[libvirt] [PATCH for 5.3.0] storage_backend_rbd: Don't fail iterating over snapshots

[Michal Privoznik]

When cloning an RBD volume we try to find a snapshot which is
not different to the image we're trying to clone. This boils down
to calling rbd_diff_iterate() or rbd_diff_iterate2() on systems
with newer ceph. These two are passed a callback -
virStorageBackendRBDIterateCb() which mistakenly states that if
a negative value is returned from the callback, the iteration is
stopped but no error is reported. Well, that is not the case.
I've looked into librbd sources and found that a negative value
does mean stop of iteration but it also means an error. That's
why any attempt to clone an RBD image results in the following
error:

  virsh # vol-clone --pool rbd_image_root coreos_2023 coreos00.disk
  error: Failed to clone vol from coreos_2023
  error: failed to iterate RBD snapshot coreos_2023 at base: Operation not
  permitted

Reported on libvirt-users list:

https://www.redhat.com/archives/libvirt-users/2019-April/msg00060.html

Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---
 src/storage/storage_backend_rbd.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/src/storage/storage_backend_rbd.c b/src/storage/storage_backend_rbd.c
index f8c968e682..3fe188551c 100644
--- a/src/storage/storage_backend_rbd.c
+++ b/src/storage/storage_backend_rbd.c
@@ -1031,14 +1031,9 @@ virStorageBackendRBDIterateCb(uint64_t offset ATTRIBUTE_UNUSED,
 {
     /*
      * Just set that there is a diff for this snapshot, we do not care where
-     *
-     * When it returns a negative number the rbd_diff_iterate() function will stop
-     *
-     * That's why we return -1, meaning that there is a difference and we can stop
-     * searching any further.
      */
     *(int*) arg = 1;
-    return -1;
+    return 0;
 }
 
 static int
-- 
2.21.0