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, 6 insertions, 5 deletions

diff --git a/modules/cairo-linear-gradient.cpp b/modules/cairo-linear-gradient.cpp
index 65717796..53e3a02a 100644
--- a/modules/cairo-linear-gradient.cpp
+++ b/modules/cairo-linear-gradient.cpp
@@ -23,6 +23,7 @@
 #include <config.h>
 
 #include "gjs/compat.h"
+#include "gjs/jsapi-util-args.h"
 #include <cairo.h>
 #include "cairo-private.h"
 
@@ -36,11 +37,11 @@ GJS_NATIVE_CONSTRUCTOR_DECLARE(cairo_linear_gradient)
 
     GJS_NATIVE_CONSTRUCTOR_PRELUDE(cairo_linear_gradient);
 
-    if (!gjs_parse_call_args(context, "LinearGradient", "ffff", argv,
-                        "x0", &x0,
-                        "y0", &y0,
-                        "x1", &x1,
-                        "y1", &y1))
+    if (!gjs_parse_call_args(context, "LinearGradient", argv, "ffff",
+                             "x0", &x0,
+                             "y0", &y0,
+                             "x1", &x1,
+                             "y1", &y1))
         return false;
 
     pattern = cairo_pattern_create_linear(x0, y0, x1, y1);