[Libguestfs] [PATCH nbdkit filters-v2 2/5] Introduce filters.

[Richard W.M. Jones]

On Fri, Jan 19, 2018 at 02:47:02PM -0600, Eric Blake wrote:
> On 01/19/2018 09:23 AM, Richard W.M. Jones wrote:
> > Filters can be placed in front of plugins to modify their behaviour.
> > 
> > This commit adds the <nbdkit-filter.h> header file, the manual page,
> > the ‘filterdir’ directory (like ‘plugindir’), the ‘filters/’ source
> > directory which will contain the actual filters, the ‘--filters’
> > parameter, and the filters backend logic.
> > ---
> 
> >  include/Makefile.am     |   4 +-
> >  include/nbdkit-filter.h | 149 +++++++++++
> >  include/nbdkit-plugin.h |   2 +
> 
> > +++ b/include/nbdkit-filter.h
> 
> > +
> > +#ifndef NBDKIT_FILTER_H
> > +#define NBDKIT_FILTER_H
> > +
> > +/* This header also defines some useful functions like nbdkit_debug
> > + * and nbdkit_parse_size which are appropriate for filters to use.
> > + */
> > +#include <nbdkit-plugin.h>
> 
> Should we stick the useful functions in a new header <nbdkit-common.h>
> that both nbdkit-plugin.h and nbdkit-filter.h include?  I'm worried that
> the games we play with NBDKIT_API_VERSION in nbdkit-plugin.h for
> maintaining back-compat there may someday have awkward interactions in
> this file.

Yes.  I'll do this in the updated (v3) patch series.

> > +
> > +#ifdef __cplusplus
> > +extern "C" {
> > +#endif
> > +
> > +#define NBDKIT_FILTER_API_VERSION 1
> 
> On the other hand, you're setting NBDKIT_FILTER_API_VERSION separately
> from NBDKIT_API_VERSION (for plugins), as we could bump one but not the
> other in a given nbdkit release (and if so, it makes for our next
> release to have PLUGIN version 2 but FILTER version 1, once filters and
> FUA flags settle, where the highest version in each file shares the same
> backend semantics).
> 
> Does it ever make sense for a single .so file to call both
> NBDKIT_REGISTER_PLUGIN() and NBDKIT_REGISTER_FILTER() in the same
> library?

I don't know that it never makes sense, but it's definitely not
intended to be used that way.

> If not, one benefit of having the common code in a separate
> nbdkit-common.h is that we could do things like:
> 
> nbdkit-common.h:
> #if !defined NBDKIT_PLUGIN_H || !defined NBDKIT_FILTER_H
> #error this file should not be directly included
> #endif
> 
> nbdkit-plugin.h:
> #ifdef NBDKIT_FILTER_H
> #error cannot mix filter and plugin in one code base
> #endif
> 
> nbdkit-filter.h:
> #ifdef NBDKIT_PLUGIN_H
> #error cannot mix filter and plugin in one code base
> #endif
> 
> > +
> > +#ifndef NBDKIT_CXX_LANG_C
> > +#ifdef __cplusplus
> > +#define NBDKIT_CXX_LANG_C extern "C"
> > +#else
> > +#define NBDKIT_CXX_LANG_C /* nothing */
> > +#endif
> > +#endif
> 
> and a common header would be a nicer place for things like
> NBDKIT_CXX_LANG_C, rather than having to repeat our #ifndef definitions
> in two files.

OK.

Rich.

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