RequestContext changed after co_await. about folly

[yiyuanliu]

I think this problem may be related to the following code in ViaIfAsync.h. If a task uses the default RequestContext, promise.context should be nullptr. It may be wrong to modify RequestContext here. I'm not sure about this, but after removing this logic, RequestContext remains consistent before and after co_await.

folly/folly/experimental/coro/ViaIfAsync.h

Lines 117 to 123 in 9079e21

	FOLLY_CORO_AWAIT_SUSPEND_NONTRIVIAL_ATTRIBUTES void await_suspend(
	coroutine_handle<promise_type> h) noexcept {
	auto& promise = h.promise();
	if (!promise.context_) {
	promise.setRequestContext(RequestContext::saveContext());
	}

[yiyuanliu]

I think this problem may be related to the following code in ViaIfAsync.h. If a task uses the default RequestContext, promise.context should be nullptr. It may be wrong to modify RequestContext here. I'm not sure about this, but after removing this logic, RequestContext remains consistent before and after co_await.

folly/folly/experimental/coro/ViaIfAsync.h

Lines 117 to 123 in 9079e21

	FOLLY_CORO_AWAIT_SUSPEND_NONTRIVIAL_ATTRIBUTES void await_suspend(
	coroutine_handle<promise_type> h) noexcept {
	auto& promise = h.promise();
	if (!promise.context_) {
	promise.setRequestContext(RequestContext::saveContext());
	}

folly/folly/experimental/coro/ViaIfAsync.h

Lines 295 to 310 in 9079e21

	// NOTE: We are using a heuristic here to determine when is the correct
	// time to capture the RequestContext. We want to capture the context just
	// before the coroutine suspends and execution is returned to the executor.
	//
	// In cases where we are awaiting another coroutine and symmetrically
	// transferring execution to another coroutine we are not yet returning
	// execution to the executor so we want to defer capturing the context until
	// the ViaCoroutine is resumed and suspends in final_suspend() before
	// scheduling the resumption on the executor.
	//
	// In cases where the awaitable may suspend without transferring execution
	// to another coroutine and will therefore return back to the executor we
	// want to capture the execution context before calling into the wrapped
	// awaitable's await_suspend() method (since it's await_suspend() method
	// might schedule resumption on another thread and could resume and destroy
	// the ViaCoroutine before the await_suspend() method returns).

It seems that an attempt is being made here to determine whether RequestContext needs to be saved in FinalAwaiter by checking whether promise.context_ is a null pointer. However, this approach overlooks cases where the coroutine uses the default RequestContext. In such cases, promise.context_ should be a null pointer as well. Perhaps we could modify the type of promise.context_ to optional<std::shared_ptr<RequestContext>>, so that we can differentiate between cases where RequestContext is not saved and cases where RequestContext is default.