r2dbc-mariadb: server message accepted with no command in flight desynchronises the connection and serves one user's rows to another

r2dbc-mariadb accepts a server packet that arrives while the client has no command outstanding, buffers it, and hands it to the next unrelated statement. Nothing correlates a response with the request it is delivered to, and the driver reads the protocol's sequence id without ever validating it. …

      this.receiverDemands.decrementAndGet();
      Exchange exchange = this.exchangeQueue.peek();

      // nothing buffered => directly emit message
      ReferenceCountUtil.retain(message);
      if (this.receiverQueue.isEmpty()
          && exchange != null
          && (exchange.hasDemand() || …