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.

Backports commit 481b002cc81ed7fc7b06e32e9d4d495d81739d14 from qemu

msvc/unicorn/unicorn/unicorn.vcxproj

@@ -211,6 +211,7 @@ copy $(SolutionDir)..\include\unicorn\*.h $(SolutionDir)distro\include\unicorn\
     <ClCompile Include="..\..\..\qemu\qobject\qfloat.c" />
     <ClCompile Include="..\..\..\qemu\qobject\qint.c" />
     <ClCompile Include="..\..\..\qemu\qobject\qlist.c" />
+    <ClCompile Include="..\..\..\qemu\qobject\qnull.c" />
     <ClCompile Include="..\..\..\qemu\qobject\qstring.c" />
     <ClCompile Include="..\..\..\qemu\qom\container.c" />
     <ClCompile Include="..\..\..\qemu\qom\cpu.c" />

msvc/unicorn/unicorn/unicorn.vcxproj.filters

@@ -98,6 +98,9 @@
     <ClCompile Include="..\..\..\qemu\qobject\qlist.c">
       <Filter>qemu\qobject</Filter>
     </ClCompile>
+    <ClCompile Include="..\..\..\qemu\qobject\qnull.c">
+      <Filter>qemu\qobject</Filter>
+    </ClCompile>
     <ClCompile Include="..\..\..\qemu\qobject\qstring.c">
       <Filter>qemu\qobject</Filter>
     </ClCompile>

msvc/unicorn/unicorn_static/unicorn_static.vcxproj

@@ -39,6 +39,7 @@
     <ClCompile Include="..\..\..\qemu\qobject\qfloat.c" />
     <ClCompile Include="..\..\..\qemu\qobject\qint.c" />
     <ClCompile Include="..\..\..\qemu\qobject\qlist.c" />
+    <ClCompile Include="..\..\..\qemu\qobject\qnull.c" />
     <ClCompile Include="..\..\..\qemu\qobject\qstring.c" />
     <ClCompile Include="..\..\..\qemu\qom\container.c" />
     <ClCompile Include="..\..\..\qemu\qom\cpu.c" />

msvc/unicorn/unicorn_static/unicorn_static.vcxproj.filters

@@ -132,6 +132,9 @@
     <ClCompile Include="..\..\..\qemu\qobject\qlist.c">
       <Filter>qemu\qobject</Filter>
     </ClCompile>
+    <ClCompile Include="..\..\..\qemu\qobject\qnull.c">
+      <Filter>qemu\qobject</Filter>
+    </ClCompile>
     <ClCompile Include="..\..\..\qemu\qobject\qstring.c">
       <Filter>qemu\qobject</Filter>
     </ClCompile>

qemu/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,
+    QTYPE_QNULL,
     QTYPE_QINT,
     QTYPE_QSTRING,
     QTYPE_QDICT,
@@ -111,4 +112,12 @@ 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 */

qemu/qobject/Makefile.objs

@@ -1 +1 @@
-util-obj-y = qint.o qstring.o qdict.o qlist.o qfloat.o qbool.o
+util-obj-y = qint.o qnull.o qstring.o qdict.o qlist.o qfloat.o qbool.o

qemu/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 = {
+    QTYPE_QNULL,
+    qnull_destroy_obj,
+};
+
+QObject qnull_ = {
+    &qnull_type,
+    1,
+};