RetroArch/network/netplay

This is RetroArch's Netplay code. RetroArch Netplay allows a second player to
be connected via the Internet, rather than local at the same computer. Netplay
in RetroArch is guaranteed* to work with perfect synchronization given a few
minor constraints:

(1) The core is deterministic,
(2) The only input devices the core interacts with are the joypad and analog sticks, and
(3) Both the core and the loaded content are identical on host and client.

Furthermore, if the core supports serialization (save states), Netplay allows
for latency and clock drift, providing both host and client with a smooth
experience.

Note that this documentation is all for (the poorly-named) "net" mode, which is
the normal mode, and not "spectator" mode, which has its own whole host of
problems.

Netplay in RetroArch works by expecting input to come delayed from the network,
then rewinding and re-playing with the delayed input to get a consistent state.
So long as both sides agree on which frame is which, it should be impossible
for them to become de-synced, since each input event always happens at the
correct frame.

In terms of the implementation, Netplay is in effect a state buffer
(implemented as a ring of buffers) and some pre- and post-frame behaviors.

Within the state buffers, there are three locations: self, other and read. Each
refers to a frame, and a state buffer corresponding to that frame. The state
buffer contains the savestate for the frame, and the input from both the local
and remote players.

Self is where the emulator believes itself to be, which may be ahead or behind
of what it's read from the peer. Generally speaking, self progresses at 1 frame
per frame, except when the network stalls, described later.

Other is where it was most recently in perfect sync: i.e., other-1 is the last
frame from which both local and remote input have been actioned. As such, other
is always less than or equal to both self and read. Since the state buffer is a
ring, other is the first frame that it's unsafe to overwrite.

Read is where it's read up to, which can be slightly ahead of other since it
can't always immediately act upon new data.

In general, other ≤ read and other ≤ self. In all likelihood, read ≤ self, but
it is both possible and supported for the remote host to get ahead of the local
host.

Pre-frame, Netplay serializes the core's state, polls for local input, and
polls for input from the other side. If the input from the other side is too
far behind, it stalls to allow the other side to catch up. To assure that this
stalling does not block the UI thread, it is implemented similarly to pausing,
rather than by blocking on the socket.

If input has not been received for the other side up to the current frame (the
usual case), the remote input is simulated in a simplistic manner.  Each
frame's local serialized state and simulated or real input goes into the frame
buffers.

During the frame of execution, when the core requests input, it receives the
input from the state buffer, both local and real or simulated remote.

Post-frame, it checks whether it's read more than it's actioned, i.e. if read >
other self > other. If so, it first checks whether its simulated remote data
was correct. If it was, it simply moves other up. If not, it rewinds to other
(by loading the serialized state there) and runs the core in replay mode with
the real data up to the least of self and read, then sets other to that.

When in Netplay mode, the callback for receiving input is replaced by
input_state_net. It is the role of input_state_net to combine the true local
input (whether live or replay) with the remote input (whether true or
simulated).

Some thoughts about "frame counts": The frame counters act like indexes into a
0-indexed array; i.e., they refer to the first unactioned frame. So, when
read_frame_count is 23, we've read 23 frames, but the last frame we read is
frame 22. With self_frame_count it's slightly more complicated, since there are
two relevant actions: Reading the data and emulating with the data. The frame
count is only incremented after the latter, so there is a period of time during
which we've actually read self_frame_count+1 frames of local input.


* Guarantee not actually a guarantee.


Netplay's command format

Netplay commands consist of a 32-bit command identifier, followed by a 32-bit
payload size, both in network byte order, followed by a payload. The command
identifiers are listed in netplay.h. The commands are described below. Unless
specified otherwise, all payload values are in network byte order.

Command: ACK
Payload: None
Description:
    Acknowledgement. Not used.

Command: NAK
Payload: None
Description:
    Negative Acknowledgement. If received, the connection is terminated. Sent
    whenever a command is malformed or otherwise not understood.

Command: INPUT
Payload:
    {
       frame number: uint32
       joypad input: uint32
       analog 1 input: uint32
       analog 2 input: uint32
       OPTIONAL state CRC: uint32
    }
Description:
    Input state for each frame. Netplay must send an INPUT command for every
    frame in order to function at all.

Command: FLIP_PLAYERS
Payload:
    {
       frame number: uint32
    }
Description:
    Flip players at the requested frame.

Command: DISCONNECT
Payload: None
Description:
    Gracefully disconnect. Not used.

Command: CRC
Payload:
    {
       frame number: uint32
       hash: uint32
    }
Description:
    Informs the peer of the correct CRC hash for the specified frame. If the
    receiver's hash doesn't match, they should send a REQUEST_SAVESTATE
    command.

Command: REQUEST_SAVESTATE
Payload: None
Description:
    Requests that the peer send a savestate.

Command: LOAD_SAVESTATE
Payload:
    {
       frame number: uint32
       serialized save state: blob (variable size)
    }
Description:
    Cause the other side to load a savestate, notionally one which the sending
    side has also loaded.