[cairo] On allowing NULL font options

[Alp Toker]

Behdad Esfahbod wrote:
> On Thu, 2008-01-17 at 17:39 -0500, Chris Wilson wrote:
>>     [cairo-font-options] Treat NULL as a default cairo_font_options_t
>>     
>>     Interpret a NULL cairo_font_options_t as the default values - i.e
>>     as if it were a fresh pointer returned by
>> cairo_font_options_create().
> 
> I thought about this before.  I think we shouldn't doing it.  Currently
> the cairo practice is: no NULL.

I agree with Behdad here, but perhaps we should be more careful never to 
return a NULL font options in external libraries like gdkcairo.

Just today I received a crasher bug report in an application. The 
problem code?

     if (GdkScreen* screen = gdk_screen_get_default())
         options = gdk_screen_get_font_options(screen);
     else
         options = defaultOptions;

     m_scaledFont = cairo_scaled_font_create(face, &mat, &ctm, options);

This looks safe, but in fact will lead to crashes because 
gdk_screen_get_font_options() is documented as being able to return NULL:

/**
  * gdk_screen_get_font_options:
  * @screen: a #GdkScreen
  *
  * Gets any options previously set with gdk_screen_set_font_options().
  *
  * Return value: the current font options, or %NULL if no default
  *  font options have been set.
  *
  * Since: 2.10
  **/

This is hard to catch because, on desktop systems, 
gdk_screen_set_font_options() almost always returns a valid 
cairo_font_options_t*.

It only seems to return NULL on obscure configurations (the user was 
testing on an embedded ARM board), so leads to crashes further down the 
line even after reasonable testing.