block: introduce bdrv_open_file_child() helper

Almost all drivers call bdrv_open_child() similarly. Let's create a
helper for this.

The only not updated drivers that call bdrv_open_child() to set
bs->file are raw-format and snapshot-access:
    raw-format sometimes want to have filtered child but
        don't set drv->is_filter to true.
    snapshot-access wants only DATA | PRIMARY

Possibly we should implement drv->is_filter_func() handler, to consider
raw-format as filter when it works as filter.. But it's another story.

Note also, that we decrease assignments to bs->file in code: it helps
us restrict modifying this field in further commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220726201134.924743-3-vsementsov@yandex-team.ru>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 3 insertions, 5 deletions

diff --git a/block/throttle.c b/block/throttle.c
index ddd450593a..131eba3ab4 100644
--- a/block/throttle.c
+++ b/block/throttle.c
@@ -78,11 +78,9 @@ static int throttle_open(BlockDriverState *bs, QDict *options,
     char *group;
     int ret;
 
-    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
-                               BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
-                               false, errp);
-    if (!bs->file) {
-        return -EINVAL;
+    ret = bdrv_open_file_child(NULL, options, "file", bs, errp);
+    if (ret < 0) {
+        return ret;
     }
     bs->supported_write_flags = bs->file->bs->supported_write_flags |
                                 BDRV_REQ_WRITE_UNCHANGED;