Re: [PATCH v3 10/10] libxl: fix caller of libxl_cpupool functions

[Juergen Gross <jgross@suse.com>]

On 07/16/2015 10:57 AM, Dario Faggioli wrote:
> [Adding Juergen]

Near miss. You took my old mail address. :-)

>
> On Wed, 2015-07-15 at 18:16 +0100, Wei Liu wrote:
>> On Tue, Jul 14, 2015 at 06:33:22PM +0100, Ian Jackson wrote:
>>> Wei Liu writes ("[PATCH v3 10/10] libxl: fix caller of libxl_cpupool functions"):
>
>>>> --- a/tools/libxl/libxl.c
>>>> +++ b/tools/libxl/libxl.c
>>>> @@ -771,8 +771,11 @@ libxl_cpupoolinfo * libxl_list_cpupool(libxl_ctx *ctx, int *nb_pool_out)
>>>>
>>>>       poolid = 0;
>>>>       for (i = 0;; i++) {
>>>> -        if (cpupool_info(gc, &info, poolid, false))
>>>> +        libxl_cpupoolinfo_init(&info);
>>>> +        if (cpupool_info(gc, &info, poolid, false)) {
>>>> +            libxl_cpupoolinfo_dispose(&info);
>>>>               break;
>>>> +        }
>>>
>>> I'm not convinced by this change.
>>>
>>> I think that this function has broken error handling: if cpupool_info
>>> fails, it simply ignores the error.
>>>
>>
>> I think the semantics of this function is to get back as many cpupool
>> info as it can.
>>
> Yes, I also think this is the case.

Indeed.

>> Unfortunately the failing of cpupool_info can either mean the cpupool id
>> does not exist or other internal errors.  So not cleaning up is the
>> right thing to do (speaking from semantics point of view).
>>
> Indeed.
>
>> I think I need to overhaul cpupool_info a bit if we want to make this
>> API better.
>>
> Well, perhaps having cpupool_info() treating specially the situation
> where the pool ID is not there may help... However, what would you do
> once you have this additional piece of information available?
>
> Maybe, depending on the error, we can cleanup the whole array? Is it
> this that we are after?

I think the best would be:

Modify cpupool_info() to return either success, internal error, or no
cpupool found.

In case of internal error libxl_list_cpupool() should clean up and
return NULL.

>
>> I'm not sure if it is possible to interleave pool ids. If so, we need a
>> way to get back the exact number of cpupools. Dario?

libxl_list_cpupool() does that (2nd parameter).

>>
> Sorry, what you mean by 'interleave pool ids'?

Not sure if this is an answer to interleaving of pool ids, but it is
possible to specify the pool id when creating a new cpupool at the
libxc interface. Even in case this is not used, pool ids can easily
be sparse after having deleted a cpupool.


Juergen