Pipereader example require internal "unsafe" class

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Hi @cosullivan , I am currently upgrading to v9.0 and had to rebuild my ListenFactory with PIPE as design is now.

In your example: https://github.com/cosullivan/SmtpServer/blob/2b455e652bb451dd3f38a6b7b84fb1b06e4dbfb3/Examples/SampleApp/Examples/CustomEndpointListenerExample.cs#L133 The StringUtil are "unsafe" because of the fixed buffers and pointers in the internal class. (I'm an old c++ programmer, so that is why I write "unsafe". You know what you are doing, so it is not unsafe... :))

By using your internal class StringUtil in an example would bind you to implement the class or SmtpProject to get it compiled using the Nuget. Wouldn't be enough to use Streamreader/Streamwriter to handle PIPEs in your SmtpServer?

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I played with your example to get the full communication between server -> client. Here is my suggestion to achieve that:

    public sealed class CustomSecurableDuplexPipe : ISecurableDuplexPipe
    {
        readonly ISecurableDuplexPipe _securableDuplexPipe;

        public CustomSecurableDuplexPipe(ISecurableDuplexPipe securableDuplexPipe)
        {
            _securableDuplexPipe = securableDuplexPipe;
        }

        public Task UpgradeAsync(X509Certificate certificate, SslProtocols protocols, CancellationToken cancellationToken = default)
        {
            return _securableDuplexPipe.UpgradeAsync(certificate, protocols, cancellationToken);
        }

        public void Dispose()
        {
            Console.WriteLine("Raw transcript of last connection:");
            foreach (var logLine in LogArray)
                Console.WriteLine(logLine);

            _securableDuplexPipe.Dispose();
        }

	//Actual variable that holds the log
        private List<String> LogArray = new List<String>();

        public PipeReader Input => new LoggingPipeReader(_securableDuplexPipe.Input, LogArray);

        public PipeWriter Output => new LoggingPipeWriter(_securableDuplexPipe.Output, LogArray);

        public bool IsSecure => _securableDuplexPipe.IsSecure;
    }

    //Quite simular to LoggingPipeReader and as simple as possible
    public sealed class LoggingPipeWriter : PipeWriter
    {
        readonly PipeWriter _delegate;
        readonly List<String> _logArray; //Store data until EOF

        public LoggingPipeWriter(PipeWriter @delegate, List<String> @logArray)
        {
            _delegate = @delegate;
            _logArray = @logArray;
        }

        public override void Advance(int bytes)
        {
            var spandata = GetSpan().Slice(0, bytes);
            String str = Encoding.UTF8.GetString(spandata);
            _logArray.Add(str);
            _delegate.Advance(bytes);
        }

        //Unused, but required overrides
        public override void CancelPendingFlush()
            => _delegate.CancelPendingFlush();

        public override void Complete(Exception exception = null)
            => _delegate.Complete(exception);

        public override ValueTask<FlushResult> FlushAsync(CancellationToken cancellationToken = default)
            => _delegate.FlushAsync(cancellationToken);

        public override Memory<byte> GetMemory(int sizeHint = 0)
            => _delegate.GetMemory(sizeHint);

        public override Span<byte> GetSpan(int sizeHint = 0)
            => _delegate.GetSpan(sizeHint);
    }


    //LoggingPipeReader with helper for getting data after each CRLF
    public sealed class LoggingPipeReader : PipeReader
    {
        readonly PipeReader _delegate;
        readonly List<String> _logArray;

        public LoggingPipeReader(PipeReader @delegate, List<String> @logArray)
        {
            _delegate = @delegate;
            _logArray = @logArray;
        }

        public override async ValueTask<ReadResult> ReadAsync(CancellationToken cancellationToken = default(CancellationToken))
        {
            ReadResult readResult = await _delegate.ReadAsync(cancellationToken);
            ReadOnlySequence<byte> buffer = readResult.Buffer;
            string tmp = ProcessLine(ref buffer); ;
            if (tmp != null)
                _logArray.Add(tmp);
            return readResult;
        }

        //Unused, but required overrides
        public override void AdvanceTo(SequencePosition consumed) => _delegate.AdvanceTo(consumed);
        public override void AdvanceTo(SequencePosition consumed, SequencePosition examined)
            => _delegate.AdvanceTo(consumed, examined);
        public override void CancelPendingRead() => _delegate.CancelPendingRead();
        public override void Complete(Exception exception = null) => _delegate.Complete(exception);
        public override bool TryRead(out ReadResult result) => _delegate.TryRead(out result);

        //Helper class, raw copy from: https://dev.to/joni2nja/evaluating-readline-using-system-io-pipelines-performance-in-c-part-2-pmf
        private static ReadOnlySpan<byte> NewLine => new[] { (byte)'\r', (byte)'\n' };
        private static string ProcessLine(ref ReadOnlySequence<byte> buffer)
        {
            string str = null;

            if (buffer.IsSingleSegment)
            {
                var span = buffer.FirstSpan;
                int consumed;
                while (span.Length > 0)
                {
                    var newLine = span.IndexOf(NewLine);

                    if (newLine == -1) break;

                    var line = span.Slice(0, newLine);
                    str = Encoding.UTF8.GetString(line);

                    // simulate string processing
                    //str = str.AsSpan().Slice(0, 5).ToString();

                    consumed = line.Length + NewLine.Length;
                    span = span.Slice(consumed);
                    buffer = buffer.Slice(consumed);
                }
            }
            else
            {
                var sequenceReader = new SequenceReader<byte>(buffer);

                while (!sequenceReader.End)
                {
                    while (sequenceReader.TryReadTo(out ReadOnlySequence<byte> line, NewLine))
                    {
                        str = Encoding.UTF8.GetString(line);

                        // simulate string processing
                        //str = str.AsSpan().Slice(0, 5).ToString();
                    }

                    buffer = buffer.Slice(sequenceReader.Position);
                    sequenceReader.Advance(buffer.Length);
                }
            }

            return str;
        }
    }

The helper for the reader here seems to be quite good. Since data is short it never triggers SequenceReader though, so that is probably useless.