On 8/15/19 10:16 AM, Markus Armbruster wrote:
> John Snow <js...@redhat.com> writes:
>
>> On 8/14/19 6:07 AM, Vladimir Sementsov-Ogievskiy wrote:
>>> To get rid of implicit filters related workarounds in future let's
>>> deprecate them now.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
>>> ---
> [...]
>>> diff --git a/blockdev.c b/blockdev.c
>>> index 36e9368e01..b3cfaccce1 100644
>>> --- a/blockdev.c
>>> +++ b/blockdev.c
>>> @@ -3292,6 +3292,11 @@ void qmp_block_commit(bool has_job_id, const char
>>> *job_id, const char *device,
>>> BlockdevOnError on_error = BLOCKDEV_ON_ERROR_REPORT;
>>> int job_flags = JOB_DEFAULT;
>>>
>>> + if (!has_filter_node_name) {
>>> + warn_report("Omitting filter-node-name parameter is deprecated, it
>>> "
>>> + "will be required in future");
>>> + }
>>> +
>>> if (!has_speed) {
>>> speed = 0;
>>> }
>>> @@ -3990,6 +3995,11 @@ void qmp_blockdev_mirror(bool has_job_id, const char
>>> *job_id,
>>> Error *local_err = NULL;
>>> int ret;
>>>
>>> + if (!has_filter_node_name) {
>>> + warn_report("Omitting filter-node-name parameter is deprecated, it
>>> "
>>> + "will be required in future");
>>> + }
>>> +
>>> bs = qmp_get_root_bs(device, errp);
>>> if (!bs) {
>>> return;
>>>
>>
>> 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.
>
> This is a compatible evolution of the QMP protocol.
>
>> 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.
>
> Yes, we should help libvirt etc. with detecting use of deprecated
> features. We discussed this at the KVM Forum 2018 BoF on deprecating
> stuff. Minutes:
>
> Message-ID: <87mur0ls8o....@dusky.pond.sub.org>
> https://lists.nongnu.org/archive/html/qemu-devel/2018-10/msg05828.html
>
> Last item is relevant here.
>
> Adding deprecation information to QMP's success response belongs to "We
> can also pass the buck to the next layer up", next to "emit a QMP
> event".
>
> Let's compare the two, i.e. "deprecation info in success response"
> vs. "deprecation event".
>
> 1. Possible triggers
>
> Anything we put in the success response should only ever apply to the
> (successful) command. So this one's limited to QMP commands.
>
> A QMP event is not limited to QMP commands. For instance, it could be
> emitted for deprecated CLI features (long after the fact, in addition to
> human-readable warnings on stderr), or when we detect use of a
> deprecated feature only after we sent the success response, say in a
> job. Neither use case is particularly convincing. Reporting use of
> deprecated CLI in QMP feels like a work-around for the CLI's
> machine-unfriendliness. Job-like commands should really check their
> arguments upfront.
>
> 2. Connection to trigger
>
> Connecting responses to commands is the QMP protocol's responsibility.
> Transmitting deprecation information in the response trivially ties it
> to the offending command.
>
> The QMP protocol doesn't tie events to anything. Thus, a deprecation
> event needs an event-specific tie to its trigger.
>
> The obvious way to tie it to a command mirrors how the QMP protocol ties
> responses to commands: by command ID. The event either has to be sent
> just to the offending monitor (currently, all events are broadcast to
> all monitors), or include a suitable monitor ID.
>
> For non-command triggers, we'd have to invent some other tie.
>
> 3. Interface complexity
>
> Tying the event to some arbitrary trigger adds complexity.
>
> Do we need non-command triggers, and badly enough to justify the
> additional complexity?
>
> 4. Implementation complexity
>
> Emitting an event could be as simple as
>
> qapi_event_send_deprecated(qmp_command_id(),
> "Omitting 'filter-node-name'");
>
> where qmp_command_id() returns the ID of the currently executing
> command. Making qmp_command_id() work is up to the QMP core. Simple
> enough as long as each QMP command runs to completion before its monitor
> starts the next one.
>
> The event is "fire and forget". There is no warning object propagated
> up the call chain into the QMP core like errors objects are.
>
> "Fire and forget" is ideal for letting arbitrary code decide "this is
> deprecated".
>
> Note the QAPI schema remains untouched.
>
> Unlike an event, which can be emitted anywhere, the success response
> gets built in the QMP core. To have the core add deprecation info to
> it, we need to get the info to the core.
>
> If deprecation info originates in command code, like errors do, we need
> to propagate it up the call chain into the QMP core like errors.
>
> Propagating errors is painful. It has caused massive churn all over the
> place.
>
> I don't think we can hitch deprecation info to the existing error
> propagation, since we need to take the success path back to the QMP
> core, not an error path.
>
> Propagating a second object for warnings... thanks, but no thanks.
>
Probably the best argument against it. Fire-and-forget avoids the
problem. Events might work just fine, but the "tie" bit seems like a yak
in need of a shave.
> The QMP core could provide a function for recording deprecation info for
> the currently executing QMP command. This is how we used to record
> errors in QMP commands, until Anthony rammed through what we have now.
> The commit messages (e.g. d5ec4f27c38) provide no justification. I
> dimly recall adamant (oral?) claims that recording errors in the Monitor
> object cannot work for us.
>
> I smell a swamp.
>
> Can we avoid plumbing deprecation info from command code to QMP core?
> Only if the QMP core itself can recognize deprecated interfaces. I
> believe it can for the cases we can expose in introspecion. Let me
> explain.
>
> Kevin recently added "features" to the QAPI schema language. The
> implementation is incomplete, but that's detail. The idea is to tack a
> "deprecated" feature to deprecated stuff in the QAPI schema.
>
That's a good idea too; but the semantics of exactly *what* was
deprecated may be hard to capture.
> Commands and arguments need to support features for that.
> Implementation should be relatively straightforward.
>
> Deprecating an argument's optionalness may require a
> "optional-deprecated" feature. I've seen more elegant designs, but I've
> also seen plenty of uglier ones.
>
> Note that features are tied to schema syntax. To express semantically
> conditional deprecation like "if you specify argument FOO, then not
> specifying argument BAR is deprecated", we'd have to add a language for
> these conditions. Uh, not now, maybe never.
>
> The primary use of having deprecation defined in the QAPI schema is
> introspection. The BoF minutes mention this, too.
>
> A secondary use could be detecting use of deprecated features right in
> the QMP core. No need for ad hoc code in commands, no need for plumbing
> information from there to the QMP core.
>
> I'd like to pursue this idea, then see how well it suits our deprecation
> needs.
>
I should clearly remember to talk to you before thinking about QMP in
public, because you've thought about it much more.
--js
