Xwayland on demand (Re: [PATCH xwayland 3/3] xwayland: Handle the case of windows being realized before redirection)

[Olivier Fourdan]

On Wed, Jan 16, 2019 at 9:55 AM Olivier Fourdan <ofourdan at redhat.com> wrote:
>
> Hi
>
> On Wed, Jan 16, 2019 at 9:35 AM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> >
> > On Tue, 15 Jan 2019 23:21:05 +0100
> > Carlos Garnacho <carlosg at gnome.org> wrote:
> >
> > > If Xwayland gets to realize a window meant for composition before the
> > > compositor redirected windows (i.e. redirect mode is not RedirectDrawManual
> > > yet), the window would stay "invisible" as we wouldn't create a
> > > wl_surface/wl_shell_surface for it at any later point.
> > >
> > > This scenario may happen if the wayland compositor raises Xwayland on
> > > demand for a X11 client being started. In this case the first data on
> > > the socket is the client's, the compositor can hardly beat that in
> > > order to redirect subwindows before the client realizes a Window.
> > >
> > > In order to handle this case, allow the late creation of a matching
> > > (shell) surface for the WindowPtr on SetWindowPixmapProc, so it is ensured
> > > to be created after the compositor set up redirection.
> > >
> > > Signed-off-by: Carlos Garnacho <carlosg at gnome.org>
> > > ---
> > >  hw/xwayland/xwayland.c | 37 +++++++++++++++++++++++++++++++++++++
> > >  hw/xwayland/xwayland.h |  1 +
> > >  2 files changed, 38 insertions(+)
> >
> > Hi Carlos,
> >
> > this reminds me of other ordering issues with starting Xwayland on
> > demand: xrdb. I presume the X resources should be merged before any app
> > X11 client is processed, so that they see the user's resources at
> > start-up. AFAIK, that cannot happen if Xwayland is started on-demand.
> > Either it is too late if app start-up triggers it, or then you launch
> > Xwayland at desktop start-up, essentially not doing on-demand anymore.
> > Have you seen people complain about that?
> >
> > Could that be fixed somehow?
> >
> > Pass the X11 app socket to Xwayland later after the XWM init is
> > complete? Stall non-XWM clients inside Xwayland by default until XWM is
> > ready?
>
> I am not sure I understand what xwayland-on-demand changes there.
>
> Considering "xrdb" is a regular X11 client, it requires a display,
> hence it must have Xwayland running.
>
> If Xwayland is not running (i.e. xrdb is the first X11 client to be
> started), then Xwayland will be started (as there is a "demand" from a
> client, the client being xrdb) and other following X11 clients will
> connect to the same display and have the resources set in xrdb
> available.
>
> I remember Ray (halfline) advocating for Xwayland running xrdb itself
> at startup to avoid this issue. But that's a bit of a special case for
> specific X11 client (xrdb) which could be considered as legacy...
> (like, people may not want to slow down Xwayland startup waiting for
> xrdb to complete when they don't actually use any X11 application
> benefiting from the X11 resources database)

There: https://bugzilla.gnome.org/show_bug.cgi?id=769644

> Maybe a more general design of a script being run automatically by
> Xwayland if present, so that users/distro-maintainers/sysadmins can
> tailor that startup script at will?

Cheers,
Olivier