Add length accessor for GRefString

Since we store the size of the allocation in the underlying ArcBox, we
can get a constant time getter for the length of the string.

docs/reference/glib/glib-sections.txt

@@ -3499,4 +3499,5 @@ g_ref_string_new
 g_ref_string_new_intern
 g_ref_string_acquire
 g_ref_string_release
+g_ref_string_length
 </SECTION>

glib/grefstring.c

@@ -173,3 +173,21 @@ g_ref_string_release (char *str)
 
   g_arc_box_release_full (str, remove_if_interned);
 }
+
+/**
+ * g_ref_string_length:
+ * @str: a reference counted string
+ *
+ * Retrieves the length of @str.
+ *
+ * Returns: the length of the given string, in bytes
+ *
+ * Since: 2.58
+ */
+gsize
+g_ref_string_length (char *str)
+{
+  g_return_val_if_fail (str != NULL && *str != '\0', 0);
+
+  return g_arc_box_get_size (str) - 1;
+}

glib/grefstring.h

@@ -33,6 +33,9 @@ char *  g_ref_string_acquire    (char       *str);
 GLIB_AVAILABLE_IN_2_58
 void    g_ref_string_release    (char       *str);
 
+GLIB_AVAILABLE_IN_2_58
+gsize   g_ref_string_length     (char       *str);
+
 typedef char GRefString;
 
 G_END_DECLS

glib/tests/refstring.c

@@ -28,6 +28,7 @@ test_refstring_base (void)
   g_test_message ("s = '%s' (%p)", s, s);
   g_assert_cmpint (strcmp (s, "hello, world"), ==, 0);
   g_assert_cmpint (strlen (s), ==, strlen ("hello, world"));
+  g_assert_cmpuint (g_ref_string_length (s), ==, strlen ("hello, world"));
 
   g_ref_string_release (s);
 }