ARRAY_SIZE: check that argument is an array

It's a familiar pattern: some code uses ARRAY_SIZE, then refactoring
changes the argument from an array to a pointer to a dynamically
allocated buffer.  Code keeps compiling but any ARRAY_SIZE calls now
return the size of the pointer divided by element size.

Let's add build time checks to ARRAY_SIZE before we allow more
of these in the code-base.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/qemu/osdep.h

@@ -198,8 +198,15 @@ extern int daemon(int, int);
 #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
 #endif
 
+/*
+ * &(x)[0] is always a pointer - if it's same type as x then the argument is a
+ * pointer, not an array.
+ */
+#define QEMU_IS_ARRAY(x) (!__builtin_types_compatible_p(typeof(x), \
+                                                        typeof(&(x)[0])))
 #ifndef ARRAY_SIZE
-#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+#define ARRAY_SIZE(x) ((sizeof(x) / sizeof((x)[0])) + \
+                       QEMU_BUILD_BUG_ON_ZERO(!QEMU_IS_ARRAY(x)))
 #endif
 
 int qemu_daemon(int nochdir, int noclose);