Re: [Qemu-devel] [PATCH] qemu-img: add check for zero-length job len

[John Snow <jsnow@redhat.com>]

On 11/02/2015 06:43 PM, Eric Blake wrote:
> 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 <jsnow@redhat.com>
>> ---
>>  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.
> 

A few lines below the context:

/* A block job may finish instantaneously without publishing any
progress,

 * so just signal completion here */
qemu_progress_print(100.f, 0);


> 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 <eblake@redhat.com>
> 
>>
>> 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.
> 

Yes, oops -- but harmless.