[Libguestfs] [PATCH libnbd v2 04/10] lib: Permit .callback = NULL, .free != NULL.

[Richard W.M. Jones]

On Thu, Aug 15, 2019 at 06:57:01AM -0500, Eric Blake wrote:
> On 8/15/19 4:56 AM, Richard W.M. Jones wrote:
> > Previously the .free function of a callback was not called if the
> > .callback field was NULL, because the callback as a whole would be
> > considered to be "null".
> > 
> > This change allows you to register callbacks where the .callback field
> > is NULL, but the .free field is != NULL, meaning that the callback is
> > freed after the last time it would have been used.
> > 
> > This is mainly convenient for language bindings where we sometimes
> > want to register a free function to clean up a persistent buffer, but
> > we don't need the associated completion callback to be actually
> > called.
> > ---
> >  docs/libnbd.pod | 15 +++++++++++++++
> >  lib/internal.h  | 18 +++++++++---------
> >  2 files changed, 24 insertions(+), 9 deletions(-)
> > 
> 
> > +++ b/lib/internal.h
> > @@ -274,20 +274,20 @@ struct command {
> >  };
> >  
> >  /* Test if a callback is "null" or not, and set it to null. */
> > -#define CALLBACK_IS_NULL(cb)     ((cb).callback == NULL)
> > +#define CALLBACK_IS_NULL(cb)     ((cb).callback == NULL && (cb).free == NULL)
> 
> Semantic change. In generator, you used CALLBACK_IS_NULL() for both
> Closure and OClosure.  For OClosure, the new semantics are still
> correct.  But for Closure, we now no longer return EFAULT when the
> callback itself is missing but a .free was provided.  This changes
> pread_structured and block_status to accept NULL for the callback; which
> is probably not a good idea.

I'm unclear on this.  This is the point of the patch - that you can
register a callback which has no callback action but still performs
the free action, and that's not considered a null callback.  (It's
considered to be a callback that does nothing and returns 0.)  Does
something bad happen to those two calls in this case?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/