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

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.