[glib] gobject: loosen property override flag restrictions

[Ryan Lortie <ryanl src gnome org>]

commit af24dbc12aa77aac3c82d39872878558cced7add
Author: Ryan Lortie <desrt desrt ca>
Date:   Tue Dec 20 14:36:29 2011 -0500

    gobject: loosen property override flag restrictions
    
    GObject enforces the following restrictions on property overrides:
    
      - must only add abilities: if the parent class supports
        readability/writability then the subclass must also support them.
        Subclasses are free to add readability/writability.
    
      - must not add additional restrictions: if the parent class doesn't
        have construct/construct-only restrictions then the subclass must
        not add them.  Subclasses are free to remove restrictions.
    
    The problem with the previous implementation is that the check against
    adding construct/construct-only restrictions was being done even if the
    property was not previously writable.  As an example:
    
      "readable" and "writable only on construct"
    
    was considered as being more restrictive than
    
      "read only".
    
    This patch tweaks the check to allow the addition of
    construct/construct-only restrictions for properties that were
    previously read-only and are now being made writable.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=666615

 gobject/gobject.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)
---
diff --git a/gobject/gobject.c b/gobject/gobject.c
index 166faac..59fcca2 100644
--- a/gobject/gobject.c
+++ b/gobject/gobject.c
@@ -1396,14 +1396,15 @@ object_interface_check_properties (gpointer func_data,
 
 #define SUBSET(a,b,mask) (((a) & ~(b) & (mask)) == 0)
 
-      /* CONSTRUCT and CONSTRUCT_ONLY add restrictions.
+      /* CONSTRUCT and CONSTRUCT_ONLY add restrictions to writability.
        * READABLE and WRITABLE remove restrictions. The implementation
        * paramspec must have less restrictive flags.
        */
       if (class_pspec &&
-	  (!SUBSET (class_pspec->flags,
-		    pspecs[n]->flags,
-		    G_PARAM_CONSTRUCT | G_PARAM_CONSTRUCT_ONLY) ||
+          (((pspecs[n]->flags & G_PARAM_WRITABLE) &&
+            !SUBSET (class_pspec->flags,
+                     pspecs[n]->flags,
+                     G_PARAM_CONSTRUCT | G_PARAM_CONSTRUCT_ONLY)) ||
 	   !SUBSET (pspecs[n]->flags,
 		    class_pspec->flags,
 		    G_PARAM_READABLE | G_PARAM_WRITABLE)))