qobject: use a QObjectBase_ struct

By moving the base fields to a QObjectBase_, QObject can be a type
which also has a 'base' field. This allows writing a generic QOBJECT()
macro that will work with any QObject type, including QObject
itself. The container_of() macro ensures that the object to cast has a
QObjectBase_ base field, giving some type safety guarantees. QObject
must have no members but QObjectBase_ base, or else QOBJECT() breaks.

QObjectBase_ is not a typedef and uses a trailing underscore to make
it obvious it is not for normal use and to avoid potential abuse.

Backports commit 3d3eacaeccaab718ea0e2ddaa578bfae9e311c59 from qemu

qemu/include/qapi/qmp/qbool.h

@@ -18,7 +18,7 @@
 #include "qapi/qmp/qobject.h"
 
 struct QBool {
-    QObject_HEAD;
+    struct QObjectBase_ base;
     bool value;
 };
 

qemu/include/qapi/qmp/qdict.h

@@ -26,7 +26,7 @@ typedef struct QDictEntry {
 } QDictEntry;
 
 struct QDict {
-    QObject_HEAD;
+    struct QObjectBase_ base;
     size_t size;
     QLIST_HEAD(,QDictEntry) table[QDICT_BUCKET_MAX];
 };

qemu/include/qapi/qmp/qlist.h

@@ -22,7 +22,7 @@ typedef struct QListEntry {
 } QListEntry;
 
 struct QList {
-    QObject_HEAD;
+    struct QObjectBase_ base;
     QTAILQ_HEAD(,QListEntry) head;
 };
 

qemu/include/qapi/qmp/qnull.h

@@ -16,7 +16,7 @@
 #include "qapi/qmp/qobject.h"
 
 struct QNull {
-    QObject base;
+    struct QObjectBase_ base;
 };
 
 extern QNull qnull_;

qemu/include/qapi/qmp/qnum.h

@@ -45,7 +45,7 @@ typedef enum {
  * convert under the hood.
  */
 struct QNum {
-    QObject base;
+    struct QObjectBase_ base;
     QNumKind kind;
     union {
         int64_t i64;

qemu/include/qapi/qmp/qobject.h

@@ -34,17 +34,21 @@
 
 #include "qapi/qapi-builtin-types.h"
 
-struct QObject {
+/* Not for use outside include/qapi/qmp/ */
+struct QObjectBase_ {
     QType type;
     size_t refcnt;
 };
 
-/* Objects definitions must include this */
-#define QObject_HEAD  \
-    QObject base
+/* this struct must have no other members than base */
+struct QObject {
+    struct QObjectBase_ base;
+};
 
-/* Get the 'base' part of an object */
-#define QOBJECT(obj) (&(obj)->base)
+#define QOBJECT(obj) ({                                         \
+    typeof(obj) _obj = (obj);                                   \
+    _obj ? container_of(&(_obj)->base, QObject, base) : NULL;   \
+})
 
 /* High-level interface for qobject_incref() */
 #define QINCREF(obj)      \
@@ -72,8 +76,8 @@ QEMU_BUILD_BUG_MSG(QTYPE__MAX != 7,
 static inline void qobject_init(QObject *obj, QType type)
 {
     assert(QTYPE_NONE < type && type < QTYPE__MAX);
-    obj->refcnt = 1;
-    obj->type = type;
+    obj->base.refcnt = 1;
+    obj->base.type = type;
 }
 
 /**
@@ -81,8 +85,9 @@ static inline void qobject_init(QObject *obj, QType type)
  */
 static inline void qobject_incref(QObject *obj)
 {
-    if (obj)
-        obj->refcnt++;
+    if (obj) {
+        obj->base.refcnt++;
+    }
 }
 
 /**
@@ -105,8 +110,8 @@ void qobject_destroy(QObject *obj);
  */
 static inline void qobject_decref(QObject *obj)
 {
-    assert(!obj || obj->refcnt);
-    if (obj && --obj->refcnt == 0) {
+    assert(!obj || obj->base.refcnt);
+    if (obj && --obj->base.refcnt == 0) {
         qobject_destroy(obj);
     }
 }
@@ -116,8 +121,8 @@ static inline void qobject_decref(QObject *obj)
  */
 static inline QType qobject_type(const QObject *obj)
 {
-    assert(QTYPE_NONE < obj->type && obj->type < QTYPE__MAX);
-    return obj->type;
+    assert(QTYPE_NONE < obj->base.type && obj->base.type < QTYPE__MAX);
+    return obj->base.type;
 }
 
 /**

qemu/include/qapi/qmp/qstring.h

@@ -17,7 +17,7 @@
 #include "qapi/qmp/qobject.h"
 
 struct QString {
-    QObject_HEAD;
+    struct QObjectBase_ base;
     char *string;
     size_t length;
     size_t capacity;

qemu/qobject/qobject.c

@@ -36,9 +36,9 @@ static void (*qdestroy[QTYPE__MAX])(QObject *) = {
 
 void qobject_destroy(QObject *obj)
 {
-    assert(!obj->refcnt);
-    assert(QTYPE_QNULL < obj->type && obj->type < QTYPE__MAX);
-    qdestroy[obj->type](obj);
+    assert(!obj->base.refcnt);
+    assert(QTYPE_QNULL < obj->base.type && obj->base.type < QTYPE__MAX);
+    qdestroy[obj->base.type](obj);
 }
 
 static bool (*qis_equal[QTYPE__MAX])(const QObject *, const QObject *) = {
@@ -60,12 +60,11 @@ bool qobject_is_equal(const QObject *x, const QObject *y)
         return true;
     }
 
-    if (!x || !y || x->type != y->type) {
+    if (!x || !y || x->base.type != y->base.type) {
         return false;
     }
 
-    assert(QTYPE_NONE < x->type && x->type < QTYPE__MAX);
+    assert(QTYPE_NONE < x->base.type && x->base.type < QTYPE__MAX);
 
-    return qis_equal[x->type](x, y);
+    return qis_equal[x->base.type](x, y);
 }
-