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, 15 insertions, 3 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index 3e1172b325..21e61427eb 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -814,8 +814,8 @@ static int qcow2_update_options(BlockDriverState *bs, QDict *options,
     return ret;
 }
 
-static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
-                      Error **errp)
+static int qcow2_do_open(BlockDriverState *bs, QDict *options, int flags,
+                         Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
     unsigned int len, i;
@@ -1205,6 +1205,18 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     return ret;
 }
 
+static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
+                      Error **errp)
+{
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
+    return qcow2_do_open(bs, options, flags, errp);
+}
+
 static void qcow2_refresh_limits(BlockDriverState *bs, Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
@@ -1785,7 +1797,7 @@ static void qcow2_invalidate_cache(BlockDriverState *bs, Error **errp)
     options = qdict_clone_shallow(bs->options);
 
     flags &= ~BDRV_O_INACTIVE;
-    ret = qcow2_open(bs, options, flags, &local_err);
+    ret = qcow2_do_open(bs, options, flags, &local_err);
     QDECREF(options);
     if (local_err) {
         error_propagate(errp, local_err);