block: Implement bdrv_append() without bdrv_swap()
Remember all parent nodes and just change the pointers there instead of swapping the contents of the BlockDriverState. Handling of snapshot=on must be moved further down in bdrv_open() because *pbs (which is the bs pointer in the BlockBackend) must already be set before bdrv_append() is called. Otherwise bdrv_append() changes the BB's pointer to the temporary snapshot, but bdrv_open() overwrites it with the read-only original image. We also need to be careful to update callers as the interface changes (becomes less insane): Previously, the meaning of the two parameters was inverted when bdrv_append() returns. Now any BDS pointers keep pointing to the same node. Signed-off-by: NKevin Wolf <kwolf@redhat.com> Reviewed-by: NMax Reitz <mreitz@redhat.com> Reviewed-by: NFam Zheng <famz@redhat.com> Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
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