demo::task supports async cancellation

with poll_context:
when stop_source.request_stop() calls poll_context::cancel(), the target
operation's io_base::cancel() is called inline. this calls
sender_awaiter::stop() which transitions from stop_state::stopping to
stopped. because request_stop() returns back to callback_t in
stop_state::stopped, complete_stopped() is called there

with uring_context:
uring_context::cancel() is asynchronous, so callback_t is still in
stop_state::stopping when request_stop() returns. run_one() eventually
sees the target operation complete with ECANCELED and calls
io_base::cancel(). when that calls sender_awaiter::stop(), the state is
still stop_state::stopping, so complete_stopped() never gets called

this causes beman.net.examples.client to exit before demo::scope gets
the stop signal from the 2 coroutines:
> ERROR: scope destroyed with live jobs: 2

to address this, callback_t now handles this async case by transitioning
back to stop_state::running after stop_source.request_stop() returns

Signed-off-by: Casey Bodley <cbodley@redhat.com>

Author: cbodley

examples/demo_task.hpp

@@ -184,8 +184,12 @@ struct task {
                 state->callback.reset();
                 state->handle->stop_state = task::stop_state::stopping;
                 state->handle->stop_source.request_stop();
-                if (state->handle->stop_state == task::stop_state::stopped)
+                if (state->handle->stop_state == task::stop_state::stopped) {
                     this->object->handle->state->complete_stopped();
+                } else {
+                    // transition back to running so sender_awaiter::stop() can safely complete later
+                    state->handle->stop_state = task::stop_state::running;
+                }
             }
         };
         using stop_token    = decltype(ex::get_stop_token(ex::get_env(::std::declval<Receiver>())));