Re: [Qemu-devel] [PATCH 04/11] snapshot: new function bdrv_snapshot_find_by_id_and_name()

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2013-6-8 16:35, Fam Zheng 写道:
> On Sat, 06/08 15:58, Wenchao Xia wrote:
>> 于 2013-6-8 15:31, Fam Zheng 写道:
>>> On Sat, 06/08 14:58, Wenchao Xia wrote:
>>>> To make it clear about id and name in searching, add this API
>>>> to distinguish them. Caller can choose to search by id or name,
>>>> *errp will be set only for exception.
>>>>
>>>> Some code are modified based on Pavel's patch.
>>>>
>>>> Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
>>>> Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
>>>> ---
>>>>   block/snapshot.c         |   74 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>   include/block/snapshot.h |    6 ++++
>>>>   2 files changed, 80 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/block/snapshot.c b/block/snapshot.c
>>>> index 6c6d9de..0a9af4e 100644
>>>> --- a/block/snapshot.c
>>>> +++ b/block/snapshot.c
>>>> @@ -48,6 +48,80 @@ int bdrv_snapshot_find(BlockDriverState *bs, QEMUSnapshotInfo *sn_info,
>>>>       return ret;
>>>>   }
>>>>
>>>> +/**
>>>> + * Look up an internal snapshot by @id and @name.
>>>> + * @bs: block device to search
>>>> + * @id: unique snapshot ID, or NULL
>>>> + * @name: snapshot name, or NULL
>>>> + * @sn_info: location to store information on the snapshot found
>>>> + * @errp: location to store error, will be set only for exception
>>>> + *
>>>> + * This function will traverse snapshot list in @bs to search the matching
>>>> + * one, @id and @name are the matching condition:
>>>> + * If both @id and @name are specified, find the first one with id @id and
>>>> + * name @name.
>>>> + * If only @id is specified, find the first one with id @id.
>>>> + * If only @name is specified, find the first one with name @name.
>>>> + * if none is specified, abort().
>>>> + *
>>>> + * Returns: true when a snapshot is found and @sn_info will be filled, false
>>>> + * when error or not found. If all operation succeed but no matching one is
>>>> + * found, @errp will NOT be set.
>>>> + */
>>>> +bool bdrv_snapshot_find_by_id_and_name(BlockDriverState *bs,
>>>> +                                       const char *id,
>>>> +                                       const char *name,
>>>> +                                       QEMUSnapshotInfo *sn_info,
>>>> +                                       Error **errp)
>>>> +{
>>>> +    QEMUSnapshotInfo *sn_tab, *sn;
>>>> +    int nb_sns, i;
>>>> +    bool ret = false;
>>>> +
>>>> +    nb_sns = bdrv_snapshot_list(bs, &sn_tab);
>>>> +    if (nb_sns < 0) {
>>>> +        error_setg_errno(errp, -nb_sns, "Failed to get a snapshot list");
>>>> +        return false;
>>>> +    } else if (nb_sns == 0) {
>>>> +        return false;
>>>> +    }
>>>> +
>>>> +    if (id && name) {
>>>> +        for (i = 0; i < nb_sns; i++) {
>>>> +            sn = &sn_tab[i];
>>>> +            if (!strcmp(sn->id_str, id) && !strcmp(sn->name, name)) {
>>>> +                *sn_info = *sn;
>>>> +                ret = true;
>>>> +                break;
>>>> +            }
>>>> +        }
>>>> +    } else if (id) {
>>>> +        for (i = 0; i < nb_sns; i++) {
>>>> +            sn = &sn_tab[i];
>>>> +            if (!strcmp(sn->id_str, id)) {
>>>> +                *sn_info = *sn;
>>>> +                ret = true;
>>>> +                break;
>>>> +            }
>>>> +        }
>>>> +    } else if (name) {
>>>> +        for (i = 0; i < nb_sns; i++) {
>>>> +            sn = &sn_tab[i];
>>>> +            if (!strcmp(sn->name, name)) {
>>>> +                *sn_info = *sn;
>>>> +                ret = true;
>>>> +                break;
>>>> +            }
>>>> +        }
>>>> +    } else {
>>>> +        /* program error */
>>>> +        abort();
>>>> +    }
>>>
>>> Looks duplicated. How about:
>>>
>>>      if (id || name) {
>>>          for (i = 0; i < nb_sns; i++) {
>>>              sn = &sn_tab[i];
>>>              if ((!id || !strcmp(sn->id_str, id)) &&
>>>                  (!name || !strcmp(sn->name, name))) {
>>>                  *sn_info = *sn;
>>>                  ret = true;
>>>                  break;
>>>              }
>>>          }
>>>      } else {
>>>          abort();
>>>      }
>>>
>>    Less code, but slightly slower since more "if" inside "for". I think
>> three "for" also show more clear about judgement logic.
>
> No I don't think if-in-for or for-in-if *here* makes any meaningful
> difference in performance, if we really need it fast, we'd better sort the
   My instinct is forbid if in for, just my custom.

> list it first and binary search. And I don't see it clearer to duplicate
> the same logic for three times, If I want to understand it, I need to
   Three cases makes work flow clear, and it is easy when I want to
change a logic in one case.

> compare if#1 and if#2 to get they are the same, and then compare #2 and
> #3 again, just to know that the three are no different.
>
   There is difference requiring reader think and extend it out into
three cases in his mind:
               if ((!id || !strcmp(sn->id_str, id)) &&
                   (!name || !strcmp(sn->name, name)))

It is a coding style issue, usually I don't mind to write more C lines 
to make things simple. Hope to get maintainer's idea.

>>
>>> And why do we have to abort here? It is not completely nonsense to me to
>>> return first snapshot with id == NULL and name == NULL.
>>>
>>    Just to tip program error. An snapshot with id == NULL and name ==
>> NULL is not possible, isn't it?.
>
> OK.
>


-- 
Best Regards

Wenchao Xia