[PATCH v7 29/47] blockdev: Use CAF in external_snapshot_prepare()

[Max Reitz]

This allows us to differentiate between filters and nodes with COW
backing files: Filters cannot be used as overlays at all (for this
function).

Signed-off-by: Max Reitz <mre...@redhat.com>
---
 blockdev.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/blockdev.c b/blockdev.c
index 1eb0fcdea2..aabe51036d 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1549,7 +1549,12 @@ static void external_snapshot_prepare(BlkActionState 
*common,
         goto out;
     }
 
-    if (state->new_bs->backing != NULL) {
+    if (state->new_bs->drv->is_filter) {
+        error_setg(errp, "Filters cannot be used as overlays");
+        goto out;
+    }
+
+    if (bdrv_cow_child(state->new_bs)) {
         error_setg(errp, "The overlay already has a backing image");
         goto out;
     }
-- 
2.26.2