block: Manipulate bs->file / bs->backing pointers in .attach/.detach

bs->file and bs->backing are a kind of duplication of part of
bs->children. But very useful diplication, so let's not drop them at
all:)

We should manage bs->file and bs->backing in same place, where we
manage bs->children, to keep them in sync.

Moreover, generic io paths are unprepared to BdrvChild without a bs, so
it's double good to clear bs->file / bs->backing when we detach the
child.

Detach is simple: if we detach bs->file or bs->backing child, just
set corresponding field to NULL.

Attach is a bit more complicated. But we still can precisely detect
should we set one of bs->file / bs->backing or not:

- if role is BDRV_CHILD_COW, we definitely deal with bs->backing
- else, if role is BDRV_CHILD_FILTERED (it must be also
  BDRV_CHILD_PRIMARY), it's a filtered child. Use
  bs->drv->filtered_child_is_backing to chose the pointer field to
  modify.
- else, if role is BDRV_CHILD_PRIMARY, we deal with bs->file
- in all other cases, it's neither bs->backing nor bs->file. It's some
  other child and we shouldn't care

OK. This change brings one more good thing: we can (and should) get rid
of all indirect pointers in the block-graph-change transactions:

bdrv_attach_child_common() stores BdrvChild** into transaction to clear
it on abort.

bdrv_attach_child_common() has two callers: bdrv_attach_child_noperm()
just pass-through this feature, bdrv_root_attach_child() doesn't need
the feature.

Look at bdrv_attach_child_noperm() callers:
  - bdrv_attach_child() doesn't need the feature
  - bdrv_set_file_or_backing_noperm() uses the feature to manage
    bs->file and bs->backing, we don't want it anymore
  - bdrv_append() uses the feature to manage bs->backing, again we
    don't want it anymore

So, we should drop this stuff! Great!

We could probably keep BdrvChild** argument to keep the int return
value, but it seems not worth the complexity.

Finally, we now set .file / .backing automatically in generic code and
want to restring setting them by hand outside of .attach/.detach.
So, this patch cleanups all remaining places where they were set.
To find such places I use:

  git grep '\->file ='
  git grep '\->backing ='
  git grep '&.*\<backing\>'
  git grep '&.*\<file\>'

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220726201134.924743-14-vsementsov@yandex-team.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

3 files changed, 5 insertions, 6 deletions

diff --git a/block/raw-format.c b/block/raw-format.c
index f337ac7569..408b20e22d 100644
--- a/block/raw-format.c
+++ b/block/raw-format.c
@@ -458,8 +458,8 @@ static int raw_open(BlockDriverState *bs, QDict *options, int flags,
         file_role = BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY;
     }
 
-    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
-                               file_role, false, errp);
+    bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
+                    file_role, false, errp);
     if (!bs->file) {
         return -EINVAL;
     }
diff --git a/block/snapshot-access.c b/block/snapshot-access.c
index 77b87c1946..0a30ec6cd9 100644
--- a/block/snapshot-access.c
+++ b/block/snapshot-access.c
@@ -82,9 +82,9 @@ static void snapshot_access_refresh_filename(BlockDriverState *bs)
 static int snapshot_access_open(BlockDriverState *bs, QDict *options, int flags,
                                 Error **errp)
 {
-    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
-                               BDRV_CHILD_DATA | BDRV_CHILD_PRIMARY,
-                               false, errp);
+    bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
+                    BDRV_CHILD_DATA | BDRV_CHILD_PRIMARY,
+                    false, errp);
     if (!bs->file) {
         return -EINVAL;
     }
diff --git a/block/snapshot.c b/block/snapshot.c
index 75e8d3a937..f3971ac2bd 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -288,7 +288,6 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
 
         /* .bdrv_open() will re-attach it */
         bdrv_unref_child(bs, *fallback_ptr);
-        *fallback_ptr = NULL;
 
         ret = bdrv_snapshot_goto(fallback_bs, snapshot_id, errp);
         open_ret = drv->bdrv_open(bs, options, bs->open_flags, &local_err);