qapi: Add qobject_to()

This is a dynamic casting macro that, given a QObject type, returns an
object as that type or NULL if the object is of a different type (or
NULL itself).

The macro uses lower-case letters because:
1. There does not seem to be a hard rule on whether qemu macros have to
   be upper-cased,
2. The current situation in qapi/qmp is inconsistent (compare e.g.
   QINCREF() vs. qdict_put()),
3. qobject_to() will evaluate its @obj parameter only once, thus it is
   generally not important to the caller whether it is a macro or not,
4. I prefer it aesthetically.

The macro parameter order is chosen with typename first for
consistency with other QAPI macros like QAPI_CLONE(), as well as
for legibility (read it as "qobject to" type "applied to" obj).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20180224154033.29559-3-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
[eblake: swap parameter order to list type first, avoid clang ubsan
warning on QOBJECT(NULL) and container_of(NULL,type,base)]
Signed-off-by: Eric Blake <eblake@redhat.com>

include/qapi/qmp/qobject.h

@@ -50,6 +50,21 @@ struct QObject {
 #define QDECREF(obj)              \
     qobject_decref(obj ? QOBJECT(obj) : NULL)
 
+/* Required for qobject_to() */
+#define QTYPE_CAST_TO_QNull     QTYPE_QNULL
+#define QTYPE_CAST_TO_QNum      QTYPE_QNUM
+#define QTYPE_CAST_TO_QString   QTYPE_QSTRING
+#define QTYPE_CAST_TO_QDict     QTYPE_QDICT
+#define QTYPE_CAST_TO_QList     QTYPE_QLIST
+#define QTYPE_CAST_TO_QBool     QTYPE_QBOOL
+
+QEMU_BUILD_BUG_MSG(QTYPE__MAX != 7,
+                   "The QTYPE_CAST_TO_* list needs to be extended");
+
+#define qobject_to(type, obj) ({ \
+    QObject *_tmp = qobject_check_type(obj, glue(QTYPE_CAST_TO_, type)); \
+    _tmp ? container_of(_tmp, type, base) : (type *)NULL; })
+
 /* Initialize an object to default values */
 static inline void qobject_init(QObject *obj, QType type)
 {
@@ -102,4 +117,18 @@ static inline QType qobject_type(const QObject *obj)
     return obj->type;
 }
 
+/**
+ * qobject_check_type(): Helper function for the qobject_to() macro.
+ * Return @obj, but only if @obj is not NULL and @type is equal to
+ * @obj's type.  Return NULL otherwise.
+ */
+static inline QObject *qobject_check_type(const QObject *obj, QType type)
+{
+    if (obj && qobject_type(obj) == type) {
+        return (QObject *)obj;
+    } else {
+        return NULL;
+    }
+}
+
 #endif /* QOBJECT_H */