[cairo] More on the future of cairo's X11 backends

[Uli Schlachter]

Hi,

a while ago [1] I proposed merging the implementations of cairo-xcb and
cairo-xlib into one. Back then there were concerns about a performance
loss [2] and I have to admit this demotivated me a bit. I don't want to
invest lots of work just to figure out that in the end the result is too
slow.

Anyway, let me take a step back: There are Xlib and XCB for talking to
an X11 server. Both are being used and for both cairo offers an API.
There are several ways how to implement something like this:

A) Have two completely separate backends with their own code paths.
B) Implement one ontop of the other.
C) Have common code and some thin wrapper around Xlib/XCB to make this work.
D) I don't know. Did I miss a further possibility?

Currently cairo has (A), which is bad for maintainability. The XCB
backend started as a copy of the Xlib backend and since then the two
diverged quite a bit. So, some things work in one, but not the other,
some bugs are only present in one of the two etcetera.

In 2013 I proposed (B): Use XGetXCBConnection() to get an
xcb_connection_t out of a Display. However, this means that control of
the X11 socket is handed back and forth between XCB and Xlib quite a
lot(?) and thus can lead to poorer performance.

Since I see no other possibilities, this means that (C) is left. What I
propose is to have thin wrappers around the X11 protocol and to add
function pointers to the cairo device struct for calling these. As an
example, XCopyArea(device->display, ....) would be replaced with
device->functions->copy_area(device, ....).

For data types (Drawable vs xcb_drawable_t etc) it does not really
matter in most cases which one is used, because both are just typedefs
for some integer type. I would plan to use the Xlib typedef.

Something like the following:

- Introduce these function pointers in cairo-xlib and make use of them
- Add an xcb-xlib-backend that implements the xcb API ontop of these
function pointers
- Eventually remove the current cairo-xcb and cairo-xlib-xcb code and
only leave the above.

Of course this is all still a bit rough around the edges, but what do
you think?

Cheers,
Uli

[1]:
https://lists.cairographics.org/archives/cairo/2013-September/024679.html
[2]
https://lists.cairographics.org/archives/cairo/2013-September/024680.html
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