CNP - Compositor Notification Protocol

CNP is designed for use between an X compositor client and other X clients who
may have redirected drawables.

The CNP helps facilitate memory and performance savings by allowing
redirected clients to reduce their memory size and bandwidth consumption.
It can also enable throttling for redirected clients.  Normally,
redirected client buffer swaps and other operations complete immediately
(because there's no vertical blank event to synchronize with).  Using
this protocol, the compositor and client can interact, and the
compositor can drive frame and drawable updates from clients.

To implement memory size and bandwidth savings, CNP aware clients can,
when connected to a CNP aware compositor, use single buffered rendering,
sending CNPDrawableReady notifications to the compositor as each frame
finishes drawing.  This eliminates the back->front copy overhead as well
as the memory consumed the extra offscreen buffer.

To implement throttling, clients simply have to stop drawing until they
receive a CNPDrawableConsumed event.  This will throttle them to the
compositor's framerate, and prevent unnecessary drawing that won't ever
be displayed to the end user.

Protocol definitions

┌───
    CNPDrawableReady
	2	1			CNPDrawableReady opcode
	2	8			length
	4	DRAWABLE		drawable
      ▶
└───

	Notifies the compositor that the drawable ID indicated by
	drawable is ready to be composited into a new frame.  Depending
	on the server implementation, it may be best for the client to
	wait until it receives a swap event message before sending this
	message to the compositor.  In other cases, simply sending it
	after a glXSwapBuffers or copy sub-buffer request is sufficient.

	Note that unless the client is using triple buffering or is
	otherwise not modifying the drawable sent to the compositor by
	this message, it should wait until it receives the
	CNPDrawableConsumed message (described below) before doing
	further drawing.  This prevents partially drawn drawables from
	being composited into an on-screen frame.

	
┌───
    CNPDrawableConsumed
	2	2			CNPDrawableConsumed opcode
	2	8			length
	4	DRAWABLE		drawable
      ▶
└───

	This message comes from the compositor and indicates to clients
	that their previously complete drawable has been consumed
	(likely copied or sourced for an on-screen frame update).  

	Upon receiving this message, clients can resume drawing to the
	drawable previously submitted to the compositor.

Ports

To implement the protocol, the compositor should listen for client
connections on a local port (how should this be advertised; D-bus?  unix
sockets?  what about remote clients?)