feat: add stream that rate limits response streams.

src/MockHttp/IO/RateLimitedStream.cs

@@ -0,0 +1,141 @@
+using System.Diagnostics;
+
+namespace MockHttp.IO;
+
+/// <summary>
+/// A stream that rate limits read IO for a provided stream, which can be used to simulate network transfer rates with large data streams.
+/// <para>
+/// While reading from the stream, the read throughput is averaged over time and throttled to match the requested bit rate as close as possible.
+/// </para>
+/// </summary>
+/// <remarks>
+/// - Does not support writing.
+/// - Do not use in real world applications, only tests, kty!
+/// - Not 100% accurate (just like real world :p).
+/// </remarks>
+public class RateLimitedStream : Stream
+{
+    private const int SampleRate = 30; // How often to take samples (per sec).
+    private const int MaxSampleCount = SampleRate * 5; // How many samples to keep track of to calculate average bit rate.
+    private const int Delay = 1000 / SampleRate;
+    internal const int MinBitRate = 128;
+    private const int MaxBufferSize = 2 << 15; // 128KB
+
+    private readonly Stopwatch _stopwatch;
+    private readonly Stream _actualStream;
+    private readonly int _byteRate;
+    private readonly Queue<Measurement> _samples;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="RateLimitedStream" />.
+    /// </summary>
+    /// <param name="actualStream">The actual stream to wrap.</param>
+    /// <param name="bitRate">The bit rate to simulate.</param>
+    /// <exception cref="ArgumentNullException">Thrown when <paramref name="actualStream" /> is null.</exception>
+    /// <exception cref="ArgumentException">Thrown when the stream is not readable or the bit rate is less than 128.</exception>
+    public RateLimitedStream(Stream actualStream, int bitRate)
+    {
+        _actualStream = actualStream ?? throw new ArgumentNullException(nameof(actualStream));
+        if (!_actualStream.CanRead)
+        {
+            throw new ArgumentException("Cannot read from stream.", nameof(actualStream));
+        }
+
+        if (bitRate <= MinBitRate)
+        {
+            throw new ArgumentOutOfRangeException(nameof(bitRate), $"Bit rate must be higher than {MinBitRate}.");
+        }
+
+        _byteRate = bitRate / 8; // We are computing bytes transferred.
+        _stopwatch = new Stopwatch();
+        _samples = new Queue<Measurement>();
+    }
+
+    /// <inheritdoc />
+    public override void Flush()
+    {
+        _actualStream.Flush();
+    }
+
+    /// <inheritdoc />
+    public override int Read(byte[] buffer, int offset, int count)
+    {
+        _stopwatch.Start();
+
+        // Do not read more from stream than what is allowed per sec and/or capped.
+        int chokeByteCount = Math.Min(Math.Min(count, _byteRate), MaxBufferSize);
+
+        int bytesRead = _actualStream.Read(buffer, offset, chokeByteCount);
+        // Add a new measurement to the queue.
+        _samples.Enqueue(new Measurement(_stopwatch.Elapsed, bytesRead));
+
+        // Throttle until average bit rate drops below threshold.
+        while (GetAverageTransferRateInBytesPerSec() > _byteRate)
+        {
+            _samples.Enqueue(new Measurement(_stopwatch.Elapsed, 0));
+            Thread.Sleep(Delay);
+        }
+
+        return bytesRead;
+    }
+
+    /// <inheritdoc />
+    public override long Seek(long offset, SeekOrigin origin)
+    {
+        return _actualStream.Seek(offset, origin);
+    }
+
+    /// <inheritdoc />
+    public override void SetLength(long value)
+    {
+        throw new NotSupportedException("Modifying stream is not supported.");
+    }
+
+    /// <inheritdoc />
+    public override void Write(byte[] buffer, int offset, int count)
+    {
+        throw new NotSupportedException("Modifying stream is not supported.");
+    }
+
+    /// <inheritdoc />
+    public override bool CanRead => _actualStream.CanRead;
+
+    /// <inheritdoc />
+    public override bool CanSeek => _actualStream.CanSeek;
+
+    /// <inheritdoc />
+    public override bool CanWrite => false;
+
+    /// <inheritdoc />
+    public override long Length => _actualStream.Length;
+
+    /// <inheritdoc />
+    public override long Position
+    {
+        get => _actualStream.Position;
+        set => _actualStream.Position = value;
+    }
+
+    /// <summary>
+    /// Gets the current average transfer rate per sec.
+    /// </summary>
+    private double GetAverageTransferRateInBytesPerSec()
+    {
+        // Remove measurements if we have too many.
+        while (_samples.Count > MaxSampleCount)
+        {
+            _samples.Dequeue();
+        }
+
+        //if (_samples.Count == 0)
+        //{
+        //    return 0;
+        //}
+
+        int bytesReadForAllSamples = _samples.Sum(m => m.BytesRead);
+        TimeSpan totalSampleTime = _stopwatch.Elapsed - _samples.First().Time;
+        return bytesReadForAllSamples / totalSampleTime.TotalSeconds;
+    }
+
+    private record struct Measurement(TimeSpan Time, int BytesRead);
+}

