block: Add and use bdrv_replace_in_backing_chain()

This cleans up the mess we left behind in the mirror code after the
previous patch. Instead of using bdrv_swap(), just change pointers.

The interface change of the mirror job that callers must consider is
that after job completion, their local BDS pointers still point to the
same node now. qemu-img must change its code accordingly (which makes it
easier to understand); the other callers stays unchanged because after
completion they don't do anything with the BDS, but just with the job,
and the job is still owned by the source BDS.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 8 insertions, 8 deletions

diff --git a/qemu-img.c b/qemu-img.c
index e678fdc403..3025776e14 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -766,12 +766,12 @@ static int img_commit(int argc, char **argv)
         goto done;
     }
 
-    /* The block job will swap base_bs and bs (which is not what we really want
-     * here, but okay) and unref base_bs (after the swap, i.e., the old top
-     * image). In order to still be able to empty that top image afterwards,
-     * increment the reference counter here preemptively. */
+    /* When the block job completes, the BlockBackend reference will point to
+     * the old backing file. In order to avoid that the top image is already
+     * deleted, so we can still empty it afterwards, increment the reference
+     * counter here preemptively. */
     if (!drop) {
-        bdrv_ref(base_bs);
+        bdrv_ref(bs);
     }
 
     run_block_job(bs->job, &local_err);
@@ -779,8 +779,8 @@ static int img_commit(int argc, char **argv)
         goto unref_backing;
     }
 
-    if (!drop && base_bs->drv->bdrv_make_empty) {
-        ret = base_bs->drv->bdrv_make_empty(base_bs);
+    if (!drop && bs->drv->bdrv_make_empty) {
+        ret = bs->drv->bdrv_make_empty(bs);
         if (ret) {
             error_setg_errno(&local_err, -ret, "Could not empty %s",
                              filename);
@@ -790,7 +790,7 @@ static int img_commit(int argc, char **argv)
 
 unref_backing:
     if (!drop) {
-        bdrv_unref(base_bs);
+        bdrv_unref(bs);
     }
 
 done: