing out non-top-frame's stackRoot is not strictly necessary
   // but it may help with debugging.

folly/tracing/AsyncStack.h

@@ -241,8 +241,9 @@ void pushAsyncStackFrameCallerCallee(
 // you are about to call/resume the caller. e.g. performing a symmetric
 // transfer to the calling coroutine in final_suspend().
 //
-// Assumes that there is actually a parent frame. If there is no parent
-// frame then use deactivateAsyncStackFrame() instead.
+// If calleeFrame.getParentFrame() is null then this method is equivalent
+// to deactivateAsyncStackFrame(), leaving no active AsyncStackFrame on
+// the current AsyncStackRoot.
 void popAsyncStackFrameCallee(folly::AsyncStackFrame& calleeFrame) noexcept;
 
 // Get a pointer to a special frame that can be used as the root-frame