'The session is not open' in ListDirectoryAsync

[m0a0k0s]

On rare occasions, I see 'The session is not open' in ListDirectoryAsync.
The last time I saw it, it was for a server where multiple instances of sftpClient might get created simultaneously with the same username/password, but working with different folders.

SSH.NET 2025.1.0 .Net 9.

My code looks like this:

try
{
    await sftpClient.ConnectAsync(cancellationToken);
	
    await sftpClient.ChangeDirectoryAsync(path, cancellationToken);
    var asyncList = sftpClient.ListDirectoryAsync(path, cancellationToken);
}
finally
{
	if (sftpClient.IsConnected)
		sftpClient.Disconnect();
}

"Type": "System.InvalidOperationException",
"HResult": -2146233079,
"Message": "The session is not open.",
"Source": "Renci.SshNet",
"StackTrace": " at Renci.SshNet.Sftp.SftpSession.SendRequest(SftpRequest request)\r\n at Renci.SshNet.Sftp.SftpSession.GetCanonicalPathAsync(String path, CancellationToken cancellationToken)\r\n at Renci.SshNet.SftpClient.ListDirectoryAsync(String path, CancellationToken cancellationToken)+MoveNext()\r\n at Renci.SshNet.SftpClient.ListDirectoryAsync(String path, CancellationToken cancellationToken)+System.Threading.Tasks.Sources.IValueTaskSource.GetResult()",
"TargetSite": "Void SendRequest(Renci.SshNet.Sftp.Requests.SftpRequest)"

[mus65]

In your example, the asyncList is not being enumerated, so you are basically disconnecting while ListDirectoyAsyncis still running in the background.

[JuergenAuer]

Hi @m0a0k0s

The last time I saw it, it was for a server where multiple instances of sftpClient might get created simultaneously with the same username/password, but working with different folders.

how many connections are created if you see "multiple instances"? The server may have a maximum of allowed connections. So if you create too much connections, the server may close some of the connections.

May be you should change your design to use only one connection ... and the problem may dissapear.

[m0a0k0s]

Yes, asyncList get enumerated, that part was skipped. From 5 to 9 instances maximum.

[JuergenAuer]

The default limit "MaxSessions" is 10. See

https://learn.microsoft.com/en-us/troubleshoot/windows-server/system-management-components/troubleshoot-openssh-connection-issues-maxstartups-maxsessions

That's the Windows SSH, but the protocol is the same.

In an environment that supports session multiplexing (multiple sessions per connection), clients connect and authenticate. However, the server drops the connection.

So you hit a limit.

And there is a probability:

Starting at the 21st connection attempt, there's a 40 percent probability that the server drops the new connection attempt.

[m0a0k0s]

SFTP server version is SSH-2.0-8.1.0.0_openssh Globalscape. As I understand, multiplexing is a client-side feature, and each SFTP client instance creates its own independent TCP connection.
I have an issue on an earlier stage when there's a single connection from each sftp client to get a list of files. Other instances could have simultaneous downloads, but after they have already gotten the list of files.