[Libguestfs] [PATCH nbdkit 6/8] data, memory: Implement extents.

[Richard W.M. Jones]

On Sat, Mar 23, 2019 at 12:05:51PM -0500, Eric Blake wrote:
> On 3/20/19 5:11 PM, Richard W.M. Jones wrote:
> > These plugins are both based on the same sparse array structure which
> > supports a simple implementation of extents.
> > ---
> 
> > +int
> > +sparse_array_extents (struct sparse_array *sa,
> > +                      uint32_t count, uint64_t offset,
> > +                      struct nbdkit_extents *extents)
> > +{
> > +  uint32_t n, type;
> > +  void *p;
> > +
> > +  while (count > 0) {
> > +    p = lookup (sa, offset, false, &n, NULL);
> > +    if (n > count)
> > +      n = count;
> 
> Why are we clamping the information we return? I'd move this clamp...

It's a copy and paste from one of the previous sparse_array_*
functions in the same file.  I agree that we should return the most
possible information and moving the clamp as you suggest is the way to
do that.

Rich.

> > +
> > +    if (p == NULL)
> > +      type = NBDKIT_EXTENT_HOLE | NBDKIT_EXTENT_ZERO;
> > +    else
> > +      type = 0; /* allocated data */
> > +    if (nbdkit_add_extent (extents, offset, n, type) == -1)
> > +      return -1;
> 
> ...here, after we've recorded the maximum amount of information possible
> into the extents array. (We need the clamp to terminate the loop, but
> that doesn't mean we have to truncate our answer)
> 
> > +
> > +    count -= n;
> > +    offset += n;
> > +  }
> > +
> > +  return 0;
> > +}
> 
> Otherwise, looks good.
> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
> 




-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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