qapi: Use QNull for a more regular visit_type_null()

Make visit_type_null() take an @obj argument like its buddies. This
helps keep the next commit simple.

Backports commit d2f95f4d482374485234790a6fc3cca29ebb7355 from qemu

qemu/include/qapi/visitor-impl.h

@@ -104,7 +104,8 @@ struct Visitor
                      Error **errp);
 
     /* Must be set to visit explicit null values.  */
-    void (*type_null)(Visitor *v, const char *name, Error **errp);
+    void (*type_null)(Visitor *v, const char *name, QNull **obj,
+                      Error **errp);
 
     /* Must be set for input visitors to visit structs, optional otherwise.
        The core takes care of the return type in the public interface. */

qemu/include/qapi/visitor.h

@@ -618,10 +618,10 @@ void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp);
  * @name expresses the relationship of the null value to its parent
  * container; see the general description of @name above.
  *
- * Unlike all other visit_type_* functions, no obj parameter is
- * needed; rather, this is a witness that an explicit null value is
- * expected rather than any other type.
+ * @obj must be non-NULL.  Input visitors set *@obj to the value;
+ * other visitors ignore *@obj.
  */
-void visit_type_null(Visitor *v, const char *name, Error **errp);
+void visit_type_null(Visitor *v, const char *name, QNull **obj,
+                     Error **errp);
 
 #endif

qemu/qapi/qapi-clone-visitor.c

@@ -127,12 +127,13 @@ static void qapi_clone_type_number(Visitor *v, const char *name, double *obj,
     /* Value was already cloned by g_memdup() */
 }
 
-static void qapi_clone_type_null(Visitor *v, const char *name, Error **errp)
+static void qapi_clone_type_null(Visitor *v, const char *name, QNull **obj,
+                                 Error **errp)
 {
     QapiCloneVisitor *qcv = to_qcv(v);
 
     assert(qcv->depth);
-    /* Nothing to do */
+    *obj = qnull();
 }
 
 static void qapi_clone_free(Visitor *v)

qemu/qapi/qapi-dealloc-visitor.c

@@ -103,8 +103,12 @@ static void qapi_dealloc_type_anything(Visitor *v, const char *name,
     }
 }
 
-static void qapi_dealloc_type_null(Visitor *v, const char *name, Error **errp)
+static void qapi_dealloc_type_null(Visitor *v, const char *name,
+                                   QNull **obj, Error **errp)
 {
+    if (obj) {
+        QDECREF(*obj);
+    }
 }
 
 static void qapi_dealloc_free(Visitor *v)

qemu/qapi/qapi-visit-core.c

@@ -280,9 +280,10 @@ void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp)
     error_propagate(errp, err);
 }
 
-void visit_type_null(Visitor *v, const char *name, Error **errp)
+void visit_type_null(Visitor *v, const char *name, QNull **obj,
+                     Error **errp)
 {
-    v->type_null(v, name, errp);
+    v->type_null(v, name, obj, errp);
 }
 
 static void output_type_enum(Visitor *v, const char *name, int *obj,

qemu/qapi/qobject-input-visitor.c

@@ -465,11 +465,13 @@ static void qobject_input_type_any(Visitor *v, const char *name, QObject **obj,
     *obj = qobj;
 }
 
-static void qobject_input_type_null(Visitor *v, const char *name, Error **errp)
+static void qobject_input_type_null(Visitor *v, const char *name,
+                                    QNull **obj, Error **errp)
 {
     QObjectInputVisitor *qiv = to_qiv(v);
     QObject *qobj = qobject_input_get_object(qiv, name, true, errp);
 
+    *obj = NULL;
     if (!qobj) {
         return;
     }
@@ -477,7 +479,10 @@ static void qobject_input_type_null(Visitor *v, const char *name, Error **errp)
     if (qobject_type(qobj) != QTYPE_QNULL) {
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE,
                    full_name(qiv, name), "null");
+        return;
     }
+
+    *obj = qnull();
 }
 
 static void qobject_input_optional(Visitor *v, const char *name, bool *present)

qemu/qapi/qobject-output-visitor.c

@@ -193,7 +193,8 @@ static void qobject_output_type_any(Visitor *v, const char *name, QObject **obj,
     qobject_output_add_obj(qov, name, *obj);
 }
 
-static void qobject_output_type_null(Visitor *v, const char *name, Error **errp)
+static void qobject_output_type_null(Visitor *v, const char *name,
+                                     QNull **obj, Error **errp)
 {
     QObjectOutputVisitor *qov = to_qov(v);
     qobject_output_add(qov, name, qnull());

qemu/qapi/string-input-visitor.c

@@ -310,14 +310,20 @@ static void parse_type_number(Visitor *v,  const char *name, double *obj,
     *obj = val;
 }
 
-static void parse_type_null(Visitor *v, const char *name, Error **errp)
+static void parse_type_null(Visitor *v, const char *name, QNull **obj,
+                            Error **errp)
 {
     StringInputVisitor *siv = to_siv(v);
 
+    *obj = NULL;
+
     if (!siv->string || siv->string[0]) {
         error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
                    "null");
+        return;
     }
+
+    *obj = qnull();
 }
 
 static void string_input_free(Visitor *v)