[PATCH 1/2] block: make bdrv_drop_intermediate() less wrong

[Vladimir Sementsov-Ogievskiy]

First, permission update loop tries to do iterations transactionally,
but the whole update is not transactional: nobody roll-back successful
loop iterations when some iteration fails.

Second, in the iteration we have nested permission update:
c->klass->update_filename may point to bdrv_child_cb_update_filename()
which calls bdrv_backing_update_filename(), which may do node reopen to
RW.

Permission update system is not prepared to nested updates, at least it
has intermediate permission-update state stored in BdrvChild
structures: has_backup_perm, backup_perm and backup_shared_perm.

So, let's first do bdrv_replace_node() (which is more transactional
than open-coded update in bdrv_drop_intermediate()) and then call
update_filename() in separate. We still do not rollback changes in case
of update_filename() failure but it's not much worse than pre-patch
behavior.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block.c | 36 +++++++++++++++---------------------
 1 file changed, 15 insertions(+), 21 deletions(-)

diff --git a/block.c b/block.c
index 9538af4884..bd9f4e534b 100644
--- a/block.c
+++ b/block.c
@@ -4958,36 +4958,30 @@ int bdrv_drop_intermediate(BlockDriverState *top, 
BlockDriverState *base,
         backing_file_str = base->filename;
     }
 
-    QLIST_FOREACH_SAFE(c, &top->parents, next_parent, next) {
-        /* Check whether we are allowed to switch c from top to base */
-        GSList *ignore_children = g_slist_prepend(NULL, c);
-        ret = bdrv_check_update_perm(base, NULL, c->perm, c->shared_perm,
-                                     ignore_children, NULL, &local_err);
-        g_slist_free(ignore_children);
-        if (ret < 0) {
-            error_report_err(local_err);
-            goto exit;
-        }
+    bdrv_replace_node(top, base, &local_err);
+    if (local_err) {
+        error_report_err(local_err);
+        goto exit;
+    }
 
-        /* If so, update the backing file path in the image file */
+    QLIST_FOREACH_SAFE(c, &base->parents, next_parent, next) {
         if (c->klass->update_filename) {
             ret = c->klass->update_filename(c, base, backing_file_str,
                                             &local_err);
             if (ret < 0) {
-                bdrv_abort_perm_update(base);
+                /*
+                 * TODO: Actually, we want to rollback all previous iterations
+                 * of this loop, and (which is almost impossible) previous
+                 * bdrv_replace_node()...
+                 *
+                 * Note, that c->klass->update_filename may lead to permission
+                 * update, so it's a bad idea to call it inside permission
+                 * update transaction of bdrv_replace_node.
+                 */
                 error_report_err(local_err);
                 goto exit;
             }
         }
-
-        /*
-         * Do the actual switch in the in-memory graph.
-         * Completes bdrv_check_update_perm() transaction internally.
-         * c->frozen is false, we have checked that above.
-         */
-        bdrv_ref(base);
-        bdrv_replace_child(c, base);
-        bdrv_unref(top);
     }
 
     if (update_inherits_from) {
-- 
2.21.3