c++boostboost-asioboost-coroutine

boost.asio spawn call handler directly from current stack

My application use boost.asio to run network related tasks that yield (coroutines).

On some occasions when my task is spawned from another spawned task, it's being called directly from the current task's callstack.

I would expect that it'll be queued and may run after the current task get suspended (if no other tasks are pending on the same io_context)

looking at the spawn code, it reaches boost::asio::dispatch which trigger async operation

  return async_initiate<NullaryToken, void()>(
      detail::initiate_dispatch(), token);

So it seems odd that my task has been called directly and not from some dispatcher, is it possible ?

Thanks

Solution

  • asio::dispatch is documented to dispatch on an executor.

    The Executor concept documents the behaviour for dispatch as:

    Effects: Creates an object f1 initialized with DECAY_COPY(forward<Func>(f)) (C++Std [thread.decaycopy]) in the current thread of execution . Calls f1() at most once. The executor may block forward progress of the caller until f1() finishes execution.

    (emphasis mine).

    Contrast this with post or defer:

    The executor shall not block forward progress of the caller pending completion of f1()

    Simply put: dispatch MAY execute the handler immediately but only if the currently executing executor implements this optimization and is compatible with the associated executor.

    See also for background/examples: