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

[Eric Blake]

On 8/15/19 7:02 AM, Richard W.M. Jones wrote:

>>>  
>>>  /* 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?

I guess nothing bad happens (for block status, you have basically turned
your command into nothing more than a success/failure check of if the
server replied, but you don't get any status; for pread_structured,
lib/rw.c actually forwards .pread to .pread_structured with
.callback=NULL under the hood).  For debug, it means you can register a
notifier for when someone else sets/clears the debug notifier, but still
get the default debug notifications.

Ok, the semantic change doesn't hurt.

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

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