[Libguestfs] [PATCH nbdkit v2 03/10] python: Implement nbdkit API version 2.

[Eric Blake]

On 11/22/19 3:30 PM, Eric Blake wrote:

>>>> +    switch (py_api_version) {
>>>> +    case 1:
>>>> +      r = PyObject_CallFunction (fn, "ONL", obj,
>>>> +                                 PyByteArray_FromStringAndSize 
>>>> (buf, count),
>>>> +                                 offset, NULL);
>>>
>>> Here, we could assert (flags == 0) (the FUA flag should not be set
>>> if the plugin uses v1 API).
>>
>> Is that true?  The plugin asserts that it wants the v1 API, but we are
>> still using the v2 C API, whatever the plugin asks for?
> 
> Hmm.  If the user implements a 'can_fua' callback that returns 1 even 
> though they forgot to declare 'api_version', then the flag can indeed be 
> set.  Perhaps that means our 'can_fua' C wrapper should have a version 
> 1/2 difference in behavior (in v1, raise an error reminding the user to 
> fix their plugin to declare 'api_version', in v2 pass back the result), 
> so that we could indeed then use the assert with a guarantee that it 
> will not trigger on an arbitrary plugin.

In fact, that would be similar to other things we've had to open-code in 
python.c to mirror what plugins.c does automatically: basically, 
plugins.c has:

   /* The plugin must use API version 2 and have .can_fua return
      NBDKIT_FUA_NATIVE before we will pass the FUA flag on. */
   if (p->plugin.can_fua) {
     r = p->plugin.can_fua (handle);
     if (r > NBDKIT_FUA_EMULATE && p->plugin._api_version == 1)
       r = NBDKIT_FUA_EMULATE;
     return r;

and so python.c has to have the same logic when dealing with a v1 plugin 
but exposing the v2 interface back to nbdkit.  Compare to things like 
py_can_write(), which has:


   /* No Python can_write callback, but there's a Python pwrite callback
    * defined, so return 1.  (In C modules, nbdkit would do this).
    */
   else if (callback_defined ("pwrite", NULL))
     return 1;

for a similar duplication in logic to what plugins.c can do.  I suspect 
I'll run into this later in the series.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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