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

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

On 1/14/20 10:35 PM, Alex Bennée wrote:
> Philippe Mathieu-Daudé <philmd@redhat.com> writes:
> 
>> 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?
> 
> It does:
> 
> static inline void g_autoptr_cleanup_gstring_free (GString *string)
> {
>          if (string)
>             g_string_free (string, TRUE);
> }

The implicit use of free_segment=TRUE was not obvious to me.
Thanks for checking it in glib source.

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

I agree. I asked because I had the other patch from Alberto in mind:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg669862.html

In this case we can not use the g_autoptr feature.

>>>>          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);
>>>>    }
>>>>      /*
>>>