quorum: Implement .bdrv_recurse_can_replace()

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200218103454.296704-9-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/quorum.c

@@ -813,6 +813,59 @@ static bool quorum_recurse_is_first_non_filter(BlockDriverState *bs,
     return false;
 }
 
+static bool quorum_recurse_can_replace(BlockDriverState *bs,
+                                       BlockDriverState *to_replace)
+{
+    BDRVQuorumState *s = bs->opaque;
+    int i;
+
+    for (i = 0; i < s->num_children; i++) {
+        /*
+         * We have no idea whether our children show the same data as
+         * this node (@bs).  It is actually highly likely that
+         * @to_replace does not, because replacing a broken child is
+         * one of the main use cases here.
+         *
+         * We do know that the new BDS will match @bs, so replacing
+         * any of our children by it will be safe.  It cannot change
+         * the data this quorum node presents to its parents.
+         *
+         * However, replacing @to_replace by @bs in any of our
+         * children's chains may change visible data somewhere in
+         * there.  We therefore cannot recurse down those chains with
+         * bdrv_recurse_can_replace().
+         * (More formally, bdrv_recurse_can_replace() requires that
+         * @to_replace will be replaced by something matching the @bs
+         * passed to it.  We cannot guarantee that.)
+         *
+         * Thus, we can only check whether any of our immediate
+         * children matches @to_replace.
+         *
+         * (In the future, we might add a function to recurse down a
+         * chain that checks that nothing there cares about a change
+         * in data from the respective child in question.  For
+         * example, most filters do not care when their child's data
+         * suddenly changes, as long as their parents do not care.)
+         */
+        if (s->children[i]->bs == to_replace) {
+            /*
+             * We now have to ensure that there is no other parent
+             * that cares about replacing this child by a node with
+             * potentially different data.
+             * We do so by checking whether there are any other parents
+             * at all, which is stricter than necessary, but also very
+             * simple.  (We may decide to implement something more
+             * complex and permissive when there is an actual need for
+             * it.)
+             */
+            return QLIST_FIRST(&to_replace->parents) == s->children[i] &&
+                QLIST_NEXT(s->children[i], next_parent) == NULL;
+        }
+    }
+
+    return false;
+}
+
 static int quorum_valid_threshold(int threshold, int num_children, Error **errp)
 {
 
@@ -1164,6 +1217,7 @@ static BlockDriver bdrv_quorum = {
 
     .is_filter                          = true,
     .bdrv_recurse_is_first_non_filter   = quorum_recurse_is_first_non_filter,
+    .bdrv_recurse_can_replace           = quorum_recurse_can_replace,
 
     .strong_runtime_opts                = quorum_strong_runtime_opts,
 };