qobject: Add a special null QObject

I'm going to fix the JSON parser to recognize null.  The obvious
representation of JSON null as (QObject *)NULL doesn't work, because
the parser already uses it as an error value.  Perhaps we should
change it to free NULL for null, but that's more than I can do right
now.  Create a special null QObject instead.

The existing QDict, QList, and QString all represent something that
is a pointer in C and could therefore be associated with NULL.  But
right now, all three of these sub-types are always non-null once
created, so the new null sentinel object is intentionally unrelated
to them.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

include/qapi/qmp/qobject.h

@@ -3,7 +3,7 @@
  *
  * Based on ideas by Avi Kivity <avi@redhat.com>
  *
- * Copyright (C) 2009 Red Hat Inc.
+ * Copyright (C) 2009, 2015 Red Hat Inc.
  *
  * Authors:
  *  Luiz Capitulino <lcapitulino@redhat.com>
@@ -37,6 +37,7 @@
 
 typedef enum {
     QTYPE_NONE,    /* sentinel value, no QObject has this type code */
+    QTYPE_QNULL,
     QTYPE_QINT,
     QTYPE_QSTRING,
     QTYPE_QDICT,
@@ -110,4 +111,12 @@ static inline qtype_code qobject_type(const QObject *obj)
     return obj->type->code;
 }
 
+extern QObject qnull_;
+
+static inline QObject *qnull(void)
+{
+    qobject_incref(&qnull_);
+    return &qnull_;
+}
+
 #endif /* QOBJECT_H */

qobject/Makefile.objs

@@ -1,3 +1,3 @@
-util-obj-y = qint.o qstring.o qdict.o qlist.o qfloat.o qbool.o
+util-obj-y = qnull.o qint.o qstring.o qdict.o qlist.o qfloat.o qbool.o
 util-obj-y += qjson.o json-lexer.o json-streamer.o json-parser.o
 util-obj-y += qerror.o

qobject/qjson.c

@@ -127,6 +127,9 @@ static void to_json_list_iter(QObject *obj, void *opaque)
 static void to_json(const QObject *obj, QString *str, int pretty, int indent)
 {
     switch (qobject_type(obj)) {
+    case QTYPE_QNULL:
+        qstring_append(str, "null");
+        break;
     case QTYPE_QINT: {
         QInt *val = qobject_to_qint(obj);
         char buffer[1024];

qobject/qnull.c

@@ -0,0 +1,29 @@
+/*
+ * QNull
+ *
+ * Copyright (C) 2015 Red Hat, Inc.
+ *
+ * Authors:
+ *  Markus Armbruster <armbru@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2.1
+ * or later.  See the COPYING.LIB file in the top-level directory.
+ */
+
+#include "qemu-common.h"
+#include "qapi/qmp/qobject.h"
+
+static void qnull_destroy_obj(QObject *obj)
+{
+    assert(0);
+}
+
+static const QType qnull_type = {
+    .code = QTYPE_QNULL,
+    .destroy = qnull_destroy_obj,
+};
+
+QObject qnull_ = {
+    .type = &qnull_type,
+    .refcnt = 1,
+};