Re: [Qemu-devel] [Qemu-block] [PATCH 2/2] qapi: deprecate implicit filters

[John Snow <jsnow@redhat.com>]

On 8/29/19 11:59 AM, Christophe de Dinechin wrote:
> 
> John Snow writes:
> [...]
>>
>> This might be OK to do right away, though.
>>
>> I asked Markus this not too long ago; do we want to amend the QAPI
>> schema specification to allow commands to return with "Warning" strings,
>> or "Deprecated" stings to allow in-band deprecation notices for cases
>> like these?
>>
>> example:
>>
>> { "return": {},
>>   "deprecated": True,
>>   "warning": "Omitting filter-node-name parameter is deprecated, it will
>> be required in the future"
>> }
>>
>> There's no "error" key, so this should be recognized as success by
>> compatible clients, but they'll definitely see the extra information.
>>
>> Part of my motivation is to facilitate a more aggressive deprecation of
>> legacy features by ensuring that we are able to rigorously notify users
>> through any means that they need to adjust their scripts.
> 
> I like this approach even if there is no consumer today. It does not
> hurt, and it is indeed a motivation to develop consumers that care.
> 
> I personally find this much easier to swallow than any kind of crash on
> deprecation, which already at the BoF seemed like a really big hammer to
> kill a fly.
> 
> CC'ing Andrea as well, because we discussed recently about how to deal
> with error checking in general, and if a new error checking framework is
> being put in place, adding deprecation to the thinking could be a good
> idea.

The most convincing argument against deprecation notices like this is
not that they won't be consumed, but that they are difficult to plumb
through the C infrastructure.

Sadly, I think I have to agree there -- we can't even really model it
like hints, because these are cases where there was no /error/ but
instead a success -- but our error propagation doesn't work on those
terms generally and we'd need a rather extensive audit to allow warnings.

We could always fudge it with a kind of global warning log: clear the
log at the beginning of a QMP interaction and if the log is non-empty
when we return, amend the return with that information.

That's not really the nicest thing to do in a multi-process,
multi-threaded, multi-stacked application, though, so...

--js