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

[Philippe Mathieu-Daudé]

On 1/14/20 10:35 PM, Alex Bennée wrote:
> 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);
> }

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/address@hidden/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 <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);
> > > >    }
> > > >      /*