Re: [Qemu-devel] [PATCH v4 04/26] qcow2: Only return status from qcow2_get_refcount

[Eric Blake <eblake@redhat.com>]

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On 12/03/2014 06:37 AM, Max Reitz wrote:
> Refcounts can theoretically be of type uint64_t; in order to be able to
> represent the full range, qcow2_get_refcount() cannot use a single
> variable to represent both all refcount values and also keep some values
> reserved for errors.
> 
> One solution would be to add an Error pointer parameter to
> qcow2_get_refcount(); however, no caller could (currently) pass that
> error message, so it would have to be emitted immediately and be
> passed to the next caller by returning -EIO or something similar.
> Therefore, an Error parameter does not offer any advantages here.
> 
> The solution applied by this patch is simpler to use. Because no caller
> would be able to pass the error message, they would have to print it and
> free it, whereas with this patch the caller only needs to pass the
> returned integer (which is often a no-op from the code perspective,
> because that integer will be stored in a variable "ret" which will be
> returned by the fail path of many callers).
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-cluster.c  |  8 ++---
>  block/qcow2-refcount.c | 79 +++++++++++++++++++++++++++-----------------------
>  block/qcow2.h          |  3 +-
>  3 files changed, 49 insertions(+), 41 deletions(-)
> 

> @@ -1670,7 +1677,7 @@ static void compare_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
>  
>              if (num_fixed) {
>                  ret = update_refcount(bs, i << s->cluster_bits, 1,
> -                                      refcount2 - refcount1,
> +                                      (int)refcount2 - (int)refcount1,

The casts look a bit odd (uint16_t already naturally promotes to int for
32-bit int), but keep it, since we'll have to re-think this operation
when a later patch widens to 64 bits anyways.

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


-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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