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Re: [Qemu-block] [Qemu-devel] [PATCH] qemu-img: add check for zero-lengt

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] qemu-img: add check for zero-length job len
Date: Mon, 2 Nov 2015 16:43:33 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 11/02/2015 04:28 PM, John Snow wrote:
> The mirror job doesn't update its total length until
> it has already started running, so we should translate
> a zero-length job-len as meaning 0%.
> Otherwise, we may get divide-by-zero faults.
> Signed-off-by: John Snow <address@hidden>
> ---
>  qemu-img.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

And indeed, this has tripped up libvirt in the past :)

My only concern is what if you truly have a 0-length job?  For example,
when doing two block-stream commands with identical arguments in a row,
the second block-stream has no work to do, but can complete instantly.

Will this result in such a job never reporting 100% complete?  If so,
that's bad.

If you can answer my concerns that we don't have a design bug, then the
code changes look correct, and you can add:

Reviewed-by: Eric Blake <address@hidden>

> diff --git a/qemu-img.c b/qemu-img.c
> index 3025776..38b4888 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -656,7 +656,8 @@ static void run_block_job(BlockJob *job, Error **errp)
>      do {
>          aio_poll(aio_context, true);
> -        qemu_progress_print((float)job->offset / job->len * 100.f, 0);
> +        qemu_progress_print(job->len ?
> +                            ((float)job->offset / job->len * 100.f) : 0.00, 
> 0);

Also, note that this promotes to double rather than float; maybe you
want to use 0.f instead of 0.00 to keep the ternary as a float?  But it
shouldn't make a difference in practice.

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

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