SFTPClient hangs forever in Windows- and Linux-based Azure App Service

We use an ASP.NET Core 3.1 Background Service to watch and import files from an Sftp Server.

We encountered two issues:

• ListDirectoryAsync hangs forever (SSH.NET 2016.1.0)
• Connect hangs forever (SSH.NET 2020.0.0-beta1)
  • After running it for about 2 days successfully)

I created a little repro application were calls to ListDirectoryAsync and Connect are involved.

public class BackgroundWorker : BackgroundService
{
    private readonly ILogger<BackgroundWorker> _logger;

    public BackgroundWorker(ILogger<BackgroundWorker> logger)
    {
        _logger = logger;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        do
        {
            var host = "";
            var username = "";
            var password = "";
            var uri = new Uri(host);
            var path = "FI/Sample";
            var parsedHost = uri.Host;

            _logger.LogInformation($"{DateTime.Now} - Creating new client ...");
            var parsedPort = uri.Port;
            var connectionInfo = new PasswordConnectionInfo(parsedHost, parsedPort, username, password)
            {
                Timeout = TimeSpan.FromSeconds(120),
            };

            var client = new SftpClient(connectionInfo);

            _logger.LogInformation($"{DateTime.Now} - Client created.");

            client.OperationTimeout = TimeSpan.FromMinutes(3);
            _logger.LogInformation($"{DateTime.Now} - Connecting ...");
            client.Connect();
            _logger.LogInformation($"{DateTime.Now} - Connected");

            var stopwatch = new Stopwatch();
            stopwatch.Start();
            _logger.LogInformation($"{DateTime.Now} - ListDirectoryAsync ...");
            var immutableArray = (await client.ListDirectoryAsync(path))
                .Where(x => x.IsRegularFile)
                .Select(x => new { x.Name, x.LastWriteTimeUtc })
                .ToImmutableArray();

            _logger.LogInformation($"{DateTime.Now} - Done({immutableArray.Length}) {stopwatch.ElapsedMilliseconds}ms");

            _logger.LogInformation($"{DateTime.Now} - Disposing ...");
            client.Dispose();
            connectionInfo.Dispose();
            _logger.LogInformation($"{DateTime.Now} - Disposed.");
            await Task.Delay(1000);

        } while (true);
    }
}

Further findings:

• We could see that both Linux and Windows-based App Services hung at the same time. So it seems that the Sftp Server might have had an issue at that time. Nevertheless SSH.NET should handle this with a timeout.
• We could also see that if we increase our polling interval hanging process were less frequent.

If you need more information, let me know.

Update:
Seems like https://github.com/sshnet/SSH.NET/blob/cefdc203d98cd890815e029bc759bc43ec5a9643/src/Renci.SshNet/Session.cs#L1945 is the problem.

Start with the repro BackgroundWorker shown in the issue and inspect Session.cs around the linked line 1945, focusing on the Connect and ListDirectoryAsync paths. Reproduce the Azure App Service hang or an unavailable SFTP server, then verify that the affected operation respects its configured timeout instead of hanging indefinitely.