[Libguestfs] [PATCH nbdkit v5 FINAL 12/19] truncate: Implement extents for beyond end of truncated region.

[Richard W.M. Jones]

On Thu, Mar 28, 2019 at 08:57:56PM -0500, Eric Blake wrote:
> On 3/28/19 11:18 AM, Richard W.M. Jones wrote:
> > ---
> >  filters/truncate/truncate.c | 55 +++++++++++++++++++++++++++++++++++++
> >  1 file changed, 55 insertions(+)
> > 
> 
> > +
> > +  /* If the entire request is beyond the end of the underlying plugin
> > +   * then this is the easy case: return a hole.
> > +   */
> > +  if (offset >= real_size_copy)
> > +    return nbdkit_add_extent (extents, offset, (uint64_t) count,
> > +                              NBDKIT_EXTENT_ZERO|NBDKIT_EXTENT_HOLE);
> 
> Why not return a hole up to the end of the file, rather than limiting
> things at the end of the client's request?

Yes, very true - I'll add a small fix for this.

> > +  /* We're asked first for extents information about the plugin, then
> > +   * possibly (if truncating larger) for the hole after the plugin.
> > +   * Since we're not required to provide all of this information, the
> > +   * easiest thing is to only return data from the plugin.  We will be
> > +   * called later about the hole.  However we do need to make sure
> > +   * that the extents array is truncated to the real size, hence we
> > +   * have to create a new extents array, ask the plugin, then copy the
> > +   * returned data to the original array.
> > +   */
> > +  extents2 = nbdkit_extents_new (0, real_size_copy);
> 
> Why 0 for start instead of offset?  You get the same result either way
> (since the copying code ignores the prefix), but it's probably a lot
> more efficient to not have to copy the extents for the prefix of the file.

Yup that's a mistake too :-(

Rich.

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