[Libguestfs] [PATCH nbdkit] server: Implement extents/can_extents calls for plugins and filters.

[Richard W.M. Jones]

On Tue, Mar 12, 2019 at 01:23:37PM +0100, Martin Kletzander wrote:
> >+=head2 C<.extents>
> >+
> >+ int zero (void *handle, uint32_t count, uint64_t offset, uint32_t flags,
> 
> s/zero/extents/

Oops.

> >+  if (f->filter.extents)
> >+    return f->filter.extents (&next_ops, &nxdata, handle,
> >+                              count, offset, flags,
> >+                              nr_extents, extents, err);
> >+  else
> >+    return f->backend.next->extents (f->backend.next, conn,
> >+                                     count, offset, flags,
> >+                                     nr_extents, extents, err);
>
> Do I understand it correctly that if a filter does not support
> extents, then its function will only be applied on the allocated
> blocks?  It makes sense, but I feel like it would be nice to have
> that mentioned somewhere.

No.  The code snippet above allows filters to intercept the extents
call and modify it (either on the way in, or the way out, or most
likely in this case both).  The conditional ‘if (f->filter.extents)’
is checking if the filter has an extents callback, which is the
callback that the filter has to provide to do this interception.

I think most filters will need to be adjusted by adding an extents
callback.  Certainly any filter which deals with offsets (most
obviously nbdkit-offset-filter, nbdkit-partition-filter) will need to
implement the .extents call and both adjust the offset parameter that
is passed down, and examine the list of extents being returned through
the filter and adjust the offsets inside that list.

This is the broad reason why we don't offer a stable API for filters
(unlike plugins).  Nor do we advise that it's a good idea to
distribute filters separately from nbdkit.  If an old filter was used
with a plugin which was using the extents API then the filter would
basically corrupt data.

> Other than that it looks OK to me, I'll try to cook up a test filter
> to test this, but mainly as a fun exercise for myself.

So note at the moment this patch doesn't really do anything.  nbdkit
server still doesn't support block status, so neither of the functions
will ever be called.  This patch was just for discussion about what
the plugin API might look like.

Rich.

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