How Can I Safely Cancel a Boost Asio Deadline Timer, Even If It\'s Already Expired?

Cancelling Boost Asio Deadline Timer Safely

This article will delve into the topic of safely cancelling a boost::asio::basic_waitable_timer by investigating a provided code example and its unexpected behavior.

The Issue

The goal is to cancel a deadline timer using the recommended approach of posting a cancellation request to the I/O service:

<code class="cpp">timer.get_io_service().post([&]{timer.cancel();})</code>

However, the provided code doesn't seem to terminate the timer as intended.

Investigating the Problem

The code uses a combination of I/O service, deadline timer, and threads to create a scenario where the timer is started and canceled concurrently.

The issue arises when the cancellation is attempted while the timer is already expired. In that case, the posted cancel() request doesn't take effect. Consequently, the completion handler for the timer continues to fire, causing the main thread to block indefinitely.

Detecting and Handling Incomplete Cancellation

To address this, we need a way to detect if the timer was not canceled. One approach is to use the expiry timepoint of the timer and assign a special "invalid" value to it when cancellation is initiated:

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

In the completion handler, if the expiry timepoint is set to this invalid value, the handler can recognize that the timer was not canceled and handle it accordingly:

<code class="cpp">void handle_timeout(const boost::system::error_code& ec)
{
    if (timer.expires_at() != Timer::time_point::min()) {
        timer.expires_from_now(std::chrono::milliseconds(10));
        timer.async_wait(&handle_timeout);
    } else {
        std::cerr <h3 id="Summary">Summary</h3>
<p>While cancellation of asynchronous operations in Boost Asio is generally safe, it's important to account for cases where the operation may have already completed before the cancellation request is received. Using additional logic to detect and handle such scenarios ensures robust cancellation of asynchronous tasks.</p></code>

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