StreamReader.ReadAsync() does not propagate CancellationToken to the underlying stream

[asmichi]

public class StreamReader : TextReader
{
    public override ValueTask<int> ReadAsync(Memory<char> buffer, CancellationToken cancellationToken = default);
}

The above overload of StreamReader.ReadAsync() cannot be canceled with cancellationToken even if the underlying stream supports cancellation because it does not propagate cancellationToken to the underlying stream

Actual Behavior

StreamReader.ReadAsync() indefinitely blocks.

Excpected Behavior

OperationCanceledException is thrown.

Environment

• .NET Core SDK 3.1.100-preview1-014459
• Windows 10 Pro 1903 (Build 18362.418)

Repro

The code below prints 0 chars read instead of canceled. See StreamReaderCancellation.zip for the full code.

using var serverPipe = new NamedPipeServerStream(
    pipeName, PipeDirection.InOut, NamedPipeServerStream.MaxAllowedServerInstances, PipeTransmissionMode.Byte, PipeOptions.Asynchronous);

// Simulate an idle connection: have another thread connect to the server and disconnect in 2000 ms.
var clientTask = Task.Run(() =>
{
    using var clientPipe = new NamedPipeClientStream(".", pipeName, PipeDirection.Out);
    clientPipe.Connect();
    Thread.Sleep(10000);
});

serverPipe.WaitForConnection();

using (var cts = new CancellationTokenSource(200))
using (var streamReader = new StreamReader(serverPipe, Encoding.UTF8, leaveOpen: true))
{
    try
    {
        var buffer = new char[256];
        var count = await streamReader.ReadAsync(buffer, cts.Token);
        Console.WriteLine("{0} chars read", count);
    }
    catch (OperationCanceledException)
    {
        Console.WriteLine("canceled");
    }
}

[asmichi]

At first glance this looks like a simple oversight of not adding a CancellationToken parameter to StreamReader.ReadBufferAsync() (patch included in the zip). I suspect there may be a reason, though.

May affect #20824.

[asmichi]

Awesome, thank you! I'm really glad this got into 3.1.