[cairo] Concerns regarding cairo-xcb backend

[Alexandre Bique]

Hi Uli,

Many thanks for the response.

As of today I'm using cairo-image + xcb-shm and the bug seems to be gone.

Regards,
Alexandre

On Sat, Dec 16, 2017 at 10:46 AM, Uli Schlachter <psychon at znc.in> wrote:

> Hi,
>
> On 12.12.2017 14:47, Alexandre Bique wrote:
> > I'm improving the Linux port of the u-he plugins (www.u-he.com) and I
> was
> > using Xlib before but I've switched to xcb now, and I'm using cairo-xcb
> > instead of cairo-xlib for the painting.
> >
> > While using the cairo-xcb backend I've come across the following
> assertion:
> >
> > cairo-xcb-screen.c:219: _get_screen_index: Assertion `!"reached"' failed.
> >
> > I've looked at the source code and I was surprised to see that the
> > cairo-xcb backend has global variables (mutex, list of
> > cairo_xcb_connection, ...), and I wonder if this is the issue in my case.
>
> cairo-xlib has such global data as well. When you create two cairo
> surfaces for the same XCB connection, you want cairo to initialise
> "everything" just once.
>
> > The application I'm working on is a software synthesizer plugin
> (VST2/VST3)
> > which a Digital Audio Workstation like Bitwig Studio (www.bitwig.com)
> can
> > use and display.
> >
> > The DAW loads the plugins, can show the plugin window (using XEmbed),
> hide
> > or close the plugin window. In case the plugin window is closed I close
> my
> > xcb connection, free my FT library handle and destroy my cairo xcb
> surface.
> >
> > So if the window is shown/closed/shown/closed/... it will initialize and
> > destroy xcb+FT+cairo_xcb_surface for each of them.
> >
> > And I wonder if this is the issue: cairo_xcb seems to not work properly
> in
> > such scenario.
> >
> > Do you have any experience or ideas to share?
>
> When you have a cairo surface using your xcb_connection*, do the following:
>
>  cairo_device_t *device = cairo_device_reference(
>         cairo_surface_get_device(surface));
>
> Then, *before* you do xcb_disconnect(connection), do:
>
>  cairo_device_finish(device);
>  cairo_device_destroy(device);
>
> The above should happen automatically when the last cairo-xcb surface is
> finished. Thus, it might be that you have a leak somewhere? (Not
> necessarily, there are non-leak cases where the above can happen)
>
> cairo-xlib does some (IMO) bad hacks so that the above is not necessary:
> It registers itself as an X11 extension with libX11. That way it gets a
> callback when you do XCloseDisplay(), but since unregistering an X11
> extension is not possible, this has some other downsides (e.g. the above
> "dance" is still necessary if cairo ends up being unloaded before the
> X11 connection is closed).
>
> Cheers,
> Uli
> --
> - Captain, I think I should tell you I've never
>   actually landed a starship before.
> - That's all right, Lieutenant, neither have I.
>



-- 
Alexandre Bique
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