glsl2: Add foreach_list_safe which works even when mutating the list.

In particular, with foreach_list_safe, one can remove and free the current
node without crashes; if new nodes are added after the current node,
they will be properly visited as well.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/list.h

@@ -421,6 +421,20 @@ struct exec_list {
 #endif
 };
 
+/**
+ * This version is safe even if the current node is removed.  If you insert
+ * new nodes before the current node, they will be processed next.
+ *
+ * \note
+ * The extra test for \c __node being \c NULL is required because after the
+ * iteration \c __prev coupld be the last node in the list.  The loop increment
+ * then causes \c __prev to point to the sentinal and \c __node to be \c NULL.
+ */ 
+#define foreach_list_safe(__node, __list)		\
+   for (exec_node * __prev = (exec_node *) (__list), * __node = (__list)->head \
+        ; __node != NULL && (__node)->next != NULL	\
+	; __prev = (__prev)->next, __node = (__prev)->next)
+
 #define foreach_list(__node, __list)			\
    for (exec_node * __node = (__list)->head		\
 	; (__node)->next != NULL 			\