[v2,1/3] block: resize backing file image during offline commit, if necessary

Currently, if an image file is logically larger than its backing file,
commiting it via 'qemu-img commit' will fail.

For instance, if we have a base image with a virtual size 10G, and a
snapshot image of size 20G, then committing the snapshot offline with
'qemu-img commit' will likely fail.

This will automatically attempt to resize the base image, if the
snapshot image to be committed is larger.

Signed-off-by: Jeff Cody <jcody@redhat.com>
---
 block.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

On 01/21/2014 09:31 AM, Jeff Cody wrote:
> Currently, if an image file is logically larger than its backing file,
> commiting it via 'qemu-img commit' will fail.

s/commiting/committing/

> 
> For instance, if we have a base image with a virtual size 10G, and a
> snapshot image of size 20G, then committing the snapshot offline with
> 'qemu-img commit' will likely fail.
> 
> This will automatically attempt to resize the base image, if the
> snapshot image to be committed is larger.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  block.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 

The commit message change is trivial, so:

Reviewed-by: Eric Blake <eblake@redhat.com>

On Tue, 01/21 11:31, Jeff Cody wrote:
> Currently, if an image file is logically larger than its backing file,
> commiting it via 'qemu-img commit' will fail.
> 
> For instance, if we have a base image with a virtual size 10G, and a
> snapshot image of size 20G, then committing the snapshot offline with
> 'qemu-img commit' will likely fail.
> 
> This will automatically attempt to resize the base image, if the
> snapshot image to be committed is larger.
> 
> Signed-off-by: Jeff Cody <jcody@redhat.com>
> ---
>  block.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 64e7d22..dea7591 100644
> --- a/block.c
> +++ b/block.c
> @@ -1876,10 +1876,10 @@ int bdrv_check(BlockDriverState *bs, BdrvCheckResult *res, BdrvCheckMode fix)
>  int bdrv_commit(BlockDriverState *bs)
>  {
>      BlockDriver *drv = bs->drv;
> -    int64_t sector, total_sectors;
> +    int64_t sector, total_sectors, length, backing_length;
>      int n, ro, open_flags;
>      int ret = 0;
> -    uint8_t *buf;
> +    uint8_t *buf = NULL;
>      char filename[PATH_MAX];
>  
>      if (!drv)
> @@ -1904,7 +1904,24 @@ int bdrv_commit(BlockDriverState *bs)
>          }
>      }
>  
> -    total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
> +    length = bdrv_getlength(bs);
> +    backing_length = bdrv_getlength(bs->backing_hd);
> +
> +    if (length < 0 || backing_length < 0) {
> +        goto ro_cleanup;
> +    }
> +
> +    /* If our top snapshot is larger than the backing file image,
> +     * grow the backing file image if possible.  If not possible,
> +     * we must return an error */
> +    if (length > backing_length) {
> +        ret = bdrv_truncate(bs->backing_hd, length);
> +        if (ret < 0) {
> +            goto ro_cleanup;
> +        }
> +    }
> +
> +    total_sectors = length >> BDRV_SECTOR_BITS;
>      buf = g_malloc(COMMIT_BUF_SECTORS * BDRV_SECTOR_SIZE);
>  
>      for (sector = 0; sector < total_sectors; sector += n) {
> -- 
> 1.8.3.1
> 
Reviewed-by: Fam Zheng <famz@redhat.com>