On 7/12/2018 12:59, Damian Jarek wrote:
Here's an example of what might happen if a composed operation doesn't maintain work guards properly: https://wandbox.org/permlink/aqsGDNJWTmFd7PdC
Why use conditional logic and extra storage in operator() when the compiler can do it for you? https://wandbox.org/permlink/HzOlDt8S6txfLNB6
Without the work_guard the coroutine never completes. If you add the work_guard, everything works correctly.
I can see it happening in your example, but I still don't really grok why this occurs. Isn't the point of composed operations to ensure that they use the same executor for all handlers? So it's the same executor as the underlying timer. When it's in a wait operation, the timer should be taking care of it. And while in the direct call context of operator() then the executor itself should know there is work in progress. So the only time where the work_guard should be having any effect is either if async_foo itself yields (which it does, but only after creating the op and making a call to async_wait, so that should keep it alive) or if the call to handler_ yields somewhere else (which it doesn't). (handler_() might internally post and yield rather than executing synchronously, especially cross-context, but in that case its executor should know that it's doing something.) So what am I missing? (I guess one of the things that I might be struggling with is that in Ye Olde Asio, as long as you always had an async_* in flight at all times then you never needed any io_service::work. Usually that was easy because you typically have a listen or read in flight.)