cancel a deadline_timer, callback triggered anyway

Question

I was suprised not to find a clock component in boost::asio (our any widely used library) so it tried making a simple, minimalistic, implementation for testing some of my co

Answer 1

First, let's show the problem reproduced:

Live On Coliru (code below)

As you can see I run it as

./a.out | grep -C5 false

This filters the output for records that print from C1's completion handler when really c1_active is false (and the completion handler wasn't expected to run)

The problem, in a nutshell, is a "logical" race condition.

It's a bit of mind bender because there's only a single thread (visible on the surface). But it's actually not too complicated.

What happens is this:

• when Clock C1 expires, it will post its completion handler onto the io_service's task queue. Which implies that it might not run immediately.

• imagine that C2 expired too, and it's completion handler now gets scheduled and executes before the one that C1 just pushed. Imagine that by some high coincidence this time, C2 decides to call stop() on C1.

• After C2's completion handler returns, C1's completion handler gets invoked.

  OOPS

  It still has ec saying "no error"... Hence the deadline timer for C1 gets rescheduled. Oops.

Background

For a more in-depth background on the guarantees that Asio (doesn't) make(s) for the order in which completion handlers get executed, see

• When do handlers for cancelled boost::asio handlers get to run?

Solutions?

The simplest solution is to realize that m_enabled could be false. Let's just add the check:

void tick(const boost::system::error_code &ec) {
    if (!ec && m_enabled) {
        m_timer.expires_at(m_timer.expires_at() + m_duration);
        m_timer.async_wait(boost::bind(&Clock::tick, this, _1));

        if (m_callback)
            m_callback(++m_count, *this);
    }
}

On my system it doesn't reproduce the problem any more :)

Reproducer

Live On Coliru

#include 
#include 
#include 

static boost::posix_time::time_duration elapsed() {
    using namespace boost::posix_time;
    static ptime const t0 = microsec_clock::local_time();
    return (microsec_clock::local_time() - t0);
}

class Clock {
  public:
    using callback_t = std::function;
    using duration_t = boost::posix_time::time_duration;

  public:
    Clock(boost::asio::io_service &io, callback_t callback = nullptr,
          duration_t duration = boost::posix_time::seconds(1), bool enable = true)
            : m_timer(io), m_duration(duration), m_callback(callback), m_enabled(false), m_count(0ul) 
    {
        if (enable)
            start();
    }

    void start() {
        if (!m_enabled) {
            m_enabled = true;
            m_timer.expires_from_now(m_duration);
            m_timer.async_wait(boost::bind(&Clock::tick, this, _1)); // std::bind _1 issue ?
        }
    }

    void stop() {
        if (m_enabled) {
            m_enabled = false;
            size_t c_cnt = m_timer.cancel();
#ifdef DEBUG
            printf("[DEBUG@%p] timer::stop : %lu ops cancelled\n", this, c_cnt);
#endif
        }
    }

    void tick(const boost::system::error_code &ec) {
        if (ec != boost::asio::error::operation_aborted) {
            m_timer.expires_at(m_timer.expires_at() + m_duration);
            m_timer.async_wait(boost::bind(&Clock::tick, this, _1));
            if (m_callback)
                m_callback(++m_count, *this);
        }
    }

    void reset_count()                     { m_count = 0ul;         } 
    size_t get_count() const               { return m_count;        } 

    void set_duration(duration_t duration) { m_duration = duration; } 
    const duration_t &get_duration() const { return m_duration;     } 

    void set_callback(callback_t callback) { m_callback = callback; } 
    const callback_t &get_callback() const { return m_callback;     } 

  private:
    boost::asio::deadline_timer m_timer;
    duration_t m_duration;
    callback_t m_callback;
    bool m_enabled;
    size_t m_count;
};

#include 

int main() {
    boost::asio::io_service ios;

    bool c1_active = true;

    Clock c1(ios, [&](int i, Clock& self)
            { 
                std::cout << elapsed() << "\t[C1 - fast] tick" << i << " (c1 active? " << std::boolalpha << c1_active << ")\n";
            },
            boost::posix_time::millisec(1)
            );

#if 1
    Clock c2(ios, [&](int i, Clock& self)
            {
                std::cout << elapsed() << "\t[C2 - slow] tick" << i << "\n";
                c1_active = (i % 2 == 0);

                if (c1_active)
                    c1.start();
                else
                    c1.stop();
            },
            boost::posix_time::millisec(10)
        );
#endif

    ios.run();
}