include: list.h: add xorg_list_present()

This function walks through the list and checks whether an entry is already
present. This check sometimes is neccessary, since trying to add an entry
twice has catastrophic consequences: iteration will become endless loop.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

include/list.h

@@ -234,6 +234,23 @@ xorg_list_is_empty(struct xorg_list *head)
     return ((head->next == NULL) || (head->next == head));
 }
 
+/**
+ * @brief check whether element already is in list
+ *
+ * @param entry The element to check for
+ * @param head The existing list.
+ * @return zero when entry isn't present in list, otherwise non-zero
+ */
+static inline int
+xorg_list_present(struct xorg_list *entry, struct xorg_list *head)
+{
+    for (struct xorg_list *l=head->next; l && (l != head); l=l->next) {
+        if (l == entry)
+            return 1;
+    }
+    return 0;
+}
+
 /**
  * Returns a pointer to the container of this list element.
  *