commit: get the overlay node before manipulating the backing chain

The 'block-commit' command has a 'top' parameter to specify the
topmost node from which the data is going to be copied.

   [E] <- [D] <- [C] <- [B] <- [A]

In this case if [C] is the top node then this is the result:

   [E] <- [B] <- [A]

[B] must be modified so its backing image string points to [E] instead
of [C]. commit_start() takes care of reopening [B] in read-write
mode, and commit_complete() puts it back in read-only mode once the
operation has finished.

In order to find [B] (the overlay node) we look for the node that has
[C] (the top node) as its backing image. However in commit_complete()
we're doing it after [C] has been removed from the chain, so [B] is
never found and remains in read-write mode.

This patch gets the overlay node before the backing chain is
manipulated.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 1471836963-28548-1-git-send-email-berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/block/commit.c b/block/commit.c
index 553e18d..a02539b 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -83,7 +83,7 @@ static void commit_complete(BlockJob *job, void *opaque)
     BlockDriverState *active = s->active;
     BlockDriverState *top = blk_bs(s->top);
     BlockDriverState *base = blk_bs(s->base);
-    BlockDriverState *overlay_bs;
+    BlockDriverState *overlay_bs = bdrv_find_overlay(active, top);
     int ret = data->ret;
 
     if (!block_job_is_cancelled(&s->common) && ret == 0) {
@@ -97,7 +97,6 @@ static void commit_complete(BlockJob *job, void *opaque)
     if (s->base_flags != bdrv_get_flags(base)) {
         bdrv_reopen(base, s->base_flags, NULL);
     }
-    overlay_bs = bdrv_find_overlay(active, top);
     if (overlay_bs && s->orig_overlay_flags != bdrv_get_flags(overlay_bs)) {
         bdrv_reopen(overlay_bs, s->orig_overlay_flags, NULL);
     }