btrfs: add assertions when deleting batches of delayed items

There are a few impossible cases that btrfs_batch_delete_items() tries to
deal with:

1) Getting a path pointing to a NULL leaf;
2) The leaf slot is pointing beyond the last item in the leaf;
3) We can't find a single item to delete.

The first case is impossible because the given path was returned by a
successful call to btrfs_search_slot(). Replace the BUG_ON() with an
ASSERT for this.

The second case is impossible because we are always called when a delayed
item matches an item in the given leaf. So add an ASSERT() for that and
if that condition is not satisfied, trigger a warning and return an error.

The third case is impossible exactly because of the same reason as the
second case. The given delayed item matches one item in the leaf, so we
know that our batch always has at least one item. Add an ASSERT to check
that, trigger a warning if that expectation fails and return an error.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/delayed-inode.c

@@ -797,20 +797,23 @@ static int btrfs_batch_delete_items(struct btrfs_trans_handle *trans,
 				    struct btrfs_delayed_item *item)
 {
 	struct btrfs_delayed_item *curr, *next;
-	struct extent_buffer *leaf;
+	struct extent_buffer *leaf = path->nodes[0];
 	struct btrfs_key key;
 	struct list_head head;
 	int nitems, i, last_item;
 	int ret = 0;
 
-	BUG_ON(!path->nodes[0]);
-
-	leaf = path->nodes[0];
+	ASSERT(leaf != NULL);
 
 	i = path->slots[0];
 	last_item = btrfs_header_nritems(leaf) - 1;
-	if (i > last_item)
-		return -ENOENT;	/* FIXME: Is errno suitable? */
+	/*
+	 * Our caller always gives us a path pointing to an existing item, so
+	 * this can not happen.
+	 */
+	ASSERT(i <= last_item);
+	if (WARN_ON(i > last_item))
+		return -ENOENT;
 
 	next = item;
 	INIT_LIST_HEAD(&head);
@@ -837,8 +840,13 @@ static int btrfs_batch_delete_items(struct btrfs_trans_handle *trans,
 		btrfs_item_key_to_cpu(leaf, &key, i);
 	}
 
-	if (!nitems)
-		return 0;
+	/*
+	 * Our caller always gives us a path pointing to an existing item, so
+	 * this can not happen.
+	 */
+	ASSERT(nitems >= 1);
+	if (nitems < 1)
+		return -ENOENT;
 
 	ret = btrfs_del_items(trans, root, path, path->slots[0], nitems);
 	if (ret)