fix: prevent addition of client connection when modbus server is not started

[LucEdfsf]

On rare circumstances, the modbus server can be stop right after accepting client connection. Therefore we can add this connection to the list while the server is down

In server.go the current code is as follow

        sock, err = ms.tcpListener.Accept()
        if err != nil {
            // if the server has just been stopped, return here
            ms.lock.Lock()
            if !ms.started {
                ms.lock.Unlock()
                return
            }
            ms.lock.Unlock()
            ms.logger.Warningf("failed to accept client connection: %v", err)
            continue
        }
(-> if ms.Stop() is called right there, lock the server, everything goes bad)
        ms.lock.Lock()
        // apply a connection limit
        if uint(len(ms.tcpClients)) < ms.conf.MaxClients {
            accepted    = true
            // add the new client connection to the pool
            ms.tcpClients   = append(ms.tcpClients, sock)
        } else {
            accepted    = false
        }
        ms.lock.Unlock()

I guess we could do

        sock, err = ms.tcpListener.Accept()
        if err != nil {
            ms.logger.Warningf("failed to accept client connection: %v", err)
            continue
        }

        ms.lock.Lock()
        // apply a connection limit
        if ms.started && uint(len(ms.tcpClients)) < ms.conf.MaxClients {
            accepted    = true
            // add the new client connection to the pool
            ms.tcpClients   = append(ms.tcpClients, sock)
        } else {
            accepted    = false
        }
        ms.lock.Unlock()

I am not familiar in github I was not able to open the dedicated branch to fix the issue, it might be clearer in a pull review

[simonvetter]

Hey Lukas, thanks for the bug report!

Nice catch. I just pushed v1.6.0 with a fix.

Mind testing it and report here? If the issue is indeed fixed, I'll let you close this.

[LucEdfsf]

Thank you for the fix, will check that it works, will close when I'm sure it's ok :)

[simonvetter]

I'm going to go ahead and close this as I believe it is fixed, but feel free to reopen if not.

Thanks again for the report!