Re: [PATCH 1/9] qapi: New special feature flag "unstable"

By convention, names starting with "x-" are experimental. The parts of external interfaces so named may be withdrawn or changed incompatibly in future releases. Drawback: promoting something from experimental to stable involves a name change. Client code needs to be updated. Moreover, the convention is not universally observed: * QOM type "input-barrier" has properties "x-origin", "y-origin". Looks accidental, but it's ABI since 4.2. * QOM types "memory-backend-file", "memory-backend-memfd", "memory-backend-ram", and "memory-backend-epc" have a property "x-use-canonical-path-for-ramblock-id" that is documented to be stable despite its name. We could document these exceptions, but documentation helps only humans. We want to recognize "unstable" in code, like "deprecated". Replace the convention by a new special feature flag "unstable". It will be recognized by the QAPI generator, like the existing feature flag "deprecated", and unlike regular feature flags. This commit updates documentation and prepares tests. The next commit updates the QAPI schema. The remaining patches update the QAPI generator and wire up -compat policy checking. Signed-off-by: Markus Armbruster <armbru(a)redhat.com&gt;