block: Add BdrvChildRole.stay_at_node

When the parents' child links are updated in bdrv_append() or
bdrv_replace_in_backing_chain(), this should affect all child links of
BlockBackends or other nodes, but not on child links held for other
purposes (like for setting permissions). This patch allows to control
the behaviour per BdrvChildRole.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>

2 files changed, 7 insertions, 0 deletions

diff --git a/block.c b/block.c
index f72a67f..9e538a5 100644
--- a/block.c
+++ b/block.c
@@ -2853,6 +2853,9 @@ static void change_parent_backing_link(BlockDriverState *from,
     BdrvChild *c, *next, *to_c;
 
     QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
+        if (c->role->stay_at_node) {
+            continue;
+        }
         if (c->role == &child_backing) {
             /* @from is generally not allowed to be a backing file, except for
              * when @to is the overlay. In that case, @from may not be replaced
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 3177b9f..a0d9328 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -441,6 +441,10 @@ typedef struct BdrvAioNotifier {
 } BdrvAioNotifier;
 
 struct BdrvChildRole {
+    /* If true, bdrv_replace_in_backing_chain() doesn't change the node this
+     * BdrvChild points to. */
+    bool stay_at_node;
+
     void (*inherit_options)(int *child_flags, QDict *child_options,
                             int parent_flags, QDict *parent_options);