LIBSRT uses wrong write-readiness epoll on a listener socket

[Mikołaj Małecki]

The epoll subscription for a listener socket uses the same method as for other modes, by waiting for write readiness. This works only due to a bug in LIBSRT, which will be fixed at some point. The event that should be checked ready on a listener socket should be the read-readiness, SRT_EPOLL_IN.

This is where the epoll is being created:

`
ret = write_eid = libsrt_epoll_create(h, fd, 1);
`

Then it is used in both branches for a listener socket and connected socket. For connecting (and rendezvous) socket it's ok, write-readiness defines connection-ready. For a listener socket it should use read-readiness, so creation of eid for the listener branch should be likely done separately, with "0" last parameter.

[Marton Balint]

I have added a patch to the ticket which fixes this, compile tested only, can you check if it works for various modes?

[Mikołaj Małecki]

Just a reminder. We would like to fix this bug in SRT, and after we do it the listener mode for SRT in ffmpeg will no longer work.

Let me clarify: the SRT documentation stated it clearly that the epoll flag in the listener socket that triggers the accept-readiness is SRT_EPOLL_IN. If this was done differently, it wasn't done according to the API documentation and it only works thanks to a bug that will soon be fixed.

To test this above fix, simply run ffmpeg with reading or sending a stream in listener mode. To make a transmission over SRT (even within the internal network of one machine) from a media file using ffmpeg -re for sending and the other command for sending it to a file (or simply receive it via ffplay).

[Marton Balint]

Can you confirm that the patch attached here fixes the API usage?

[Balling]

We would like to fix this bug in SRT, and after we do it the listener mode for SRT in ffmpeg

Fix the bug in LIBSRT.

[Mikołaj Małecki]

Alright then. I take it as a good deal.

[Max]

To avoid possible misunderstanding, the libSRT triggers the SRT_EPOLL_IN event on the accept-readiness. This means the patch is valid and can be merged without changes to libSRT.
The libSRT, however, also signals the SRT_EPOLL_OUT event on accept-readiness, on which ffmpeg is currently relying. This behavior is wrong and is to be removed from libSRT.

[Marton Balint]

Fixed in 87677c2195e86b126c3438439a05d0a46ae5bb50.