libretro netplay API

[GregorR]

There's some push from both sides to make an API in libretro to allow cores to have some feedback from netplay. This is further complicated by the fact that netplay can be enabled or disabled at any time, and so any such API has to be "live"; we need an actual callback into the core.

In particular, the following features are wanted:

• Knowing which devices are connected, and which devices are local. Note that since netplay allows one client to have multiple devices or one device to be controlled by multiple clients, this is all bitmaps. Known use case: Link cable implementation by emulating both machines.
• Knowing when a savestate is requested by netplay. Known use cases: (1) fbalpha normally discards some "unimportant" information (such as PRNG seeds) when it saves, and they want to only save that when replayable rewind is important; this is currently breaking netplay on fbalpha. (2) I want heavy cores to be able to save "differentially", using the existing savestate buffer and overwriting only the parts that have actually changed. This could make saving vastly faster, helping to bring e.g. PSX into the netplay fold.
• Knowing when a frame is real and visible to users, vs requested for netplay replay and thus squelched. Known use case: I'm told that the way PSX and N64 work, one could save significant time by simply not executing the graphics and audio processor cycles, which would also help to bring such heavyweight cores down.

In addition, there is some mumbling about piggybacking on some features of RA's netplay while actually implementing the core's own netplay, in cores where that makes sense. e.g. it would be nice for cores that implement netplay to be able to use RA's lobby. This one would be more invasive and is not currently on my "actually important" list.

Obviously we can't modify retro_serialize or retro_run, so the normal execution path would have to go something like this:

netplay_callback(NETPLAY_IS_ABOUT_TO_SAVE, ...);
retro_serialize(...);
netplay_callback(NETPLAY_IS_DONE_SAVING, ...);
...
netplay_callback(NETPLAY_IS_ABOUT_TO_RUN_HIDDEN_FRAMES, ...);
retro_run();
retro_run();
retro_run();
netplay_callback(NETPLAY_IS_DONE_RUNNING_HIDDEN_FRAMES, ...);

If any other use cases, perhaps demanding different feedback, are known, please post them here. It would be nice to cover all known use cases.

[furiadeoso]

Hi, @GregorR ! Maybe this actual working line can help to start all those improvements:

https://github.com/libretro/RetroArch/issues/7299 https://github.com/libretro/RetroArch/pull/7379

Glad to help if it's needed! Best Regards!

[LibretroAdmin]

@Cthulhu-throwaway Some old issue with some ideas regarding netplay. Not sure if any of it is worthwile.