[mutter/wip/carlosg/xwayland-on-demand: 14/32] x11: Make error trap API NULL safe

[Carlos Garnacho <carlosg src gnome org>]

commit a03c6f83ca6690882684c307e414721372bde693
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sun Dec 30 12:19:24 2018 +0100

    x11: Make error trap API NULL safe
    
    There's quite a few places where error traps are used on high level
    code (i.e. not calling X11 itself) in order to coalesce the error
    handling for all X11 operations deeper in the callstack.
    
    This does not play well if display->x11_display is nullable. In
    order to avoid adding notions about it in code that's otherwise not
    related to MetaX11Display, make the error trap API accept NULL by
    itself, so it's safe to call with no X11 display.
    
    ATM we just lie and say "Success" if we are popping an error value
    if the is no X11 display present. This makes sense if the X11 display
    was nonexistent before pushing the trap, as no X11 calls should
    happen then. The returned error should be different if the X11 display
    became NULL while inside the error trap, but the X11 connection will
    die anyway, so this is currently moot.

 src/x11/meta-x11-errors.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/src/x11/meta-x11-errors.c b/src/x11/meta-x11-errors.c
index 506769464..3dc542da2 100644
--- a/src/x11/meta-x11-errors.c
+++ b/src/x11/meta-x11-errors.c
@@ -49,17 +49,23 @@
 void
 meta_x11_error_trap_push (MetaX11Display *x11_display)
 {
+  if (!x11_display)
+    return;
   gdk_x11_display_error_trap_push (x11_display->gdk_display);
 }
 
 void
 meta_x11_error_trap_pop (MetaX11Display *x11_display)
 {
+  if (!x11_display)
+    return;
   gdk_x11_display_error_trap_pop_ignored (x11_display->gdk_display);
 }
 
 int
 meta_x11_error_trap_pop_with_return (MetaX11Display *x11_display)
 {
+  if (!x11_display)
+    return BadImplementation;
   return gdk_x11_display_error_trap_pop (x11_display->gdk_display);
 }