Exception "There is no valid request available" when any client reconnect.

[eryk955]

Hi
I have big problem with creating ModbusTCP Server. Server works, data flows, but when client reconnect (like PLC, because don't like to keep alive connection) FluentModbus call exception "System.Exception" and "There is no valid request available.". So in short, exception occurs, when any client tried to reconnect.

I checked this also on Node-Red ModbusTCP Client. Init connection is ok, but when Connected, VS call exception like above. I cannot find reason why this happens, so please, help :)

Tried also RequestValidator, but this cannot catch this problem. Problem still happens when Server is asynchronus.

[Apollo3zehn]

Hi, thanks for the bug report. Could you please provide also the stack trace of the exception? I would then see what methods were invoked before the exception happened. Thanks :-)

[eryk955]

Sure, here you go :)

[Apollo3zehn]

Thanks! I think the break; statement in the following line should be replaced with a return false;:

FluentModbus/src/FluentModbus/Server/ModbusTcpRequestHandler.cs

Lines 171 to 175 in bbd7dcd

	else
	{
	Length = 0;
	break;
	}

Why? Because this else block is only executed when the TCP connection has been closed and in that case the return value should be false. If it is true instead, the method WriteResponse is being invoked here:

FluentModbus/src/FluentModbus/Server/ModbusTcpRequestHandler.cs

Lines 58 to 64 in bbd7dcd

	if (await TryReceiveRequestAsync())
	{
	IsReady = true; // WriteResponse() can be called only when IsReady = true
	if (ModbusServer.IsAsynchronous)
	WriteResponse();
	}

And if Length <= 0, it will throw the error you mentioned:

FluentModbus/src/FluentModbus/Server/ModbusRequestHandler.cs

Lines 68 to 69 in bbd7dcd

	if (!(IsReady && Length > 0))
	throw new Exception(ErrorMessage.ModbusTcpRequestHandler_NoValidRequestAvailable);

It would be great if you could try it with the above suggested fix :-)

[eryk955]

This seems to work, but another problem occured :)

[epasinetti]

Here same problem, in some cases the server Tcp stop working and connections from clients are not accepeted.
Also the connections counter is not very relayable.