[libvirt] Exposing feature deprecation to machine clients (was: [Qemu-devel] [PATCH 2/2] qapi: deprecate implicit filters)

John Snow <jsnow(a)redhat.com&gt; writes: [...] This is a compatible evolution of the QMP protocol. 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.fsf(a)dusky.pond.sub.org&gt; 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. 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. 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.