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This is a fork of RetroArch intended to fix its wonderful-but-imperfect Netplay.

Done and merged upstream:

• Removed the "here there be dragons" sign that prevented anyone else from looking at this code.
• Added a README that lightly documents how RetroArch's Netplay works.
• Netplay ring buffer hardened to prevent overruns, which previously caused insane desyncs.
• Ripped out UDP. Everything is done over TCP now. The previous implementation demanded reliability and in-order guarantees anyway, so UDP was just silly. Note that the protocol has also changed in more fundamental ways, so it's not really possible to make a "UDP or TCP" option except by having two incompatible implementations of Netplay.
• Removed all limits on the size of the replay buffer. You want to allow 10 seconds of drift? 10 seconds of drift it is!
• Made stalling work without blocking the UI thread.
• General bugfixes to make Netplay bulletproof.

Done:

• Remote pausing. The Netplay support for remote pausing is in the netplay-new-features branch. The frontend support for telling Netplay that it's paused is in netplay-new-features-iffy, because it uses an ugly polling approach, and there may be something better.
• Stalling without rerunning frames. Netplay is now one of the many reasons the frontend may pause.

Todo:

• Infinite testing, of course!
• Support for loading state over Netplay
  • Support for launching Netplay on an existing session?
• UDP as redundancy for TCP: Still send input over TCP, but send it over UDP as well, and use the UDP data whenever possible. That gets the speed of UDP with the reliability of TCP.
• Fix spectator mode. It used to slowly lose time. Lately, it doesn't even work. I haven't touched it, so it continues to not work.
• General "must-rewind" feature, in particular so that player flipping becomes deterministic.
• Frame CRCing as a last resort for sync (probably only necessary for cores with nondeterminism. If they have HEAVY nondeterminism, still useless)
  • Rewindless mode. This is strictly-wrong Netplay, but with frame CRCing, can work fine. If one disabled both rewinds and frame CRCing, one would be left with ZSNES-style netplay (no attempt at sync)
• Maybe: Support for input grappling (two players controlling the same input device). Useful for e.g. handhelds.
• Super-future: >2 player netplay. Not as difficult as it seems, just need an array of buffers, frames and replay.
• Super-future: Netplay input support for devices other than the simple retropad.