Re: [PATCH v12 13/14] stream: skip filters when writing backing file name to QCOW2 header

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

27.10.2020 19:42, Andrey Shinkevich wrote:
> On 27.10.2020 19:21, Vladimir Sementsov-Ogievskiy wrote:
>> 27.10.2020 19:01, Andrey Shinkevich wrote:
>>> On 27.10.2020 18:09, Vladimir Sementsov-Ogievskiy wrote:
>>>> 22.10.2020 21:13, Andrey Shinkevich wrote:
>>>>> Avoid writing a filter JSON file name and a filter format name to QCOW2
>>>>> image when the backing file is changed after the block stream job.
>>>>> A user is still able to assign the 'backing-file' parameter for a
>>>>> block-stream job keeping in mind the possible issue mentioned above.
>>>>> If the user does not specify the 'backing-file' parameter, QEMU will
>>>>> assign it automatically.
>>>>>
>>>>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
>>>>> ---
>>>>>   block/stream.c | 15 +++++++++++++--
>>>>>   blockdev.c     |  9 ++-------
>>>>>   2 files changed, 15 insertions(+), 9 deletions(-)
>>>>>
>>>>> diff --git a/block/stream.c b/block/stream.c
>>>>> index e0540ee..1ba74ab 100644
>>>>> --- a/block/stream.c
>>>>> +++ b/block/stream.c
>>>>> @@ -65,6 +65,7 @@ static int stream_prepare(Job *job)
>>>>>       BlockDriverState *bs = blk_bs(bjob->blk);
>>>>>       BlockDriverState *unfiltered_bs = bdrv_skip_filters(bs);
>>>>>       BlockDriverState *base = bdrv_filter_or_cow_bs(s->above_base);
>>>>> +    BlockDriverState *base_unfiltered = NULL;
>>>>>       Error *local_err = NULL;
>>>>>       int ret = 0;
>>>>> @@ -75,8 +76,18 @@ static int stream_prepare(Job *job)
>>>>>           const char *base_id = NULL, *base_fmt = NULL;
>>>>>           if (base) {
>>>>>               base_id = s->backing_file_str;
>>>>> -            if (base->drv) {
>>>>> -                base_fmt = base->drv->format_name;
>>>>> +            if (base_id) {
>>>>> +                if (base->drv) {
>>>>> +                    base_fmt = base->drv->format_name;
>>>>
>>>> hmm. this doesn't make real sense: so, we assume that user specified backing_file_str, which may not relate to base, but we use base->drv->format_name? But it may be name of the filter driver, which would be wrong..
>>>>
>>>> Any ideas?
>>>>
>>>> 1. we can use base_fmt=NULL, to provoke probing on next open of the qcow2 file..
>>>
>>> I would choose this item #1 but have to check the probing code logic... Particularly, I do not remember now if the probing is able to recognize a protocol.
>>> The logic for the format_name in the QEMU existent code (I has kept it here in the patch) is a slippery way for an imprudent user. That's why I staked on the backing_file_str deprication in the previous version.
>>>
>>>> 2. we can do probing now
>>>> 3. we can at least check, if backing_file_str == 
>>>
>>> Not bad for the sanity check but we will search a node by the file name again - not good ((
>>
>> Not search, but only check one very likely option.
> 
> Yes, just strcmp(). And why a user may not merely specify a desired backing file as the base?

*shrung*

> 
>>
>> Additionally to 1. or 3. (or combined), or even keeping things as is (i.e. wrong, but it is preexisting), we can:
>>
>>   - add backing-format argument to qapi as pair for backing-file
>>   - deprecate using backing-file without backing-format.
>>
>> Then, after deprecation period we'll have correct code. This may be done in separate.
>>
>>>
>>>> base_unfiltered->filename, in this case we can use base_unfiltered->drv->format_name
>>>>
>>>>
>>>>> +                }
>>>>> +            } else {
>>>>> +                base_unfiltered = bdrv_skip_filters(base);
>>>>> +                if (base_unfiltered) {
>>>>> +                    base_id = base_unfiltered->filename;
>>>>> +                    if (base_unfiltered->drv) {
>>>>> +                        base_fmt = base_unfiltered->drv->format_name;
>>>>> +                    }
>>>>> +                }
>>>>>               }
>>>>>           }
>>>>>           bdrv_set_backing_hd(unfiltered_bs, base, &local_err);
>>>>> diff --git a/blockdev.c b/blockdev.c
>>>>> index c917625..0e9c783 100644
>>>>> --- a/blockdev.c
>>>>> +++ b/blockdev.c
>>>
>>> [...]
>>>
>>>>> -    stream_start(has_job_id ? job_id : NULL, bs, base_bs, base_name,
>>>>> +    stream_start(has_job_id ? job_id : NULL, bs, base_bs,
>>>>> +                 has_backing_file ? backing_file : NULL,
>>>>
>>>> backing_file should be NULL if has_backing_file is false, so you can use just backing_file instead of ternary operator.
>>>>
>>>
>>> Yes, if reliable. I has kept the conformation with the ternary operator at the first parameter above.
>>>
>>> Andrey
>>>
>>>>>                    job_flags, has_speed ? speed : 0, on_error,
>>>>>                    filter_node_name, &local_err);
>>>>>       if (local_err) {
>>>>>
>>>>
>>>>
>>
>>


-- 
Best regards,
Vladimir