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CSHARP-6220: Prevent cancellation-token retention in WaitAsync #2124

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Merged
papafe merged 3 commits into from
Oct 20, 2026
Merged

CSHARP-6220: Prevent cancellation-token retention in WaitAsync #2124

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957b7a8
CSHARP-6220: Prevent cancellation-token retention in WaitAsync
mohammadshahsavari Sep 12, 2026
bef7253
CSHARP-6220: Avoid linked CTS for non-cancelable tokens
mohammadshahsavari Sep 16, 2026
9405f2d
CSHARP-6220: Simplify timeout cancellation cleanup
mohammadshahsavari Sep 19, 2026
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18 changes: 14 additions & 4 deletions src/MongoDB.Driver/Core/Misc/TaskExtensions.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -66,8 +66,13 @@ static async Task WaitAsyncCore(Task task, TimeSpan timeout, CancellationToken c
{
if (!task.IsCompleted)
{
var timeoutTask = Task.Delay(timeout, cancellationToken);
await Task.WhenAny(task, timeoutTask).ConfigureAwait(false);
using (var timeoutCancellationTokenSource = cancellationToken.CanBeCanceled ?

@papafe papafe Sep 16, 2026

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Do we need to have a special case from non-cancelable token sources?

@mohammadshahsavari mohammadshahsavari Sep 19, 2026

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The previous special case created a regular CTS, which wasn’t necessary. I’ve updated it so non-cancelable tokens don’t create a CTS at all. the conditional now only avoids that unnecessary allocation.

CancellationTokenSource.CreateLinkedTokenSource(cancellationToken) : null)
{
var timeoutTask = Task.Delay(timeout, timeoutCancellationTokenSource?.Token ?? cancellationToken);
await Task.WhenAny(task, timeoutTask).ConfigureAwait(false);
timeoutCancellationTokenSource?.Cancel();
}
}

if (task.IsCompleted)
Expand All @@ -94,8 +99,13 @@ static async Task<TResult> WaitAsyncCore(Task<TResult> task, TimeSpan timeout, C
{
if (!task.IsCompleted)
{
var timeoutTask = Task.Delay(timeout, cancellationToken);
await Task.WhenAny(task, timeoutTask).ConfigureAwait(false);
using (var timeoutCancellationTokenSource = cancellationToken.CanBeCanceled ?
CancellationTokenSource.CreateLinkedTokenSource(cancellationToken) : null)
{
var timeoutTask = Task.Delay(timeout, timeoutCancellationTokenSource?.Token ?? cancellationToken);
await Task.WhenAny(task, timeoutTask).ConfigureAwait(false);
timeoutCancellationTokenSource?.Cancel();
}
}

if (task.IsCompleted)
Expand Down
63 changes: 63 additions & 0 deletions tests/MongoDB.Driver.Tests/Core/Misc/TaskExtensionsTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,9 @@
*/

using System;
#if !NET6_0_OR_GREATER
using System.Reflection;
#endif
using System.Threading;
using System.Threading.Tasks;
using FluentAssertions;
Expand Down Expand Up @@ -105,6 +108,26 @@ await Record.ExceptionAsync(() => task.WaitAsync(TimeSpan.FromMilliseconds(5), C
exception.Should().BeOfType<TimeoutException>();
}

#if !NET6_0_OR_GREATER
[Theory]
[ParameterAttributeData]
public async Task WaitAsync_should_release_cancellation_token_registration_when_task_completes([Values(true, false)] bool generic)
{
using var cancellationTokenSource = new CancellationTokenSource();
var taskCompletionSource = new TaskCompletionSource<int>();
Task waitTask = generic ?
taskCompletionSource.Task.WaitAsync(Timeout.InfiniteTimeSpan, cancellationTokenSource.Token) :
((Task)taskCompletionSource.Task).WaitAsync(Timeout.InfiniteTimeSpan, cancellationTokenSource.Token);

GetCancellationCallbackCount(cancellationTokenSource).Should().Be(1);

taskCompletionSource.SetResult(42);
await waitTask;

GetCancellationCallbackCount(cancellationTokenSource).Should().Be(0);
}
#endif

[Theory]
[ParameterAttributeData]
public async Task WaitAsyncTResult_should_throw_on_negative_timeout([Values(true, false)] bool isPromiseTask)
Expand Down Expand Up @@ -210,6 +233,46 @@ private Task<TResult> CreateSubject<TResult>(TResult result, bool isPromise, Exc

return isPromise ? tcs.Task : Task.FromException<TResult>(exception);
}

#if !NET6_0_OR_GREATER
private static int GetCancellationCallbackCount(CancellationTokenSource cancellationTokenSource)
{
const BindingFlags bindingFlags = BindingFlags.Instance | BindingFlags.NonPublic;
var callbackListsField = typeof(CancellationTokenSource).GetField("m_registeredCallbacksLists", bindingFlags);
var callbackLists = (Array)callbackListsField.GetValue(cancellationTokenSource);
if (callbackLists == null)
{
return 0;
}

var count = 0;
foreach (var callbackList in callbackLists)
{
if (callbackList == null)
{
continue;
}

var fragment = callbackList.GetType().GetField("m_head", bindingFlags).GetValue(callbackList);
while (fragment != null)
{
var fragmentType = fragment.GetType();
var callbacks = (Array)fragmentType.GetField("m_elements", bindingFlags).GetValue(fragment);
foreach (var callback in callbacks)
{
if (callback != null)
{
count++;
}
}

fragment = fragmentType.GetField("m_next", bindingFlags).GetValue(fragment);
}
}

return count;
}
#endif
}
}

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