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

[Eric Blake]

On 3/29/19 3:33 AM, Richard W.M. Jones wrote:

>>> +  /* 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 :-(

Thanks for the quick fix. I realized that it is not just an efficiency
problem, but an actual bug - if the plugin returns exactly one extent,
it ends up not covering the start offset of the original extents, at
which point the truncate filter returns success with 0 extents instead
of the requirement of at least 1.  While the 'end' parameter does not
drive correct results, the 'start' parameter does :)


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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