How to Safely Cancel Boost.Asio\'s basic_waitable_timer and Prevent Unexpected Behavior?

Cancelling Deadline Timers Safely in Boost.Asio

Boost.Asio's basic_waitable_timer enables the scheduling of asynchronous timeouts. While it provides a cancel() method to cancel a pending operation, safe cancellation requires careful handling to prevent unexpected behavior.

The Problem: Non-Robust Cancellation

In the provided code, cancellation via the post() function doesn't always work as expected. The timer keeps starting new operations even after a cancellation attempt. This occurs because the cancel() call only cancels asynchronous operations that are currently in flight. If the timer expires before the cancellation is invoked, subsequent calls to async_wait() will continue as normal.

Diagnosing the Issue: Handler Tracking

To understand the problem, handler tracking can be enabled to visualize the flow of events. By inspecting the resulting trace, it becomes apparent that the cancellation attempt is executed after the timer has already expired, resulting in the subsequent operations being unaffected.

Detecting Expired Timers

To detect expired timers before cancellation, the following approach can be used:

• Check the expiration timepoint of the timer.
• If the timepoint is set to a specific "invalid" value (e.g., Timer::clock_type::time_point::min()), it indicates that the timer has been shut down and should not perform any further operations.

Robust Cancellation with Shutdown Signalling

To handle expired timers gracefully, the cancellation logic can be modified to set the timer's expiration timepoint to this "invalid" value. This will signal the completion handler to stop future executions.

<code class="cpp">timer.get_io_service().post([]() {
    timer.expires_at(Timer::time_point::min());
});</code>

In the completion handler, the "invalid" timepoint can be checked to detect shutdown:

<code class="cpp">if (timer.expires_at() != Timer::time_point::min()) {
    // Continue normal operation...
} else {
    std::cerr << "handle_timeout: detected shutdown\n";
}</code>

Conclusion

Cancelling basic_waitable_timer instances requires careful consideration of the timer's state. Detecting expired timers and signalling shutdown through a dedicated expiration timepoint allows for robust and predictable cancellation behavior.

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