rbd: record overall image request result

If any image object request produces a non-zero result, preserve
that as the result of the overall image request.  If multiple
objects have non-zero results, save only the first one.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -214,6 +214,7 @@ struct rbd_img_request {
 	spinlock_t		completion_lock;/* protects next_completion */
 	u32			next_completion;
 	rbd_img_callback_t	callback;
+	int			result;	/* first nonzero obj_request result */
 
 	u32			obj_request_count;
 	struct list_head	obj_requests;	/* rbd_obj_request structs */
@@ -1488,6 +1489,7 @@ static struct rbd_img_request *rbd_img_request_create(
 	spin_lock_init(&img_request->completion_lock);
 	img_request->next_completion = 0;
 	img_request->callback = NULL;
+	img_request->result = 0;
 	img_request->obj_request_count = 0;
 	INIT_LIST_HEAD(&img_request->obj_requests);
 	kref_init(&img_request->kref);
@@ -1552,13 +1554,16 @@ static void rbd_img_obj_callback(struct rbd_obj_request *obj_request)
 		if (!obj_request_done_test(obj_request))
 			break;
 
-		rbd_assert(obj_request->xferred <= (u64) UINT_MAX);
-		xferred = (unsigned int) obj_request->xferred;
-		result = (int) obj_request->result;
-		if (result)
+		rbd_assert(obj_request->xferred <= (u64)UINT_MAX);
+		xferred = (unsigned int)obj_request->xferred;
+		result = obj_request->result;
+		if (result) {
 			rbd_warn(NULL, "obj_request %s result %d xferred %u\n",
 				img_request->write_request ? "write" : "read",
 				result, xferred);
+			if (!img_request->result)
+				img_request->result = result;
+		}
 
 		more = blk_end_request(img_request->rq, result, xferred);
 		which++;