## Is Cancelling Boost Asio Deadline Timer Always Safe? A Look into Common Pitfalls and Robust Solutions.

Safely Cancelling Boost Asio Deadline Timer

Introduction

Ensuring the safe cancellation of boost::asio::basic_waitable_timer objects is a critical aspect of developing reliable asynchronous programs. This article delves into a common approach for cancelling timers safely and addresses a specific issue encountered when using this approach.

The Problem

According to a discussion on Stack Overflow, the following code is claimed to cancel a boost::asio::basic_waitable_timer safely:

timer.get_io_service().post([&]{timer.cancel();})

However, this approach is not always effective, causing the timer to continue running indefinitely in some cases.

Investigation

An investigation using Boost Asio's handler tracking feature revealed that the cancellation was indeed not taking effect in certain situations. Specifically, the completion handler for the async_wait operation was being invoked even after the timer had been cancelled, indicating that the asynchronous operation was still in progress.

Root Cause

The root cause of the issue is that the timer.cancel() function only cancels asynchronous operations that are currently in flight. If the timer has already expired or if there is no asynchronous operation associated with the timer when the cancel is called, the cancel will have no effect.

Detecting the Issue

To detect this condition, one can manually check the timer's expiry time before cancelling. If the expiry time is in the past, it indicates that the timer has already expired and will not be affected by the cancel operation.

<code class="cpp">if (timer.expires_from_now() >= std::chrono::steady_clock::duration(0)) {
  timer.cancel();
} else {
  std::cout <h3 id="Robust-Shutdown">Robust Shutdown</h3>
<p>To ensure a robust shutdown of the timer, it is recommended to use a special value for the timer's expiry time to signal that the timer is invalid and should not execute any further asynchronous operations. This value can be checked in the completion handler to handle shutdown gracefully.</p>
<pre class="brush:php;toolbar:false"><code class="cpp">timer.get_io_service().post([](){
  std::cerr <h3 id="Conclusion">Conclusion</h3>
<p>The cancellation of boost::asio::basic_waitable_timer objects must be handled carefully to maintain program reliability. By using the correct cancellation mechanisms and detecting potential issues, such as timers that have already expired, developers can ensure robust and controlled behavior in asynchronous operations.</p></code>

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