Re: [PATCH] qcow2: Use a GString in report_unsupported_feature()

[Alex Bennée]

Philippe Mathieu-Daudé <address@hidden> writes:

> On 1/14/20 7:08 PM, Alex Bennée wrote:
>> Alberto Garcia <address@hidden> writes:
>> 
>>> This is a bit more efficient than having to allocate and free memory
>>> for each item.
>>>
>>> The default size (60) is enough for all the existing incompatible
>>> features.
>>>
>>> Suggested-by: Philippe Mathieu-Daudé <address@hidden>
>>> Signed-off-by: Alberto Garcia <address@hidden>
>>> ---
>>>   block/qcow2.c | 24 ++++++++++++------------
>>>   1 file changed, 12 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/block/qcow2.c b/block/qcow2.c
>>> index cef9d72b3a..ecf6827420 100644
>>> --- a/block/qcow2.c
>>> +++ b/block/qcow2.c
>>> @@ -453,16 +453,15 @@ static void 
>>> cleanup_unknown_header_ext(BlockDriverState *bs)
>>>   static void report_unsupported_feature(Error **errp, Qcow2Feature *table,
>>>                                          uint64_t mask)
>>>   {
>>> -    char *features = g_strdup("");
>>> -    char *old;
>>> +    GString *features = g_string_sized_new(60);
>>         g_autoptr(GString) features = g_string_sized_new(60);
>> will save you the clean-up later.
>
> Does this work with g_string_free() too?

It does:

static inline void g_autoptr_cleanup_gstring_free (GString *string)
{
        if (string)
           g_string_free (string, TRUE);
}

If you want to keep the allocated string but drop the GString you are
still best doing that yourself.

>
>>>         while (table && table->name[0] != '\0') {
>>>           if (table->type == QCOW2_FEAT_TYPE_INCOMPATIBLE) {
>>>               if (mask & (1ULL << table->bit)) {
>>> -                old = features;
>>> -                features = g_strdup_printf("%s%s%.46s", old, *old ? ", " : 
>>> "",
>>> -                                           table->name);
>>> -                g_free(old);
>>> +                if (features->len > 0) {
>>> +                    g_string_append(features, ", ");
>>> +                }
>>> +                g_string_append_printf(features, "%.46s",
>>>       table->name);
>> We have a number of cases of this sort of idiom in the code base. I
>> wonder if it calls for a utility function:
>>         qemu_append_with_sep(features, ", ", "%.46s", table->name)
>
> Good idea for https://wiki.qemu.org/Contribute/BiteSizedTasks
>
>> Anyway not mandatory for this patch so with the autoptr fix:
>> Reviewed-by: Alex Bennée <address@hidden>
>> 
>>>                   mask &= ~(1ULL << table->bit);
>>>               }
>>>           }
>>> @@ -470,14 +469,15 @@ static void report_unsupported_feature(Error **errp, 
>>> Qcow2Feature *table,
>>>       }
>>>         if (mask) {
>>> -        old = features;
>>> -        features = g_strdup_printf("%s%sUnknown incompatible feature: %" 
>>> PRIx64,
>>> -                                   old, *old ? ", " : "", mask);
>>> -        g_free(old);
>>> +        if (features->len > 0) {
>>> +            g_string_append(features, ", ");
>>> +        }
>>> +        g_string_append_printf(features,
>>> +                               "Unknown incompatible feature: %" PRIx64, 
>>> mask);
>>>       }
>>>   -    error_setg(errp, "Unsupported qcow2 feature(s): %s",
>>> features);
>>> -    g_free(features);
>>> +    error_setg(errp, "Unsupported qcow2 feature(s): %s", features->str);
>>> +    g_string_free(features, TRUE);
>>>   }
>>>     /*
>> 


-- 
Alex Bennée