void sequencer::enqueue(mailbox_element_ptr what, execution_unit* context) {
auto down_msg_handler = [&](down_msg& dm) {
// quit if either `f` or `g` are no longer available
cleanup(std::move(dm.reason), context);
};
if (handle_system_message(*what, context, false, down_msg_handler))
return;
strong_actor_ptr f;
strong_actor_ptr g;
error err;
shared_critical_section([&] {
f = f_;
g = g_;
err = fail_state_;
});
if (!f) {
// f and g are invalid only after the sequencer terminated
bounce(what, err);
return;
}
// process and forward the non-system message;
// store `f` as the next stage in the forwarding chain
what->stages.push_back(std::move(f));
// forward modified message to `g`
g->enqueue(std::move(what), context);
}
bool handle_system_message(mailbox_element& x, execution_unit* context,
bool trap_exit, F& down_msg_handler) {
auto& content = x.content();
if (content.type_token() == make_type_token<down_msg>()) {
if (content.shared()) {
auto vptr = content.copy(0);
down_msg_handler(vptr->get_mutable_as<down_msg>());
} else {
down_msg_handler(content.get_mutable_as<down_msg>(0));
}
return true;
}
return handle_system_message(x, context, trap_exit);
}