Incorrect handling of SendTo() return value

Several SS14 players have reported on Discord and elsewhere that they sometimes get stuck when connecting to the official servers at the "Connection Established" window, where the client seems to hang. Because this Heisenbug is difficult to reproduce, it seems to have evaded detection and squashing. I believe I have finally chased down this bug.

Steps to Reproduce

Get a server with a very slow upstream connection.
Have a friend connect to your server.
Sometimes observe this error on the server console:

[ERRO] net: "0.0.0.0": "Failed to send packet: System.Net.Sockets.SocketException (55): No buffer space available
   at System.Net.Sockets.Socket.UpdateStatusAfterSocketErrorAndThrowException(SocketError error, String callerName)
   at System.Net.Sockets.Socket.SendTo(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags, EndPoint remoteEP)
   at Lidgren.Network.NetPeer.ActuallySendPacket(Byte[] data, Int32 numBytes, IPEndPoint target, Boolean& connectionReset) in /space-station-14/RobustToolbox/Lidgren.Network/Lidgren.Network/Lidgren.Network/NetPeer.LatencySimulation.cs:line 212"

What's happening here is that SendTo() is returning less bytes than the caller wanted it to send, which, for very good reasons, can happen on Windows and Unix.

Specifically, the OS guesstimates a good size for the transmit buffer, and if this buffer gets filled, the OS tells the application that it has only done a partial send. The bug is that it is up to the caller, i.e. the application, to attempt to re-send those bits.

Workaround

Increase the OS's default buffer size
Increase Robust's buffer size by calling setsockopt() to make its buffer bigger
Don't send so much stuff to the client just as the client is connecting. This is probably more trouble than it's worth.
Get a faster uplink on the server

Fix Basically, the problem is this:

int bytesSent = NetFastSocket.SendTo(m_socket, data, 0, numBytes, SocketFlags.None, realTarget);
if (numBytes != bytesSent)
    LogWarning("Failed to send the full " + numBytes + "; only " + bytesSent + " bytes sent in packet!");

Lidgren must attempt to re-send any bytes that didn't get sent as a result of an incomplete SendTo().

space-wizards / SpaceWizards.Lidgren.Network

Incorrect handling of SendTo() return value #6