test/MockHttp.Tests/RateLimitedStreamTests.cs

@@ -0,0 +1,178 @@
+#nullable enable
+using System.Diagnostics;
+using FluentAssertions;
+using MockHttp.IO;
+using Moq;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace MockHttp;
+
+public class RateLimitedStreamTests : IDisposable
+{
+    private readonly ITestOutputHelper _testOutputHelper;
+    private const int DataSizeInBytes = 512 * 1024; // 512 KB
+    private const int BitRate = 1024000; // 1024 kbps = 128 KB/s
+    private static readonly TimeSpan ExpectedTotalTime = TimeSpan.FromSeconds(DataSizeInBytes / ((double)BitRate / 8)); // ~ 4sec
+
+    private readonly MemoryStream _actualStream;
+    private readonly RateLimitedStream _sut;
+
+    private readonly byte[] _content = Enumerable.Range(0, DataSizeInBytes)
+        .Select((_, index) => (byte)(index % 256))
+        .ToArray();
+
+    public RateLimitedStreamTests(ITestOutputHelper testOutputHelper)
+    {
+        _testOutputHelper = testOutputHelper;
+        _actualStream = new MemoryStream(_content);
+        _sut = new RateLimitedStream(_actualStream, BitRate);
+    }
+
+    [Fact]
+    public void When_reading_from_stream_it_should_be_rate_limited()
+    {
+        using var ms = new MemoryStream();
+
+        byte[] buffer = new byte[4096];
+        var sw = Stopwatch.StartNew();
+        long totalBytesRead = 0;
+        int bytesRead;
+        int readCount = 0;
+        while ((bytesRead = _sut.Read(buffer, 0, buffer.Length)) > 0)
+        {
+            totalBytesRead += bytesRead;
+            readCount++;
+            _testOutputHelper.WriteLine("Read: {0:000}, Time: {1}, Total bytes read: {2}/{3}", readCount, sw.Elapsed, totalBytesRead, DataSizeInBytes);
+            ms.Write(buffer, 0, bytesRead);
+        }
+
+        sw.Elapsed.Should().BeGreaterThanOrEqualTo(ExpectedTotalTime);
+        ms.ToArray().Should().BeEquivalentTo(_content, opts => opts.WithStrictOrdering());
+    }
+
+    [Theory]
+    [InlineData(100, 100)]
+    [InlineData(1000, 1000 - 768)]
+    public void Given_that_position_is_set_when_reading_next_byte_it_should_return_expected_and_advance(long newPosition, byte expectedByte)
+    {
+        // Act
+        _sut.Position = newPosition;
+        int nextByte = _sut.ReadByte();
+
+        // Assert
+        nextByte.Should().Be(expectedByte);
+        _sut.Position.Should().Be(newPosition + 1);
+    }
+
+    [Fact]
+    public void Given_that_actual_stream_is_null_when_creating_instance_it_should_throw()
+    {
+        Stream? actualStream = null;
+
+        // Act
+        Func<RateLimitedStream> act = () => new RateLimitedStream(actualStream!, 1024);
+
+        // Assert
+        act.Should()
+            .ThrowExactly<ArgumentNullException>()
+            .WithParameterName(nameof(actualStream));
+    }
+
+    [Fact]
+    public void Given_that_actual_stream_is_not_readable_when_creating_instance_it_should_throw()
+    {
+        var actualStream = new Mock<Stream>();
+        actualStream
+            .Setup(m => m.CanRead)
+            .Returns(false)
+            .Verifiable();
+
+        // Act
+        Func<RateLimitedStream> act = () => new RateLimitedStream(actualStream.Object, 1024);
+
+        // Assert
+        act.Should()
+            .ThrowExactly<ArgumentException>()
+            .WithMessage("Cannot read from stream.*")
+            .WithParameterName(nameof(actualStream));
+        actualStream.Verify();
+    }
+
+    [Theory]
+    [InlineData(-1)]
+    [InlineData(0)]
+    [InlineData(127)]
+    public void Given_that_bitRate_is_invalid_when_creating_instance_it_should_throw(int bitRate)
+    {
+        // Act
+        Func<RateLimitedStream> act = () => new RateLimitedStream(Stream.Null, bitRate);
+
+        // Assert
+        act.Should()
+            .ThrowExactly<ArgumentOutOfRangeException>()
+            .WithMessage($"Bit rate must be higher than {RateLimitedStream.MinBitRate}.*")
+            .WithParameterName(nameof(bitRate));
+    }
+
+    [Fact]
+    public void It_should_set_have_expected_properties()
+    {
+        _actualStream.Should().BeOfType<MemoryStream>();
+        _sut.CanRead.Should().BeTrue();
+        _sut.CanWrite.Should().BeFalse("even though actual stream supports writing, we do not");
+        _sut.CanSeek.Should().BeTrue();
+
+        _actualStream.Length.Should().NotBe(0);
+        _sut.Length.Should().Be(_actualStream.Length);
+
+        _actualStream.Position = 1000;
+        _sut.Position.Should().Be(1000);
+        _sut.Position = 100;
+        _actualStream.Position.Should().Be(100);
+
+        _sut.Seek(2000, SeekOrigin.Current);
+        _actualStream.Position.Should().Be(2100);
+    }
+
+    [Fact]
+    public void When_writing_it_should_throw()
+    {
+        // Act
+        Action act = () => _sut.Write(_content, 0, 1024);
+
+        // Assert
+        act.Should().ThrowExactly<NotSupportedException>();
+    }
+
+    [Fact]
+    public void When_setting_length_it_should_throw()
+    {
+        // Act
+        Action act = () => _sut.SetLength(1024);
+
+        // Assert
+        act.Should().ThrowExactly<NotSupportedException>();
+    }
+
+    [Fact]
+    public void When_flushing_it_should_flush_underlying()
+    {
+        var streamMock = new Mock<MemoryStream> { CallBase = true };
+        using MemoryStream? stream = streamMock.Object;
+        var sut = new RateLimitedStream(stream, 1024);
+
+        // Act
+        sut.Flush();
+
+        // Assert
+        streamMock.Verify(m => m.Flush(), Times.Once);
+    }
+
+    public void Dispose()
+    {
+        _actualStream?.Dispose();
+        _sut?.Dispose();
+    }
+}
+#nullable restore