[Spice-devel] [PATCH spice-gtk 1/3] clipboard: accept grab only from the side with keyboard focus

[Marc-André Lureau]

Hi

On Fri, Mar 15, 2019 at 4:43 PM Jakub Janku <jjanku at redhat.com> wrote:
>
> Hi,
>
> On Thu, Mar 14, 2019 at 6:18 PM Marc-André Lureau
> <marcandre.lureau at gmail.com> wrote:
> >
> > Hi
> >
> > On Thu, Feb 28, 2019 at 8:12 PM Jakub Janků <jjanku at redhat.com> wrote:
> > >
> > > If two grab messages in opposite directions "meet" on their way
> > > to their destinations, we end up in a state when both spice-gtk
> > > and spice-vdagent think that the other component can provide
> > > clipboard data. As a consequence of this, neither of them can.
> > >
> > > This is a bug in the spice-protocol. To mitigate the issue,
> > > accept grab only from the side that currently has keyboard focus,
> > > this means:
> > > (1) spice-gtk has focus ==>
> > >     * accept grabs from vdagent,
> > >     * ignore grabs from other apps running in the host
> > > (2) spice-gtk doesn't have focus ==>
> > >     * accept grabs from other apps running in the host
> > >     * ignore grabs from vdagent
> > >
> > > The check in clipboard_owner_change() is X11-specific,
> > > because on Wayland, spice-gtk is first notified about the
> > > owner-change once it gains focus.
> > >
> > > The check in clipboard_grab() can be generic.
> > > Note that on Wayland, calling gtk_clipboard_set_with_owner()
> > > while not having focus doesn't have any effect anyway,
> > > only focused clients can set clipboard.
> > >
> > > With this patch, the race can still occur around the time
> > > when focus changes (rare, but possible), on X11 as well as Wayland.
> > >
> > > Related: https://gitlab.freedesktop.org/spice/spice-gtk/issues/82
> > > Related: https://bugzilla.redhat.com/show_bug.cgi?id=1594876
> > >
> > > Signed-off-by: Jakub Janků <jjanku at redhat.com>
> > > ---
> > >
> > > Victor, half of this patch is basically yours,
> > > so feel free to add signed-off or something,
> > > in the case this gets upstream :)
> > >
> > > ---
> > >  src/spice-gtk-session.c | 13 +++++++++++++
> > >  1 file changed, 13 insertions(+)
> > >
> > > diff --git a/src/spice-gtk-session.c b/src/spice-gtk-session.c
> > > index b48f92a..7b7370c 100644
> > > --- a/src/spice-gtk-session.c
> > > +++ b/src/spice-gtk-session.c
> > > @@ -680,6 +680,13 @@ static void clipboard_owner_change(GtkClipboard        *clipboard,
> > >          s->clip_hasdata[selection] = FALSE;
> > >          return;
> > >      }
> > > +
> > > +    if (GDK_IS_X11_DISPLAY(gdk_display_get_default()) &&
> > > +        spice_gtk_session_get_keyboard_has_focus(self)) {
> > > +        SPICE_DEBUG("%s: spice-gtk has keyboard focus, "
> > > +                    "ignoring grab from other app", __FUNCTION__);
> > > +        return;
> > > +    }
> > >  #endif
> >
> > This will break clipboard managers interactions, which may take the
> > clipboard content, save it and/or modify it.
>
> Depends on the implementation of the given clipboard manager, I'd say.
> I tried the "clipboard indicator" you're using. Seems like no problem there :)

without, and with that patch I suppose

> >
> > >
> > >      s->clip_hasdata[selection] = TRUE;
> > > @@ -823,6 +830,12 @@ static gboolean clipboard_grab(SpiceMainChannel *main, guint selection,
> > >      cb = get_clipboard_from_selection(s, selection);
> > >      g_return_val_if_fail(cb != NULL, FALSE);
> > >
> > > +    if (!spice_gtk_session_get_keyboard_has_focus(self)) {
> > > +        SPICE_DEBUG("%s: spice-gtk doesn't have keyboard focus, "
> > > +                    "ignoring grab from guest agent", __FUNCTION__);
> > > +        return FALSE;
> > > +    }
> >
> >
> > Beside automation, the cursor alone may easily create a new clipboard
> > content which won't be available to the client side (the auto-grab may
> > fail to follow cursor etc).
> >
> > It's a bit unclear why it's not X11 specific but for client side
> > change it is, this could deserve a code comment.
>
> Tried to describe that in the commit log. I could add a comment in the
> code as well.

yes, please

> >
> > All in all, this feels weak and breaks some legitimate cases.
> >
> > I am not very strongly against this, as I understand it may help with
> > some races we discussed,
>
> Is this an ack or nack?

If you ask me, it's a nack at this point, as I don't have a way to
reproduce the problem at this point and I clearly see problematic case
created by the patch. Iow, it looks like a regression to me.

> Seems like we're just going round in circles now...

Hopefully I am not the only one reviewing and questioning this patch

> > but it feels like the problem is elsewhere
>
> sorry, but that's very vague, I have no idea what you're referring to

Some clipboard managers seem to work fine. So what is klipper doing?

>
> > and we need a better solution to prevent the race from happening.
> >
> > I haven't read the bug reports: this kind of workaround needs a
> > description of a broken use case (not a theoretical description of a
> > race that "never" happen in practice).
>
> I described a broken case in the previous email:
> KDE with klipper, "prevent empty clipboard" enabled + slow network

I guess I will have to install KDE to understand and reproduce the issue.

Rgardless if this patch is accepted or not, we should work on a real
solution to prevent the race.