Add auto subscribe support

Add a new property to ubus_subscriber called new_obj_cb. It gets called
everytime a new object appears on the bus. If the callback returns true,
libubus will automatically subscribe to the notifications.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

libubus-sub.c

@@ -31,13 +31,66 @@ const struct ubus_method watch_method __hidden = {
 	.handler = ubus_subscriber_cb,
 };
 
+static void
+ubus_auto_sub_event_handler_cb(struct ubus_context *ctx,  struct ubus_event_handler *ev,
+			       const char *type, struct blob_attr *msg)
+{
+	enum {
+		EVENT_ID,
+		EVENT_PATH,
+		__EVENT_MAX
+	};
+
+	static const struct blobmsg_policy event_policy[__EVENT_MAX] = {
+		[EVENT_ID] = { .name = "id", .type = BLOBMSG_TYPE_INT32 },
+		[EVENT_PATH] = { .name = "path", .type = BLOBMSG_TYPE_STRING },
+	};
+
+	struct blob_attr *tb[__EVENT_MAX];
+	struct ubus_subscriber *s;
+	const char *path;
+	int id;
+
+	blobmsg_parse(event_policy, __EVENT_MAX, tb, blob_data(msg), blob_len(msg));
+
+	if (!tb[EVENT_ID] || !tb[EVENT_PATH])
+		return;
+
+	path = blobmsg_get_string(tb[EVENT_PATH]);
+	id = blobmsg_get_u32(tb[EVENT_ID]);
+
+	list_for_each_entry(s, &ctx->auto_subscribers, list)
+		if (s->new_obj_cb(ctx, s, path))
+			ubus_subscribe(ctx, s, id);
+}
+
+static void
+ubus_auto_sub_lookup(struct ubus_context *ctx, struct ubus_object_data *obj,
+		     void *priv)
+{
+	struct ubus_subscriber *s = priv;
+
+	if (s->new_obj_cb(ctx, s, obj->path))
+		ubus_subscribe(ctx, s, obj->id);
+}
+
 int ubus_register_subscriber(struct ubus_context *ctx, struct ubus_subscriber *s)
 {
 	struct ubus_object *obj = &s->obj;
 
+	INIT_LIST_HEAD(&s->list);
 	obj->methods = &watch_method;
 	obj->n_methods = 1;
 
+	if (s->new_obj_cb) {
+		struct ubus_event_handler *ev = &ctx->auto_subscribe_event_handler;
+		list_add(&s->list, &ctx->auto_subscribers);
+		ev->cb = ubus_auto_sub_event_handler_cb;
+		if (!ev->obj.id)
+			ubus_register_event_handler(ctx, ev, "ubus.object.add");
+		ubus_lookup(ctx, NULL, ubus_auto_sub_lookup, s);
+	}
+
 	return ubus_add_object(ctx, obj);
 }
 

libubus.c

@@ -298,6 +298,7 @@ int ubus_connect_ctx(struct ubus_context *ctx, const char *path)
 
 	INIT_LIST_HEAD(&ctx->requests);
 	INIT_LIST_HEAD(&ctx->pending);
+	INIT_LIST_HEAD(&ctx->auto_subscribers);
 	avl_init(&ctx->objects, ubus_cmp_id, false, NULL);
 	if (ubus_reconnect(ctx, path)) {
 		free(ctx->msgbuf.data);

libubus.h

@@ -63,6 +63,7 @@ typedef void (*ubus_notify_complete_handler_t)(struct ubus_notify_request *req,
 typedef void (*ubus_notify_data_handler_t)(struct ubus_notify_request *req,
 					   int type, struct blob_attr *msg);
 typedef void (*ubus_connect_handler_t)(struct ubus_context *ctx);
+typedef bool (*ubus_new_object_handler_t)(struct ubus_context *ctx, struct ubus_subscriber *sub, const char *path);
 
 #define UBUS_OBJECT_TYPE(_name, _methods)		\
 	{						\
@@ -139,10 +140,12 @@ struct ubus_object {
 };
 
 struct ubus_subscriber {
+	struct list_head list;
 	struct ubus_object obj;
 
 	ubus_handler_t cb;
 	ubus_remove_handler_t remove_cb;
+	ubus_new_object_handler_t new_obj_cb;
 };
 
 struct ubus_event_handler {
@@ -170,6 +173,9 @@ struct ubus_context {
 	struct ubus_msghdr_buf msgbuf;
 	uint32_t msgbuf_data_len;
 	int msgbuf_reduction_counter;
+
+	struct list_head auto_subscribers;
+	struct ubus_event_handler auto_subscribe_event_handler;
 };
 
 struct ubus_object_data {
@@ -302,6 +308,8 @@ int ubus_register_subscriber(struct ubus_context *ctx, struct ubus_subscriber *o
 static inline int
 ubus_unregister_subscriber(struct ubus_context *ctx, struct ubus_subscriber *obj)
 {
+	if (!list_empty(&obj->list))
+		list_del_init(&obj->list);
 	return ubus_remove_object(ctx, &obj->obj);
 }