timing may cause socket to enter invalid state which cannot reconnect

[paul-szczepanek-arm]

I'm running FreeRTOS in virtual environment that creates unusual timing issues which is probably why I ran into this but I believe this could also happen on real hardware.

The scenario is that I lose a connection and I try to shutdown the socket and reconnect. This fails because the socket is already in the wrong state and FreeRTOS_shutdown returns pdFREERTOS_ERRNO_EOPNOTSUPP causing the AWS stack to abort execution and failing to reconnect later as the the socket was not fully shutdown.

The reason pdFREERTOS_ERRNO_EOPNOTSUPP is returned is because the socket that loses connection can have its state change away from established before FreeRTOS_shutdown is called.

    BaseType_t FreeRTOS_shutdown( Socket_t xSocket,
                                  BaseType_t xHow )
    {
        FreeRTOS_Socket_t * pxSocket = ( FreeRTOS_Socket_t * ) xSocket;
        BaseType_t xResult;

        if( prvValidSocket( pxSocket, FREERTOS_IPPROTO_TCP, pdTRUE ) == pdFALSE )
        {
            /*_RB_ Is this comment correct?  The socket is not of a type that
             * supports the listen() operation. */
            xResult = -pdFREERTOS_ERRNO_EOPNOTSUPP;
        }
        else if( pxSocket->u.xTCP.ucTCPState != ( uint8_t ) eESTABLISHED )
        {
            /*_RB_ Is this comment correct?  The socket is not of a type that
             * supports the listen() operation. */
            xResult = -pdFREERTOS_ERRNO_EOPNOTSUPP;
        }

The condition else if( pxSocket->u.xTCP.ucTCPState != ( uint8_t ) eESTABLISHED ) is entered.

https://github.com/sifive/Amazon-FreeRTOS/blob/3cb6cc8d0bf2551eee5eda602501b89a5d5791b2/libraries/freertos_plus/standard/freertos_plus_tcp/source/FreeRTOS_Sockets.c#L2745

The comment inside the condition makes no sense to me. It is fine for the FreeRTOS_shutdown to not do anything in this case but it should return a more useful error, something like maybe pdFREERTOS_ERRNO_ENOTCONN. This way the stack can disregard the error and interpret it as shutdown not needed so I can continue safely instead of aborting. Right now SOCKETS_Shutdown in lib/amazon_freertos/libraries/abstractions/secure_sockets/freertos_plus_tcp/iot_secure_sockets.c has no way to distinguish between the two and passes an error further up the stack causing execution to be aborted leaving the socket in unusable state.

[paul-szczepanek-arm]

Sorry, posted on the forum instead and will reopen if staff accepts it on the forum.

[htibosch]

The scenario is that I lose a connection and I try to shutdown the socket and reconnect.

I think that the biggest confusion for developers in using TCP is about shutdown(). Once an API returns a negative value, you do not have to call shutdown anymore. That is because the connection is closed already.

The API shutdown() puts the socket in a closing state. It will try to deliver the last data, and it will wait for any promised data. And yes, FreeRTOS_shutdown() can only be called while a socket is still connected.

The API shutdown() is only called when something special is happening: a HTTP client that has obtained the desired information, a TCP server that wants to end a connection with a client in a gracious way.

If you want to actively shut down a connection, I recommend this kind of code:

    FreeRTOS_shutdown( xSocket, FREERTOS_SHUT_RDWR );
    while( timeout == pdFALSE )
    {
        int32_t rc = FreeRTOS_recv( xSocket )
        if( ( rc < 0 ) && ( rc != -pdFREERTOS_ERRNO_EAGAIN ) )
        {
            /* The TCP connection is broken. */
            break;
        }
    }