add optional 2nd stage initialization to -object/object-add commands

Introduces USER_CREATABLE interface that must be implemented by
objects which are designed to created with -object CLI option or
object-add QMP command.

Interface provides an ability to do an optional second stage
initialization of the object created with -object/object-add
commands. By providing complete() callback, which is called
after the object properties were set.

It allows to:
 * prevents misusing of -object/object-add by filtering out
   objects that are not designed for it.
 * generalize second stage backend initialization instead of
   adding custom APIs to perform it
 * early error detection of backend initialization at -object/
   object-add time rather than through a proxy DEVICE object
   that tries to use backend.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

backends/rng.c

@@ -12,6 +12,7 @@
 
 #include "sysemu/rng.h"
 #include "qapi/qmp/qerror.h"
+#include "qom/object_interfaces.h"
 
 void rng_backend_request_entropy(RngBackend *s, size_t size,
                                  EntropyReceiveFunc *receive_entropy,
@@ -83,6 +84,10 @@ static const TypeInfo rng_backend_info = {
     .instance_init = rng_backend_init,
     .class_size = sizeof(RngBackendClass),
     .abstract = true,
+    .interfaces = (InterfaceInfo[]) {
+        { TYPE_USER_CREATABLE },
+        { }
+    }
 };
 
 static void register_types(void)

include/qom/object_interfaces.h

@@ -0,0 +1,62 @@
+#ifndef OBJECT_INTERFACES_H
+#define OBJECT_INTERFACES_H
+
+#include "qom/object.h"
+
+#define TYPE_USER_CREATABLE "user-creatable"
+
+#define USER_CREATABLE_CLASS(klass) \
+     OBJECT_CLASS_CHECK(UserCreatableClass, (klass), \
+                        TYPE_USER_CREATABLE)
+#define USER_CREATABLE_GET_CLASS(obj) \
+     OBJECT_GET_CLASS(UserCreatableClass, (obj), \
+                      TYPE_USER_CREATABLE)
+#define USER_CREATABLE(obj) \
+     INTERFACE_CHECK(UserCreatable, (obj), \
+                     TYPE_USER_CREATABLE)
+
+
+typedef struct UserCreatable {
+    /* <private> */
+    Object Parent;
+} UserCreatable;
+
+/**
+ * UserCreatableClass:
+ * @parent_class: the base class
+ * @complete: callback to be called after @obj's properties are set.
+ *
+ * Interface is designed to work with -object/object-add/object_add
+ * commands.
+ * Interface is mandatory for objects that are designed to be user
+ * creatable (i.e. -object/object-add/object_add, will accept only
+ * objects that inherit this interface).
+ *
+ * Interface also provides an optional ability to do the second
+ * stage * initialization of the object after its properties were
+ * set.
+ *
+ * For objects created without using -object/object-add/object_add,
+ * @user_creatable_complete() wrapper should be called manually if
+ * object's type implements USER_CREATABLE interface and needs
+ * complete() callback to be called.
+ */
+typedef struct UserCreatableClass {
+    /* <private> */
+    InterfaceClass parent_class;
+
+    /* <public> */
+    void (*complete)(UserCreatable *uc, Error **errp);
+} UserCreatableClass;
+
+/**
+ * user_creatable_complete:
+ * @obj: the object whose complete() method is called if defined
+ * @errp: if an error occurs, a pointer to an area to store the error
+ *
+ * Wrapper to call complete() method if one of types it's inherited
+ * from implements USER_CREATABLE interface, otherwise the call does
+ * nothing.
+ */
+void user_creatable_complete(Object *obj, Error **errp);
+#endif

qmp.c

@@ -27,6 +27,7 @@
 #include "qapi/qmp/qobject.h"
 #include "qapi/qmp-input-visitor.h"
 #include "hw/boards.h"
+#include "qom/object_interfaces.h"
 
 NameInfo *qmp_query_name(Error **errp)
 {
@@ -554,6 +555,17 @@ void object_add(const char *type, const char *id, const QDict *qdict,
         }
     }
 
+    if (!object_dynamic_cast(obj, TYPE_USER_CREATABLE)) {
+        error_setg(&local_err, "object '%s' isn't supported by object-add",
+                   id);
+        goto out;
+    }
+
+    user_creatable_complete(obj, &local_err);
+    if (local_err) {
+        goto out;
+    }
+
     object_property_add_child(container_get(object_get_root(), "/objects"),
                               id, obj, &local_err);
 out:

qom/Makefile.objs

@@ -1,2 +1,3 @@
 common-obj-y = object.o container.o qom-qobject.o
 common-obj-y += cpu.o
+common-obj-y += object_interfaces.o

qom/object_interfaces.c

@@ -0,0 +1,32 @@
+#include "qom/object_interfaces.h"
+#include "qemu/module.h"
+
+void user_creatable_complete(Object *obj, Error **errp)
+{
+
+    UserCreatableClass *ucc;
+    UserCreatable *uc =
+        (UserCreatable *)object_dynamic_cast(obj, TYPE_USER_CREATABLE);
+
+    if (!uc) {
+        return;
+    }
+
+    ucc = USER_CREATABLE_GET_CLASS(uc);
+    if (ucc->complete) {
+        ucc->complete(uc, errp);
+    }
+}
+
+static void register_types(void)
+{
+    static const TypeInfo uc_interface_info = {
+        .name          = TYPE_USER_CREATABLE,
+        .parent        = TYPE_INTERFACE,
+        .class_size = sizeof(UserCreatableClass),
+    };
+
+    type_register_static(&uc_interface_info);
+}
+
+type_init(register_types)

vl.c

@@ -170,6 +170,7 @@ int main(int argc, char **argv)
 
 #include "ui/qemu-spice.h"
 #include "qapi/string-input-visitor.h"
+#include "qom/object_interfaces.h"
 
 //#define DEBUG_NET
 //#define DEBUG_SLIRP
@@ -2816,9 +2817,21 @@ static int object_create(QemuOpts *opts, void *opaque)
         return -1;
     }
 
+    if (!object_dynamic_cast(obj, TYPE_USER_CREATABLE)) {
+        error_setg(&local_err, "object '%s' isn't supported by -object",
+                   id);
+        goto out;
+    }
+
+    user_creatable_complete(obj, &local_err);
+    if (local_err) {
+        goto out;
+    }
+
     object_property_add_child(container_get(object_get_root(), "/objects"),
                               id, obj, &local_err);
 
+out:
     object_unref(obj);
     if (local_err) {
         qerror_report_err(local_err);