Iceshrimp.NET/Iceshrimp.Backend/Core/Helpers/EventHelpers.cs

namespace Iceshrimp.Backend.Core.Helpers;

public sealed class AsyncAutoResetEvent(bool signaled)
{
	private readonly Queue<TaskCompletionSource> _queue = new();

	public Task WaitAsync(CancellationToken cancellationToken = default)
	{
		lock (_queue)
		{
			if (signaled)
			{
				signaled = false;
				return Task.CompletedTask;
			}

			var tcs = new TaskCompletionSource();
			if (cancellationToken.CanBeCanceled)
			{
				// If the token is cancelled, cancel the waiter.
				var registration =
					cancellationToken.Register(() => tcs.TrySetCanceled(), false);

				// If the waiter completes or faults, unregister our interest in cancellation.
				tcs.Task.ContinueWith(
				                      _ => registration.Unregister(),
				                      cancellationToken,
				                      TaskContinuationOptions.OnlyOnRanToCompletion |
				                      TaskContinuationOptions.NotOnFaulted,
				                      TaskScheduler.Default);
			}

			_queue.Enqueue(tcs);
			return tcs.Task;
		}
	}

	public void Set()
	{
		TaskCompletionSource? toRelease = null;

		lock (_queue)
		{
			if (_queue.Count > 0)
			{
				toRelease = _queue.Dequeue();
			}
			else if (!signaled)
			{
				signaled = true;
			}
		}

		// It's possible that the TCS has already been cancelled.
		toRelease?.TrySetResult();
	}
}