Peter Xu <pet...@redhat.com> writes:
> Signed-off-by: Peter Xu <pet...@redhat.com>
> ---
> include/qom/object_interfaces.h | 47 +++++++++++++++++++++++++++++++++
> qom/object.c | 3 +++
> qom/object_interfaces.c | 24 +++++++++++++++++
> qom/qom-qmp-cmds.c | 22 ++++++++++++---
> system/qdev-monitor.c | 7 +++++
> 5 files changed, 100 insertions(+), 3 deletions(-)
>
> diff --git a/include/qom/object_interfaces.h b/include/qom/object_interfaces.h
> index 02b11a7ef0..9b2cc0e554 100644
> --- a/include/qom/object_interfaces.h
> +++ b/include/qom/object_interfaces.h
> @@ -177,4 +177,51 @@ bool user_creatable_del(const char *id, Error **errp);
> */
> void user_creatable_cleanup(void);
>
> +#define TYPE_SINGLETON "singleton"
> +
> +typedef struct SingletonClass SingletonClass;
> +DECLARE_CLASS_CHECKERS(SingletonClass, SINGLETON, TYPE_SINGLETON)
> +
> +/**
> + * SingletonClass:
> + *
> + * @parent_class: the base class
> + * @get_instance: fetch the singleton instance if it is created,
> + * NULL otherwise.
> + *
> + * Singleton class describes the type of object classes that can only
> + * provide one instance for the whole lifecycle of QEMU. It will fail the
> + * operation if one attemps to create more than one instance.
> + *
> + * One can fetch the single object using class's get_instance() callback if
> + * it was created before. This can be useful for operations like QMP
> + * qom-list-properties, where dynamically creating an object might not be
> + * feasible.
> + */
> +struct SingletonClass {
> + /* <private> */
> + InterfaceClass parent_class;
> + /* <public> */
> + Object *(*get_instance)(Error **errp);
> +};
> +
> +/**
> + * object_class_is_singleton:
> + *
> + * @class: the class to detect singleton
> + *
> + * Returns: true if it's a singleton class, false otherwise.
> + */
> +bool object_class_is_singleton(ObjectClass *class);
> +
> +/**
> + * singleton_get_instance:
> + *
> + * @class: the class to fetch singleton instance
> + *
> + * Returns: the object* if the class is a singleton class and the singleton
> + * object is created, NULL otherwise.
> + */
> +Object *singleton_get_instance(ObjectClass *class);
A non-null return value can become dangling when the object gets
destroyed. This could conceivably happen in another thread.
The obviously safe interface would take a reference the caller must
unref when done.
> +
> #endif
> diff --git a/qom/object.c b/qom/object.c
> index 11424cf471..ded299ae1a 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -553,6 +553,9 @@ static void object_initialize_with_type(Object *obj,
> size_t size, TypeImpl *type
> g_assert(type->abstract == false);
> g_assert(size >= type->instance_size);
>
> + /* Singleton class can only create one object */
> + g_assert(!singleton_get_instance(type->class));
> +
> memset(obj, 0, type->instance_size);
> obj->class = type->class;
> object_ref(obj);
> diff --git a/qom/object_interfaces.c b/qom/object_interfaces.c
> index e0833c8bfe..6766060d0a 100644
> --- a/qom/object_interfaces.c
> +++ b/qom/object_interfaces.c
> @@ -354,6 +354,23 @@ void user_creatable_cleanup(void)
> object_unparent(object_get_objects_root());
> }
>
> +bool object_class_is_singleton(ObjectClass *class)
> +{
> + return !!object_class_dynamic_cast(class, TYPE_SINGLETON);
> +}
> +
> +Object *singleton_get_instance(ObjectClass *class)
> +{
> + SingletonClass *singleton =
> + (SingletonClass *)object_class_dynamic_cast(class, TYPE_SINGLETON);
> +
> + if (!singleton) {
> + return NULL;
> + }
> +
> + return singleton->get_instance(&error_abort);
> +}
> +
> static void register_types(void)
