[PULL 15/15] block: make bdrv_drop_intermediate() less wrong

[Max Reitz <mreitz@redhat.com>]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

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_common() (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.

Note that bdrv_replace_node_common() does check for frozen children,
so corresponding check is dropped in bdrv_drop_intermediate().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201106124241.16950-4-vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block.c | 54 ++++++++++++++++++++++++------------------------------
 1 file changed, 24 insertions(+), 30 deletions(-)

diff --git a/block.c b/block.c
index 9a945a058d..f1cedac362 100644
--- a/block.c
+++ b/block.c
@@ -4910,9 +4910,11 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
 {
     BlockDriverState *explicit_top = top;
     bool update_inherits_from;
-    BdrvChild *c, *next;
+    BdrvChild *c;
     Error *local_err = NULL;
     int ret = -EIO;
+    g_autoptr(GSList) updated_children = NULL;
+    GSList *p;
 
     bdrv_ref(top);
     bdrv_subtree_drained_begin(top);
@@ -4926,14 +4928,6 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
         goto exit;
     }
 
-    /* This function changes all links that point to top and makes
-     * them point to base. Check that none of them is frozen. */
-    QLIST_FOREACH(c, &top->parents, next_parent) {
-        if (c->frozen) {
-            goto exit;
-        }
-    }
-
     /* If 'base' recursively inherits from 'top' then we should set
      * base->inherits_from to top->inherits_from after 'top' and all
      * other intermediate nodes have been dropped.
@@ -4950,36 +4944,36 @@ 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;
-        }
+    QLIST_FOREACH(c, &top->parents, next_parent) {
+        updated_children = g_slist_prepend(updated_children, c);
+    }
+
+    bdrv_replace_node_common(top, base, false, &local_err);
+    if (local_err) {
+        error_report_err(local_err);
+        goto exit;
+    }
+
+    for (p = updated_children; p; p = p->next) {
+        c = p->data;
 
-        /* If so, update the backing file path in the image file */
         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) {
-- 
2.28.0