Re: [Qemu-devel] [PATCH RFC v2 29/47] qapi: Replace dirty is_c_ptr() by method c_null()

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 07/29/2015 02:32 AM, Markus Armbruster wrote:
>
>>>> 2. We can leak retval only when qmp_FOO() returns non-null and local_err
>>>>    is non-null.  This must not happen, because:
>>>>
>>>>    a. local_err must be null before the call, and
>>>>
>>>>    b. the call must not return non-null when it sets local_err.
>>>
>>> We don't state that contract anywhere, but I doubt any of the qmp_FOO()
>>> functions violate it, so it is worth making it part of the contract.
>> 
>> It's a general Error API rule: set an error exactly on failure.  It
>> applies to any function returning errors through an Error **errp
>> parameter, and we generally don't bother to spell it out for the
>> individual functions.
>> 
>> The part that needs to be spelling out is what success and failure mean.
>> A qmp_FOO() returning an object returns null on failure.
>> 
>>>>    We could right after out: assert(!local_err || !retval).  Not sure
>>>>    it's worthwhile.
>>>
>>> I think it IS worthwhile, because it would catch buggy callers.  Not
>> 
>> We use the same assumption all over the place, without asserting it.
>
> Okay, you've got a point there.
>
>> 
>> Let's drop the useless initializer.  As explained above, I don't like
>> the assertion for reasons explained above, but if you want it, I'm
>> willing to take it anyway, in a separate follow-up patch.
>> 
>> I'd prefer to drop the initializer myself myself (with you credited in
>> the commit message), because it's certainly less total work, and quite
>> possibly less work for just for me.
>
> No need to add assertions.  Maybe worth a patch to add a comment
> somewhere, maybe as a new section in docs/qapi-code-gen.txt, documenting
> how to write a handler and hook it into qmp-commands.hx, and what
> conditions the handler must obey.  And I'm fine with you dropping the
> initializer as part of your v3 series.

Sounds like a plan.  Thanks!