> {
> static const TypeInfo uc_interface_info = {
> @@ -362,7 +379,14 @@ static void register_types(void)
> .class_size = sizeof(UserCreatableClass),
> };
>
> + static const TypeInfo singleton_interface_info = {
> + .name = TYPE_SINGLETON,
> + .parent = TYPE_INTERFACE,
> + .class_size = sizeof(SingletonClass),
> + };
> +
> type_register_static(&uc_interface_info);
> + type_register_static(&singleton_interface_info);
> }
>
> type_init(register_types)
> diff --git a/qom/qom-qmp-cmds.c b/qom/qom-qmp-cmds.c
> index e91a235347..ecc1cf781c 100644
> --- a/qom/qom-qmp-cmds.c
> +++ b/qom/qom-qmp-cmds.c
> @@ -126,6 +126,7 @@ ObjectPropertyInfoList *qmp_device_list_properties(const
> char *typename,
> ObjectProperty *prop;
> ObjectPropertyIterator iter;
> ObjectPropertyInfoList *prop_list = NULL;
> + bool create;
>
> klass = module_object_class_by_name(typename);
> if (klass == NULL) {
> @@ -141,7 +142,15 @@ ObjectPropertyInfoList *qmp_device_list_properties(const
> char *typename,
> return NULL;
> }
>
> - obj = object_new(typename);
> + /* Avoid creating multiple instances if the class is a singleton */
> + create = !object_class_is_singleton(klass) ||
> + !singleton_get_instance(klass);
> +
> + if (create) {
> + obj = object_new(typename);
If the class is not a singleton or else if no instance exists, we create
a temporary instance.
> + } else {
> + obj = singleton_get_instance(klass);
If the class is a singleton and the instance exists, we use that
instead.
Any properties the instance has created dynamically after object_new()
are visible to introspection. This is not the case when we create a
temporary instance. Such subtle differences are problematic. If we
decide we're okay with this one, we need to document it.
> + }
>
> object_property_iter_init(&iter, obj);
> while ((prop = object_property_iter_next(&iter))) {
> @@ -172,7 +181,9 @@ ObjectPropertyInfoList *qmp_device_list_properties(const
> char *typename,
> QAPI_LIST_PREPEND(prop_list, info);
> }
>
> - object_unref(obj);
> + if (create) {
> + object_unref(obj);
> + }
>
> return prop_list;
> }
> @@ -199,7 +210,12 @@ ObjectPropertyInfoList *qmp_qom_list_properties(const
> char *typename,
> return NULL;
> }
>
> - if (object_class_is_abstract(klass)) {
> + /*
> + * Abstract classes are not for instantiations, meanwhile avoid
> + * creating temporary singleton objects because it can cause conflicts
> + * if there's already one created.
> + */
> + if (object_class_is_abstract(klass) || object_class_is_singleton(klass))
> {
> object_class_property_iter_init(&iter, klass);
> } else {
> obj = object_new(typename);
If the class is a singleton, we treat it as if it was abstract. Its
instance properties, if any, are not visible in qom-list-properties.
This is a defect. If you make an existing class with instance
properties a singleton, the defect is a regression.
> diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
> index 44994ea0e1..1310f35c9f 100644
> --- a/system/qdev-monitor.c
> +++ b/system/qdev-monitor.c
> @@ -36,6 +36,7 @@
> #include "qemu/option.h"
> #include "qemu/qemu-print.h"
> #include "qemu/option_int.h"
> +#include "qom/object_interfaces.h"
> #include "sysemu/block-backend.h"
> #include "migration/misc.h"
> #include "qemu/cutils.h"
> @@ -643,6 +644,12 @@ DeviceState *qdev_device_add_from_qdict(const QDict
> *opts,
> return NULL;
> }
>
> + if (singleton_get_instance(OBJECT_CLASS(dc))) {
> + error_setg(errp, "Class '%s' only supports one instance",
> + driver);
> + return NULL;
> + }
> +
> /* find bus */
> path = qdict_get_try_str(opts, "bus");
> if (path != NULL) {
