Question

Sometimes boost::asio seems to disconnect before I want it to, i.e. before the server properly handles the disconnect. I’m not sure how this is possible because the client seems to think its fully sent the message, yet when the server emits the error its not even read the message header… During testing this only happens maybe 1 in 5 times, the server receives the client shut down message, and disconnects the client cleanly.

The error: “An existing connection was forcibly closed by the remote host”

The client disconnecting:

void disconnect()
{
    boost::system::error_code error;
    //just creates a simple buffer with a shutdown header
    boost::uint8_t *packet = createPacket(PC_SHUTDOWN,0);
    //sends it
    if(!sendBlocking(socket,packet,&error))
    {
        //didnt get here in my tests, so its not that the write failed...
        logWrite(LOG_ERROR,"server",
            std::string("Error sending shutdown message.\n")
            + boost::system::system_error(error).what());
    }

    //actaully disconnect
    socket.close();
    ioService.stop();
}
bool sendBlocking(boost::asio::ip::tcp::socket &socket,
    boost::uint8_t *data, boost::system::error_code* error)
{
    //get the length section from the message
    boost::uint16_t len = *(boost::uint16_t*)(data - 3);
    //send it
    asio::write(socket, asio::buffer(data-3,len+3),
        asio::transfer_all(), *error);
    deletePacket(data);
    return !(*error);
}

The server:

void Client::clientShutdown()
{
    //not getting here in problem cases
    disconnect();
}
void Client::packetHandler(boost::uint8_t type, boost::uint8_t *data,
    boost::uint16_t len, const boost::system::error_code& error)
{
    if(error)
    {
        //error handled here
        delete[] data;
        std::stringstream ss;
        ss << "Error recieving packet.\n";
        ss << logInfo() << "\n";
        ss << "Error: " << boost::system::system_error(error).what();
        logWrite(LOG_ERROR,"Client",ss.str());

        disconnect();
    }
    else
    {
        //call handlers based on type, most will then call startRead when
        //done to get the next packet. Note however, that clientShutdown
        //does not
        ...
    }
}



void startRead(boost::asio::ip::tcp::socket &socket, PacketHandler handler)
{
    boost::uint8_t *header = new boost::uint8_t[3];
    boost::asio::async_read(socket,boost::asio::buffer(header,3),
        boost::bind(&handleReadHeader,&socket,handler,header, 
        boost::asio::placeholders::bytes_transferred,boost::asio::placeholders::error));
}
void handleReadHeader(boost::asio::ip::tcp::socket *socket, PacketHandler handler,
    boost::uint8_t *header, size_t len, const boost::system::error_code& error)
{
    if(error)
    {
        //error "thrown" here, len always = 0 in problem cases...
        delete[] header;
        handler(0,0,0,error);
    }
    else
    {
        assert(len == 3);
        boost::uint16_t payLoadLen  = *((boost::uint16_t*)(header + 0));
        boost::uint8_t  type        = *((boost::uint8_t*) (header + 2));
        delete[] header;
        boost::uint8_t *payLoad = new boost::uint8_t[payLoadLen];

        boost::asio::async_read(*socket,boost::asio::buffer(payLoad,payLoadLen),
            boost::bind(&handleReadBody,socket,handler,
            type,payLoad,payLoadLen,
            boost::asio::placeholders::bytes_transferred,boost::asio::placeholders::error));
    }
}
void handleReadBody(ip::tcp::socket *socket, PacketHandler handler,
    boost::uint8_t type, boost::uint8_t *payLoad, boost::uint16_t len,
    size_t readLen, const boost::system::error_code& error)
{
    if(error)
    {
        delete[] payLoad;
        handler(0,0,0,error);
    }
    else
    {
        assert(len == readLen);
        handler(type,payLoad,len,error);
        //delete[] payLoad;
    }
}

Answer 1

I think you should probably have a call to socket.shutdown(boost::asio::ip::tcp::socket::shutdown_both, ec) in there before the call to socket.close().

The boost::asio documentation for basic_stream_socket::close states:

For portable behaviour with respect to graceful closure of a connected socket, call shutdown() before closing the socket.

This should ensure that any pending operations on the socket are properly cancelled and any buffers are flushed prior to the call to socket.close.