Cancellation of FtpConnection.ReadFromStreamAsync causes UnobservedTaskException

[cjzzz]

In FtpConnection.ReadFromStreamAsync, readTask is started using the cancellation token passed into the method, but tcs is also created which has its result set when the cancellation token is cancelled:

protected override async Task<int> ReadFromStreamAsync(byte[] buffer, int offset, int length, CancellationToken cancellationToken)
{
    var readTask = Stream
       .ReadAsync(buffer, offset, length, cancellationToken);

    var tcs = new TaskCompletionSource<object?>();
    using var registration = cancellationToken.Register(() => tcs.TrySetResult(null));
    var resultTask = await Task.WhenAny(readTask, tcs.Task)
       .ConfigureAwait(false);

    if (cancellationToken.IsCancellationRequested)
    {
        Logger?.LogTrace("Cancelled through CancellationToken");
        return 0;
    }
...

This can cause resultTask to complete when tcs.Task is complete, and the exception from the cancellation of readTask is never observed. causing an UnobservedTaskException.

[fubar-coder]

Nice catch!