qapi: Introduce a first class 'null' type

I expect the 'null' type to be useful mostly for members of alternate
types.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/devel/qapi-code-gen.txt

@@ -282,6 +282,7 @@ The following types are predefined, and map to C as follows:
   size      uint64_t   like uint64_t, except StringInputVisitor
                        accepts size suffixes
   bool      bool       JSON true or false
+  null      QNull *    JSON null
   any       QObject *  any JSON value
   QType     QType      JSON string matching enum QType values
 
@@ -536,10 +537,11 @@ can only express a choice between types represented differently in
 JSON.  If a branch is typed as the 'bool' built-in, the alternate
 accepts true and false; if it is typed as any of the various numeric
 built-ins, it accepts a JSON number; if it is typed as a 'str'
-built-in or named enum type, it accepts a JSON string; and if it is
-typed as a complex type (struct or union), it accepts a JSON object.
-Two different complex types, for instance, aren't permitted, because
-both are represented as a JSON object.
+built-in or named enum type, it accepts a JSON string; if it is typed
+as the 'null' built-in, it accepts JSON null; and if it is typed as a
+complex type (struct or union), it accepts a JSON object.  Two
+different complex types, for instance, aren't permitted, because both
+are represented as a JSON object.
 
 The example alternate declaration above allows using both of the
 following example objects:

include/qapi/qmp/qobject.h

@@ -93,9 +93,9 @@ static inline QType qobject_type(const QObject *obj)
     return obj->type;
 }
 
-typedef struct QNull {
+struct QNull {
     QObject base;
-} QNull;
+};
 
 extern QNull qnull_;
 

include/qemu/typedefs.h

@@ -89,6 +89,7 @@ typedef struct QEMUSGList QEMUSGList;
 typedef struct QEMUTimer QEMUTimer;
 typedef struct QEMUTimerListGroup QEMUTimerListGroup;
 typedef struct QObject QObject;
+typedef struct QNull QNull;
 typedef struct RAMBlock RAMBlock;
 typedef struct Range Range;
 typedef struct SerialState SerialState;

scripts/qapi.py

@@ -20,6 +20,7 @@
 from ordereddict import OrderedDict
 
 builtin_types = {
+    'null':     'QTYPE_QNULL',
     'str':      'QTYPE_QSTRING',
     'int':      'QTYPE_QNUM',
     'number':   'QTYPE_QNUM',
@@ -1056,6 +1057,7 @@ def json_type(self):
 
     def alternate_qtype(self):
         json2qtype = {
+            'null':    'QTYPE_QNULL',
             'string':  'QTYPE_QSTRING',
             'number':  'QTYPE_QNUM',
             'int':     'QTYPE_QNUM',
@@ -1515,7 +1517,8 @@ def _def_predefineds(self):
                   ('uint64', 'int',     'uint64_t'),
                   ('size',   'int',     'uint64_t'),
                   ('bool',   'boolean', 'bool'),
-                  ('any',    'value',   'QObject' + pointer_suffix)]:
+                  ('any',    'value',   'QObject' + pointer_suffix),
+                  ('null',   'null',    'QNull' + pointer_suffix)]:
             self._def_builtin_type(*t)
         self.the_empty_object_type = QAPISchemaObjectType(
             'q_empty', None, None, None, [], None)

tests/qapi-schema/qapi-schema-test.json

@@ -93,7 +93,8 @@
 { 'struct': 'WrapAlternate',
   'data': { 'alt': 'UserDefAlternate' } }
 { 'alternate': 'UserDefAlternate',
-  'data': { 'udfu': 'UserDefFlatUnion', 'e': 'EnumOne', 'i': 'int' } }
+  'data': { 'udfu': 'UserDefFlatUnion', 'e': 'EnumOne', 'i': 'int',
+            'n': 'null' } }
 
 { 'struct': 'UserDefC',
   'data': { 'string1': 'str', 'string2': 'str' } }

tests/qapi-schema/qapi-schema-test.out

@@ -64,6 +64,7 @@ alternate UserDefAlternate
     case udfu: UserDefFlatUnion
     case e: EnumOne
     case i: int
+    case n: null
 object UserDefB
     member intb: int optional=False
     member a-b: bool optional=True

tests/test-qobject-input-visitor.c

@@ -583,6 +583,11 @@ static void test_visitor_in_alternate(TestInputVisitorData *data,
     g_assert_cmpint(tmp->u.e, ==, ENUM_ONE_VALUE1);
     qapi_free_UserDefAlternate(tmp);
 
+    v = visitor_input_test_init(data, "null");
+    visit_type_UserDefAlternate(v, NULL, &tmp, &error_abort);
+    g_assert_cmpint(tmp->type, ==, QTYPE_QNULL);
+    qapi_free_UserDefAlternate(tmp);
+
     v = visitor_input_test_init(data, "{'integer':1, 'string':'str', "
                                 "'enum1':'value1', 'boolean':true}");
     visit_type_UserDefAlternate(v, NULL, &tmp, &error_abort);

tests/test-qobject-output-visitor.c

@@ -422,6 +422,16 @@ static void test_visitor_out_alternate(TestOutputVisitorData *data,
 
     qapi_free_UserDefAlternate(tmp);
 
+    visitor_reset(data);
+    tmp = g_new0(UserDefAlternate, 1);
+    tmp->type = QTYPE_QNULL;
+    tmp->u.n = qnull();
+
+    visit_type_UserDefAlternate(data->ov, NULL, &tmp, &error_abort);
+    g_assert_cmpint(qobject_type(visitor_get(data)), ==, QTYPE_QNULL);
+
+    qapi_free_UserDefAlternate(tmp);
+
     visitor_reset(data);
     tmp = g_new0(UserDefAlternate, 1);
     tmp->type = QTYPE_QDICT;