Add srt_shutdown() to the SRT API

[gegles]

The standard POSIX socket API has a int shutdown(int socket, int how) method that allows "all or part of a full-duplex connection on the socket associated with the file descriptor socket to be shut down".

There is no srt_shutdown() equivalent. Would it be possible to add? Any thoughts as to how I could emulate this for now? Would sending an empty msg do the trick? (haven't tried yet).

This is especially needed/useful when building some kind of proxy to/from SRT to another protocol... I wonder how stransmit deals with that? Shutting down half connections in the right order is critical for proper behavior...

[ethouris]

Not sure how we would implement this because currently the only way that comes to my mind is to make a shutdown direction flag which will be set by this function so that the transmission in particular direction will be denied (error reported from particular sending or receiving function). That would be the only visible part of it and of course it will not affect the underlying UDP sockets at all, especially that they can be used by multiple SRT sockets at a time, of which not all wanted to be shut down.

Please tell me if making just this thing - set a flag that will enforce error on calls to srt_send* or srt_recv* respectively - and effectively making the socket unusable without closing it, will serve this solution best. It would be also enlightening if you describe more how you're going to use it and what it should help you with better than just closing the socket.

[gegles]

I think that would be a great start, but may not be enough. I don't know SRT's inner working well enough to say.

My understanding of how this works (in TCP), is that when one side sends its SHUTDOWN msg, it triggers an EOF on the receiving side. The receiving side can't read anymore after that, but can still send as much data as it wants and only when it decides it is done send its SHUTDOWN message. The initial sender should still be receiving until it gets that final SHUTDOWN... Once both sending/receiving have been shutdown, each side can then call close to properly dispose of the socket...

Is this possible to do with SRT?

[ethouris]

Everything is possible within the ranges of what userspace software can do, the question is only how much effort.

In this case the trouble is that the information must be sent from the closed sender to the received, without breaking the connection. As this is UDP, this must use some cyclic-sending with confirm-response. I have some ideas how to implement that, but I must consult with the management first. Note of course that this mechanism will only work when both sides are of minimum version that supports it.