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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 1/14/20 7:08 PM, Alex Bennée wrote:
> Alberto Garcia <berto@igalia.com> 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 <philmd@redhat.com>
>> Signed-off-by: Alberto Garcia <berto@igalia.com>
>> ---
>>   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?

>>   
>>       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 <alex.bennee@linaro.org>
> 
>>                   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);
>>   }
>>   
>>   /*
> 
>