How to Safely Cancel a boost::asio::basic_waitable_timer?

Cancelling Boost Asio Deadline Timer Safely

This article addresses the proper cancellation of boost::asio::basic_waitable_timer timers.

Problem:

Initially, the proposed solution to cancel a timer via timer.get_io_service().post([&]{timer.cancel();}) might appear to fail. The timer does not abort even after calling cancel().

Reason:

The issue lies in the timer's behavior post-cancellation. It does not terminate outstanding asynchronous operations and simply starts a new asynchronous wait when the completion handler is invoked.

Solution:

To cancel a timer safely, it's essential to check if any asynchronous operations are currently in progress. This check can be performed by comparing the timer's expiry time with the current time:

<code class="cpp">timer.get_io_service().post([](){
    if (timer.expires_from_now() >= std::chrono::steady_clock::duration(0)) {
        timer.cancel();
    }
});</code>

Enhanced Cancellation:

In addition to canceling the current operation, it can be beneficial to communicate a shutdown signal to the completion handler. This can be achieved by setting the timer's expiry time to a special value like Timer::clock_type::time_point::min().

<code class="cpp">timer.expires_at(Timer::clock_type::time_point::min());</code>

This allows the completion handler to identify the shutdown state:

<code class="cpp">void handle_timeout(const boost::system::error_code&amp; ec)
{
    if (timer.expires_at() != Timer::time_point::min()) {
        // timer is not in shutdown state
        // ...
    } else {
        // timer is in shutdown state
        std::cerr << "handle_timeout: detected shutdown\n";
    }
}</code>

By implementing these techniques, the cancellation of boost::asio::basic_waitable_timer timers becomes both safe and robust.

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