jsapi-util: Rooting-safe gjs_parse_call_args()

This removes the old unused gjs_parse_args(), and moves
gjs_parse_call_args() into new files jsapi-util-args.cpp and
jsapi-util-args.h.

In order to ensure that JSObjects unpacked from gjs_parse_call_args()
land in GC roots, we need to add some type safety to its variable
arguments. Instead of accepting a JSObject** pointer for the "o" format
character, it must accept a JS::MutableHandleObject.

We can do this (and make the whole thing type safe as well) by using C++
templates instead of C varargs. Now we can issue a compile-time error
when an unknown type is passed in as a return location for an argument,
in particular JSObject**.

This also fixes some undefined behaviour: the old signature of
gjs_parse_call_args() placed the varargs parameter right after the
JS::CallArgs& parameter, and using a reference parameter in va_start() is
undefined. Here's a good explanation of what can go wrong:
http://stackoverflow.com/a/222288/172999

https://bugzilla.gnome.org/show_bug.cgi?id=742249

1 files changed, 3 insertions, 2 deletions

diff --git a/modules/cairo-surface.cpp b/modules/cairo-surface.cpp
index df288245..e5635957 100644
--- a/modules/cairo-surface.cpp
+++ b/modules/cairo-surface.cpp
@@ -24,6 +24,7 @@
 
 #include "gjs/compat.h"
 #include "gi/foreign.h"
+#include "gjs/jsapi-util-args.h"
 #include <cairo.h>
 #include <cairo-gobject.h>
 #include "cairo-private.h"
@@ -65,8 +66,8 @@ writeToPNG_func(JSContext *context,
     char *filename;
     cairo_surface_t *surface;
 
-    if (!gjs_parse_call_args(context, "writeToPNG", "s", argv,
-                        "filename", &filename))
+    if (!gjs_parse_call_args(context, "writeToPNG", argv, "F",
+                             "filename", &filename))
         return false;
 
     surface = gjs_cairo_surface_get_surface(context, obj);