[libvirt] [PATCH 2/3] Routine to truncate virBuffer

[Eric Blake]

On 11/06/2011 06:58 AM, Bharata B Rao wrote:
> Routine to truncate virBuffer
>
> From: Bharata B Rao<bharata at linux.vnet.ibm.com>
>
> Add a helper to truncate virBuffer.
>
> /**
>   * virBufferTruncate:

> +++ b/src/util/buf.c
> @@ -123,6 +123,31 @@ virBufferGrow(virBufferPtr buf, unsigned int len)
>   }
>
>   /**
> + * virBufferTruncate:
> + * @buf:  the buffer
> + * @len:  number of bytes by which the buffer is truncated
> + *
> + * Truncate the buffer by @len bytes.
> + *
> + * Returns zero on success or -1 on error
> + */
> +int
> +virBufferTruncate(virBufferPtr buf, unsigned int len)

What good is returning an error, given that callers already have to use 
virBufferError for detecting other errors, and given that you aren't 
marking the function as ATTRIBUTE_RETURN_CHECK to require callers to 
respect that error?  One of the points of the virBuffer API is that most 
functions can return void (for example, see virBufferAdjustIndent), 
because the user doesn't care about error checking until the end.

> +{
> +    if (buf->error)
> +        return -1;
> +
> +    if ((signed)(buf->use - len)<  0) {

I tend to write the cast as (int), not (signed), if a cast is even 
needed.  You don't catch all possible overflows, though - if buf->use is 
2, and len is 4294967295 (aka (unsigned)-1), then you've proceeded to 
expand(!) buf->use to 3, skipping an uninitialized byte.  A better 
filter would be:

if (len > buf->use)

> +        virBufferSetError(buf, -1);
> +        return -1;
> +    }
> +
> +    buf->use -= len;
> +    buf->content[buf->use] = '\0';

This looks correct, once you filter out invalid len first.

> +++ b/tests/virbuftest.c
> @@ -123,7 +123,8 @@ static int testBufAutoIndent(const void *data ATTRIBUTE_UNUSED)
>       virBufferAddChar(buf, '\n');
>       virBufferEscapeShell(buf, " 11");
>       virBufferAddChar(buf, '\n');
> -
> +    virBufferAsprintf(buf, "%d", 12);
> +    virBufferTruncate(buf, 4);

That gives no change in the expected output.  I'd almost rather see:

virBufferAsprintf(buf, "%d", 123);
virBufferTruncate(buf, 1);

as well as an update to the expected output to see output ending in "12".

You're on the right track, but I don't think it is worth applying this 
until after 0.9.7 is released.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org