Btrfs has changed to delete subvolume/snapshot asynchronously, which means that
after umount itself, if we've already deleted 'ext2_saved', rollback can still
be completed.

So this adds a check for ROOT_BACKREF before checking ROOT_ITEM since
ROOT_BACKREF is immediately not in the btree after ioctl(BTRFS_IOC_SNAP_DESTROY)
returns.

Signed-off-by: Liu Bo <bo.li....@oracle.com>
---
 btrfs-convert.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/btrfs-convert.c b/btrfs-convert.c
index 802930c..f8a6c16 100644
--- a/btrfs-convert.c
+++ b/btrfs-convert.c
@@ -2591,6 +2591,22 @@ static int do_rollback(const char *devname)
        btrfs_init_path(&path);
 
        key.objectid = CONV_IMAGE_SUBVOL_OBJECTID;
+       key.type = BTRFS_ROOT_BACKREF_KEY;
+       key.offset = BTRFS_FS_TREE_OBJECTID;
+       ret = btrfs_search_slot(NULL, root->fs_info->tree_root, &key, &path, 0,
+                               0);
+       btrfs_release_path(&path);
+       if (ret > 0) {
+               fprintf(stderr, "unable to open subvol %llu\n",
+                       (unsigned long long)key.objectid);
+               goto fail;
+       } else if (ret < 0) {
+               fprintf(stderr, "unable to open subvol %llu ret %d\n",
+                       (unsigned long long)key.objectid, ret);
+               goto fail;
+       }
+
+       key.objectid = CONV_IMAGE_SUBVOL_OBJECTID;
        key.type = BTRFS_ROOT_ITEM_KEY;
        key.offset = (u64)-1;
        image_root = btrfs_read_fs_root(root->fs_info, &key);
-- 
1.8.2.1

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