Src/Support/Google.Apis.Core/Util/TaskExtensions.cs

/*
Copyright 2020 Google Inc

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

using System.Threading;
using System.Threading.Tasks;

namespace Google.Apis.Util
{
    // Note: this is duplicated between Google.Apis.Auth and Google.Apis.Core so it can stay internal. Please
    // change both at the same time.
    internal static class TaskExtensions
    {
        /// <summary>
        /// Returns a task which can be cancelled by the given cancellation token, but otherwise observes the original
        /// task's state. This does *not* cancel any work that the original task was doing, and should be used carefully.
        /// </summary>
        internal static Task<T> WithCancellationToken<T>(this Task<T> task, CancellationToken cancellationToken)
        {
            if (!cancellationToken.CanBeCanceled)
            {
                return task;
            }

            return ImplAsync();

            // Separate async method to allow the above optimization to avoid creating any new state machines etc.
            async Task<T> ImplAsync()
            {
                var cts = new TaskCompletionSource<T>();
                using (cancellationToken.Register(() => cts.TrySetCanceled()))
                {
                    var completedTask = await Task.WhenAny(task, cts.Task).ConfigureAwait(false);
                    return await completedTask.ConfigureAwait(false);
                }
            }
        }
    }
}