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TimeoutAsync does not propegate the correct cancellation token

See original GitHub issue

Summary: When cancelling the execution of a timeout policy and then catching the operation cancelled exception, the cancellation token in the exception does not match the one that was cancelled. This makes it impossible to check which cancellation token triggered the exception.

Expected behavior: OperationCanceledException.CancellationToken should equal the input cancellation token.

Actual behaviour: OperationCanceledException.CancellationToken does not equal the input cancellation token.

Steps / Code to reproduce the problem:

using System.Diagnostics;
using Polly;
using Polly.Timeout;

namespace PollyBug
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            // Create an async timeout policy
            var timeoutPolicy = Policy.TimeoutAsync(TimeSpan.FromSeconds(10), TimeoutStrategy.Optimistic);

            // Create a cancellation token
            var cancellationTokenSource = new CancellationTokenSource();
            var cancellationToken = cancellationTokenSource.Token;

            // Execute the timeout policy with a cancellable task
            var task = timeoutPolicy.ExecuteAsync(c => Task.Delay(TimeSpan.FromSeconds(10), c), cancellationToken);

            // Cancel the task
            cancellationTokenSource.Cancel();

            try
            {
                // Await the task to throw the cancellation exception
                await task;
            }
            catch (OperationCanceledException exception)
            {
                // The cancellation tokens do not match!
                Debug.Assert(exception.CancellationToken == cancellationToken);
            }
        }
    }
}

Stack trace:

   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable.ConfiguredTaskAwaiter.GetResult()
   at Polly.AsyncPolicy.<>c__DisplayClass40_0.<<ImplementationAsync>b__0>d.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at Polly.Timeout.AsyncTimeoutEngine.<ImplementationAsync>d__0`1.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Polly.Timeout.AsyncTimeoutEngine.<ImplementationAsync>d__0`1.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable.ConfiguredTaskAwaiter.GetResult()
   at Polly.AsyncPolicy.<ExecuteAsync>d__12.MoveNext()
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
   at PollyBug.Program.<Main>d__0.MoveNext() in C:\Users\Rick\source\repos\PollyBug\Program.cs:line 27