block: Attach bs->file only during .bdrv_open()

The way that attaching bs->file worked was a bit unusual in that it was
the only child that would be attached to a node which is not opened yet.
Because of this, the block layer couldn't know yet which permissions the
driver would eventually need.

This patch moves the point where bs->file is attached to the beginning
of the individual .bdrv_open() implementations, so drivers already know
what they are going to do with the child. This is also more consistent
with how driver-specific children work.

For a moment, bdrv_open() gets its own BdrvChild to perform image
probing, but instead of directly assigning this BdrvChild to the BDS, it
becomes a temporary one and the node name is passed as an option to the
drivers, so that they can simply use bdrv_open_child() to create another
reference for their own use.

This duplicated child for (the not opened yet) bs is not the final
state, a follow-up patch will change the image probing code to use a
BlockBackend, which is completely independent of bs.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/block/qcow.c b/block/qcow.c
index 4534515db9..038b05ab1b 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -106,6 +106,12 @@ static int qcow_open(BlockDriverState *bs, QDict *options, int flags,
     QCowHeader header;
     Error *local_err = NULL;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
     if (ret < 0) {
         goto fail;