[Libguestfs] [PATCH nbdkit] blocksize: Export block size constraints

[Eric Blake]

On Sun, Feb 20, 2022 at 08:56:37PM +0000, Richard W.M. Jones wrote:
> This filter is a little unusual because it allows clients to send a
> wider range of request sizes than the underlying plugin allows.
> Therefore we advertise the widest possible minimum and maximum block
> size to clients.
> 
> We still need to pick a suitable preferred block size assuming the
> plugin itself doesn't advertise one.
> ---
>  filters/blocksize/blocksize.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/filters/blocksize/blocksize.c b/filters/blocksize/blocksize.c
> index a6fa00cb..46e759b0 100644
> --- a/filters/blocksize/blocksize.c
> +++ b/filters/blocksize/blocksize.c
> @@ -156,6 +156,29 @@ blocksize_get_size (nbdkit_next *next,
>    return ROUND_DOWN (size, minblock);
>  }
>  
> +/* Block size constraints.
> + *
> + * Note that the purpose of this filter is to allow clients to send a
> + * wider range of request sizes than the underlying plugin permits,
> + * and therefore this callback advertises the full availability of the
> + * filter, likely widening the constraints from the plugin.
> + */
> +static int
> +blocksize_block_size (nbdkit_next *next, void *handle,
> +                      uint32_t *minimum, uint32_t *preferred, uint32_t *maximum)
> +{
> +  if (next->block_size (next, minimum, preferred, maximum) == -1)
> +    return -1;
> +
> +  if (*preferred == 0)
> +    *preferred = MAX (4096, minblock);

Hmm.  Even if minblock > *maximum reported by the plugin,...

> +
> +  *minimum = 1;
> +  *maximum = 0xffffffff;

...our advertisement to the client does not trigger any asserts.

ACK.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org