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

[Philippe Mathieu-Daudé]

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?

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