The future of cairo's X11 backends

Hey everyone,

since 2004 (commit 810037bc), cairo has two separate X11 backends which shared
no backend-specific code. Time passed and the xcb backend bitrotted while the
xlib one was developed further. In 2010 (commit 1236c410), Chris rewrote and
improved the xcb backend a lot and IMHO since then it is pretty much usable.

However, there is no good reason for having these two separate. Since 2004, Xlib
has XGetXCBConnection() which allows mixing Xlib and xcb operations on the same
socket. The xcb-support in Xlib isn't optional since version 1.4.0 from 2010.
Chris used this for cairo's "xlib-xcb backend" which implements the cairo-xlib
API and uses xcb surfaces "behind the back" for all operations.

Obviously it would be great if cairo just had a single X11 backend which means
less code, less maintenance and more happy people. This is where I am trying to get.

The ugly approach for this would be to have a thin abstraction layer which hides
all the differences between Xlib and xcb.

My preferred approach instead is something like the current cairo-xlib-xcb code,
mostly because it is simpler and the only downside that I can see is that cairo
now depends on an xcb-enabled Xlib.

How can we get there?

Since cairo-xlib is what everyone uses, I would like to keep that code.
Hopefully, no new bugs are introduced and nobody outside of cairo notices that
anything changed. Also, it seems to me that Chris experimented with cairo-xcb
and those experiments lead to the compositor API which cairo-xlib now uses and
thus cairo-xlib is technically superior.

(The only things that I know that cairo-xcb does that aren't in cairo-xlib are
_cairo_surface_attach_snapshot() (which causes lots of problems) and different
approach to shared memory synchronization. None of that should be missed.)

My plan would be to make cairo-xlib use xcb and XGetXCBConnection() for
everything, dump cairo-xcb and re-implement the cairo-xcb API on top of the code
that is left. This can easily be done incrementally. Basically it is "dump
cairo-xcb and turn cairo-xlib into a new cairo-xcb". I guess this means that the
now-common source files and functions need to be renamed to cairo-x11-*.c and
cairo_x11_*. :-)

What do others think about this plan? Anything that I miss, any objections,
anyone who still uses cairo with Xlib 1.0?

Cheers,
Uli
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