rotor v0.22 and thread unsafety

There is not so well explained BUILD_THREAD_UNSAFE rotor build option. Technically it means, that boost's intrusive ptr is used in thread-unsafe manner, i.e. the underlying reference counter is not atomic. Accordingly, all objects (in our case messages and actors from rotor) cannot be accessed from different threads concurrently.

It should be explicitly mentioned, that rotor's cross-thread messaging facility also cannot be used, otherwise there is notorious UB (undefined behavior).

Practically that usually means, that you are building single-threaded application, most likely a service with asio/ev backend.

Why you might need that feature? Performance is the answer, i.e. when you need rotor supervising/messaging facilities in single-threaded app. According to my measurements, with the thread-unsafety you'll get ~30.8 millions of messages per second instead of ~23.5 with the feauture disabled, i.e. ~30% of performance boost for free.

The question arises, then, how to stop that single threaded application? With the thread safety it can be done via launching additional thread, which monitors some atomic flag, and, once it detects that it is set, it sends shutdown signal to the root supervisor. The atomic flag is set externally, i.e. in signal handler (NB: you cannot send shutdown message within signal handler as all memory allocations are prohibited).

...
struct sigaction action;
memset(&action, 0, sizeof(action));
action.sa_handler = [](int) { shutdown_flag = true; };
auto r = sigaction(SIGINT, &action, nullptr);
...
auto console_thread = std::thread([&] {
    while (!shutdown_flag) {
        std::this_thread::sleep_for(std::chrono::milliseconds(100));
    }
    supervisor->shutdown();
});

supervisor->start();
io_context.run();
console_thread.join();

However, this is no longer possible with thread-unsafety option. What can be done instead, is periodical flag checking from the rotor thread itself, i.e. using timers from root supervisor. I found that I use that feature frequently, so I decided to include it in rotor: in supervisor builder there is needed to specify reference to the shutdown_flag and the frequency of checking it, i.e.:

rth::system_context_thread_t ctx;
auto timeout = r::pt::milliseconds{100};
auto sup = ctx.create_supervisor<rth::supervisor_thread_t>()
               .timeout(timeout)
               .shutdown_flag(shutdown_flag, timeout / 2)
               .finish();

(You still need to have to set it externally, like in the example above with sigaction call). When it detects the flag is set to true it shuts self down.

The full example of usage can be seen at examples/thread/sha512.cpp

BUILD_THREAD_UNSAFE is turned off by default. You should explicitly turn it on if you are knowning what you are doing.