boost::asio::deadline_timer doesn't wake up (stress scenario) - c++

Question

I'm using a deadline_timer as an asynchronous event and I'm running into a situation where, after some time, the thread waiting on the event never seems to be woken up (despite more calls to cancel()). I've been able to reproduce this using some sample code that I've pasted below; it's not exactly consistent but I have seen what I think is the same issue I'm experiencing.
boost::asio::io_service io_service;
boost::asio::deadline_timer timer(io_service);
timer.expires_at(boost::posix_time::pos_infin);
int num_events = 0;
auto waiter = [&timer, &num_events](boost::asio::yield_context context) {
while (true) {
std::cout << "waiting on event" << std::endl;
boost::system::error_code e;
timer.async_wait(context[e]);
std::cout << "got event (" << e << ")" << std::endl;
++num_events;
}
};
boost::asio::spawn(io_service, std::move(waiter));
boost::thread thread(boost::bind(&boost::asio::io_service::run, &io_service));
for (auto i = 0; i < 500000; ++i) {
timer.cancel();
std::cout << i << std::endl;
}
Am I doing something here that's unsupported and inadvertently hitting some race condition? The error code from the wait() never looks troublesome (even on the very last time it's woken up before it never seems to again). EDIT: I've also noticed the original bug on 3 different platforms (Windows, Mac and Linux) but the above test I've been using to reproduce has been on Windows.

Answer 1

The deadline_timer object is not threadsafe.
You're canceling it from another thread than the one that's posting the async_wait. This means the calls can race.
I'm not sure how this can completely inhibit the callback, in your sample. It seems to me that the program should /just/ quit because the tight loop to 500000 finishes quickly (doing many redundant cancels that never get processed, because the coroutine would e.g. not even have posted the new async_wait).
So maybe you mean, "why don't I get 500000 events".
UPDATE
After the comment, here's a trivial transformation that shows how you are gonna be fine calling members on the timer from within an actor. Note: this critically hinges on the idea that the io_service is run from a single thread only!
#include <boost/asio.hpp>
#include <boost/asio/spawn.hpp>
#include <boost/make_shared.hpp>
#include <boost/thread.hpp>
#include <iostream>
using boost::thread;
using boost::asio::io_service;
int main() {
boost::asio::io_service io_service;
boost::asio::deadline_timer timer(io_service);
timer.expires_at(boost::posix_time::pos_infin);
boost::atomic_bool shutdown(false);
int num_events = 0;
auto waiter = [&timer, &num_events, &shutdown](boost::asio::yield_context context) {
while (!shutdown) {
std::cout << "waiting on event" << std::endl;
boost::system::error_code e;
timer.async_wait(context[e]);
std::cout << "got event (" << e.message() << ")" << std::endl;
++num_events;
}
};
boost::asio::spawn(io_service, std::move(waiter));
boost::thread thread(boost::bind(&boost::asio::io_service::run, &io_service));
for (auto i = 0; i < 5000; ++i) {
io_service.post([&timer, i]{
std::cout << i << std::endl;
timer.cancel();
});
}
io_service.post([&]{
shutdown = true;
timer.cancel();
});
thread.join();
std::cout << "Check: " << num_events << " events counted\n";
}
Also, it looks like you just wanted to signal a background task. As given you can simplify the program like:
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#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include <boost/make_shared.hpp>
#include <iostream>
using boost::thread;
using boost::asio::io_service;
int main() {
io_service svc;
int num_events = 0;
auto work = boost::make_shared<io_service::work>(svc); // keep svc running
boost::thread thread(boost::bind(&io_service::run, &svc));
for (auto i = 0; i < 500000; ++i) {
svc.post([&num_events,i]{
std::cout << "got event (" << i << ")" << std::endl;
++num_events;
});
}
work.reset();
thread.join();
std::cout << "Check: " << num_events << " events counted\n";
}
This does print all 500000 events:
got event (0)
got event (1)
got event (3)
...
got event (499998)
got event (499999)
Check: 500000 events counted