Refactor the function to replace child at last. Thus we don't need to
revert it and code is simplified.

block-copy state initialization being done before replacing the child
doesn't need any drained section.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
---
 block/copy-before-write.c | 33 +++++++++++----------------------
 1 file changed, 11 insertions(+), 22 deletions(-)

diff --git a/block/copy-before-write.c b/block/copy-before-write.c
index 7a6c15f141..adbbc64aa9 100644
--- a/block/copy-before-write.c
+++ b/block/copy-before-write.c
@@ -172,7 +172,6 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
     int ret;
     BDRVCopyBeforeWriteState *state;
     BlockDriverState *top;
-    bool appended = false;
 
     assert(source->total_sectors == target->total_sectors);
 
@@ -196,8 +195,7 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                       BDRV_CHILD_DATA, errp);
     if (!state->target) {
         error_prepend(errp, "Cannot attach target child: ");
-        bdrv_unref(top);
-        return NULL;
+        goto fail;
     }
 
     bdrv_ref(source);
@@ -206,18 +204,8 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
                                   errp);
     if (!top->file) {
         error_prepend(errp, "Cannot attach file child: ");
-        bdrv_unref(top);
-        return NULL;
-    }
-
-    bdrv_drained_begin(source);
-
-    ret = bdrv_replace_node(source, top, errp);
-    if (ret < 0) {
-        error_prepend(errp, "Cannot append copy-before-write filter: ");
         goto fail;
     }
-    appended = true;
 
     state->bcs = block_copy_state_new(top->file, state->target, false, compress,
                                       errp);
@@ -225,21 +213,22 @@ BlockDriverState *bdrv_cbw_append(BlockDriverState *source,
         error_prepend(errp, "Cannot create block-copy-state: ");
         goto fail;
     }
-    *bcs = state->bcs;
 
+    bdrv_drained_begin(source);
+    ret = bdrv_replace_node(source, top, errp);
     bdrv_drained_end(source);
+    if (ret < 0) {
+        error_prepend(errp, "Cannot append copy-before-write filter: ");
+        goto fail;
+    }
+
+    *bcs = state->bcs;
 
     return top;
 
 fail:
-    if (appended) {
-        bdrv_cbw_drop(top);
-    } else {
-        bdrv_unref(top);
-    }
-
-    bdrv_drained_end(source);
-
+    block_copy_state_free(state->bcs);
+    bdrv_unref(top);
     return NULL;
 }
 
-- 
2.29.2