Select-based TCP server disconnection Processing

The camera and the mobile phone use a Transit server to forward data. If either party unexpectedly exits, the Select function execution will be abnormal.

You do not need to regularly check each socket descriptor on the server. The solution is as follows:

1) a heartbeat packet is sent every 30 seconds during video playback on the mobile phone and forwarded to camera by the server. If the heartbeat packet is not received for more than 30 seconds, camera interrupts the transmission and notifies the server to release the mobile socket descriptor.

2) when the camera is disconnected, the SELECT command gets a change in the fd_set of the camera, but the Recv returns an error. In this case, the server releases the socket descriptor of Camara directly.