Upcoming backup-top filter wants to operate like usual implicit filter
node with fall-through to backing child. But also needs additional
target child, let's support that.

On the other hand, after backup completion (before job dismiss) filter
is still attached to job blk, but don't have any children. Support this
too.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index 57216f4115..3f4de3ae32 100644
--- a/block.c
+++ b/block.c
@@ -6200,9 +6200,20 @@ void bdrv_refresh_filename(BlockDriverState *bs)
     }
 
     if (bs->implicit) {
-        /* For implicit nodes, just copy everything from the single child */
+        /*
+         * For implicit nodes, just copy everything from the single child or
+         * from backing, if there are several children.
+         * If there are no children for some reason (filter is still attached
+         * to block-job blk, but already removed from backing chain of device)
+         * do nothing.
+         */
         child = QLIST_FIRST(&bs->children);
-        assert(QLIST_NEXT(child, next) == NULL);
+        if (!child) {
+            return;
+        } else if (QLIST_NEXT(child, next)) {
+            assert(bs->backing);
+            child = bs->backing;
+        }
 
         pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
                 child->bs->exact_filename);
-- 
2.18.0

On 29.05.19 17:46, Vladimir Sementsov-Ogievskiy wrote:
> Upcoming backup-top filter wants to operate like usual implicit filter
> node with fall-through to backing child. But also needs additional
> target child, let's support that.
> 
> On the other hand, after backup completion (before job dismiss) filter
> is still attached to job blk, but don't have any children. Support this
> too.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 57216f4115..3f4de3ae32 100644
> --- a/block.c
> +++ b/block.c
> @@ -6200,9 +6200,20 @@ void bdrv_refresh_filename(BlockDriverState *bs)
>      }
>  
>      if (bs->implicit) {
> -        /* For implicit nodes, just copy everything from the single child */
> +        /*
> +         * For implicit nodes, just copy everything from the single child or
> +         * from backing, if there are several children.
> +         * If there are no children for some reason (filter is still attached
> +         * to block-job blk, but already removed from backing chain of device)
> +         * do nothing.
> +         */
>          child = QLIST_FIRST(&bs->children);
> -        assert(QLIST_NEXT(child, next) == NULL);
> +        if (!child) {
> +            return;
> +        } else if (QLIST_NEXT(child, next)) {
> +            assert(bs->backing);
> +            child = bs->backing;
> +        }
>  
>          pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
>                  child->bs->exact_filename);

Reviewed-by: Max Reitz <mreitz@redhat.com>

(To be changed to bdrv_filtered_rw_bs(), of course)