As it already said in the comment, we don't want to create loops in
parent->child relations. So, when we try to append @to to @c, we should
check that @c is not in @to children subtree, and we should check it
recursively, not only the first level. The patch provides BFS-based
search, to check the relations.
This is needed for further fleecing-hook filter usage: we need to
append it to source, when the hook is already a parent of target, and
source may be in a backing chain of target (fleecing-scheme). So, on
appending, the hook should not became a child (direct or through
children subtree) of the target.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
block.c | 33 ++++++++++++++++++++++++++++-----
1 file changed, 28 insertions(+), 5 deletions(-)
diff --git a/block.c b/block.c
index 4f5ff2cc12..8f04f293da 100644
--- a/block.c
+++ b/block.c
@@ -3536,7 +3536,7 @@ void bdrv_close_all(void)
static bool should_update_child(BdrvChild *c, BlockDriverState *to)
{
- BdrvChild *to_c;
+ GList *queue = NULL, *pos;
if (c->role->stay_at_node) {
return false;
@@ -3572,13 +3572,36 @@ static bool should_update_child(BdrvChild *c,
BlockDriverState *to)
* if A is a child of B, that means we cannot replace A by B there
* because that would create a loop. Silently detaching A from B
* is also not really an option. So overall just leaving A in
- * place there is the most sensible choice. */
- QLIST_FOREACH(to_c, &to->children, next) {
- if (to_c == c) {
- return false;
+ * place there is the most sensible choice.
+ *
+ * We would also create a loop in any cases where @c is only
+ * indirectly referenced by @to. Prevent this by returning false
+ * if @c is found (by breadth-first search) anywhere in the whole
+ * subtree of @to.
+ */
+
+ pos = queue = g_list_append(queue, to);
+ while (pos) {
+ BlockDriverState *v = pos->data;
+ BdrvChild *c2;
+
+ QLIST_FOREACH(c2, &v->children, next) {
+ if (c2 == c) {
+ g_list_free(queue);
+ return false;
+ }
+
+ if (g_list_find(queue, c2->bs)) {
+ continue;
+ }
+
+ queue = g_list_append(queue, c2->bs);
}
+
+ pos = pos->next;
}
+ g_list_free(queue);
return true;
}
--
2.18.0
