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Re: [PATCH v2] qcow2: Use a GString in report_unsupported_feature()

From: Max Reitz
Subject: Re: [PATCH v2] qcow2: Use a GString in report_unsupported_feature()
Date: Thu, 16 Jan 2020 13:19:34 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1

On 15.01.20 14:56, Alberto Garcia wrote:
> 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 or the "Unknown incompatible feature" message.

That doesn’t make sense to me.  The existing incompatible features are
known to qemu, and as such will never be printed here.

But just because it doesn’t make sense doesn’t make it wrong.  I suppose
we can assume that if new features are added, they will have a similar
length like the ones we have now.

(Well, it does make “60” a weirdly specific number, but whatever.)

((I’d just have created an empty string.  Considering this is part of
the explanation for a fatal error, nobody cares whether this function
takes a couple of microseconds more.  Or maybe a length of 47, so it can
definitely hold exactly one incompatible feature name.))

> Suggested-by: Philippe Mathieu-Daudé <address@hidden>
> Signed-off-by: Alberto Garcia <address@hidden>
> Reviewed-by: Alex Bennée <address@hidden>
> ---
>  block/qcow2.c | 23 +++++++++++------------
>  1 file changed, 11 insertions(+), 12 deletions(-)

Thanks, applied to my block branch:


> diff --git a/block/qcow2.c b/block/qcow2.c
> index cef9d72b3a..e29fc07068 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c


> @@ -470,14 +469,14 @@ 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);

Existing problem: This can lead to stuff like

“Unsupported qcow2 feature(s): feat1, feat2, Unknown incompatible
feature: 0xec0”

(i.e., singular “feature” when there are multiple unknown features, and
capitalization of “Unknown”)


But whatever.  It’s unlikely there’s going to be more than one
incompatible feature, and it’s extremely unlikely it won’t have a string


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