Simple C# channel-based token bucket limiter

RESULT.md

Timings

0.001096, 0.101212, 0.2020205, 0.3030929, 0.4042182, 0.5052511, 0.6066207, 0.7077272, 0.8092904, 0.9107451

TokenBucketLimiter.cs

using System.Collections.Concurrent;
using System.Diagnostics;
using System.Threading.Channels;

namespace SimpleRequestLimiting;

public static class TokenBucketLimiter
{
    private enum Void { }

    public static Func<CancellationToken, Task> CreateBucketLimiter(double rate, int maxBucketSize,
        CancellationToken refillCancellationToken)
    {
        var chan = Channel.CreateBounded<Void>(maxBucketSize);
        _ = RefillAsync(chan.Writer, rate, refillCancellationToken);
        return async cancellationToken => { await chan.Reader.ReadAsync(cancellationToken); };
    }

    private static async Task RefillAsync(ChannelWriter<Void> w, double rate, CancellationToken cancellationToken)
    {
        var delaySeconds = 1.0 / rate;
        while (true)
        {
            await w.WriteAsync(new Void(), cancellationToken);
            await Task.Delay(TimeSpan.FromSeconds(delaySeconds), cancellationToken);
        }
    }
}

public class Tests
{
    [Test]
    public async Task RequestRateIsLimited()
    {
        var cts = new CancellationTokenSource(TimeSpan.FromSeconds(1));
        var cancellationToken = cts.Token;
        var limiter = TokenBucketLimiter.CreateBucketLimiter(10, 2, cancellationToken);

        var timings = new ConcurrentQueue<double>();

        var sw = Stopwatch.StartNew();
        var workers = Enumerable.Range(0, 100).Select(async _ =>
        {
            while (!cancellationToken.IsCancellationRequested)
            {
                try
                {
                    await limiter(cancellationToken);
                    timings.Enqueue(sw.Elapsed.TotalSeconds);
                }
                catch (OperationCanceledException)
                {
                }
            }
        }).ToList();

        await Task.WhenAll(workers);

        TestContext.WriteLine(string.Join(", ", timings));
    }
}