Re: [PATCH RFC 0/5] Subject: [PATCH RFC 0/5] qapi: Add feature flags to enum members

[Vladimir Sementsov-Ogievskiy]

Great! Thanks for working on this!

15.09.2021 22:24, Markus Armbruster wrote:
> PATCH 1+2 add feature flags to enum members.  Awkward due to an
> introspection design mistake; see PATCH 1 for details.  Feedback
> welcome, in particular from management application guys.
>
> PATCH 3+4 implement policy deprecated-input={reject,crash} for enum
> values.
>
> Policy deprecated-output=hide is not implemented, because we can't
> hide a value without hiding the entire member, which is almost
> certainly more than the requester of this policy bargained for.
> Perhaps we want a new policy deprecated-output=crash to help us catch
> unwanted use of deprecated enum values.  Thoughts?

I think crash policy for output is doubtful. If we have query-* command that returns a 
lot of things and some of them are deprecated the whole command will always crash..  
Deprecated is "use" of the deprecated field, but we can't control does user use 
a specific field or not.

If some deprecated output is a consequence of deprecated input, we'd better 
always show it.. Or in other words, we shouldn't deprecate such output, 
deprecating of the corresponding input is enough.

So, we are saying about showing some internal state to the user. And part of 
this state becomes deprecated because internal implementation changed. I think 
the only correct thing to do is handle deprecated=hide by hand, in the place 
where we set this deprecated field. Only at this place we can decide, should we 
simulate old deprecated output value or not. For this we'll need a possibility 
to get current policy at any place, but that doesn't seem to be a problem, I 
see, it's just a global variable compat_policy.

> PATCH 5 puts the new feature flags to use.  It makes sense only on top
> of Vladimir's deprecation of drive-backup.  See its commit message for
> a reference.
>
> Based on my "[PATCH 00/22] qapi: Remove simple unions from the schema
> language".
>
> Based-on: Message-Id: <20210913123932.3306639-1-armbru@redhat.com>
>
> Markus Armbruster (5):
>    qapi: Enable enum member introspection to show more than name
>    qapi: Add feature flags to enum members
>    qapi: Move compat policy from QObject to generic visitor
>    qapi: Implement deprecated-input={reject,crash} for enum values
>    block: Deprecate transaction type drive-backup
>
>   docs/devel/qapi-code-gen.rst                  |  4 ++-
>   qapi/compat.json                              |  3 +++
>   qapi/introspect.json                          | 23 ++++++++++++++--
>   qapi/transaction.json                         |  5 +++-
>   include/qapi/qobject-input-visitor.h          |  4 ---
>   include/qapi/qobject-output-visitor.h         |  4 ---
>   include/qapi/util.h                           |  6 ++++-
>   include/qapi/visitor-impl.h                   |  3 +++
>   include/qapi/visitor.h                        |  9 +++++++
>   qapi/qapi-visit-core.c                        | 27 ++++++++++++++++---
>   qapi/qmp-dispatch.c                           |  4 +--
>   qapi/qobject-input-visitor.c                  | 14 +---------
>   qapi/qobject-output-visitor.c                 | 14 +---------
>   scripts/qapi/expr.py                          |  3 ++-
>   scripts/qapi/introspect.py                    | 19 ++++++++++---
>   scripts/qapi/schema.py                        | 22 +++++++++++++--
>   scripts/qapi/types.py                         | 17 +++++++++++-
>   tests/qapi-schema/doc-good.json               |  5 +++-
>   tests/qapi-schema/doc-good.out                |  3 +++
>   tests/qapi-schema/doc-good.txt                |  3 +++
>   .../qapi-schema/enum-dict-member-unknown.err  |  2 +-
>   tests/qapi-schema/qapi-schema-test.json       |  3 ++-
>   tests/qapi-schema/qapi-schema-test.out        |  1 +
>   tests/qapi-schema/test-qapi.py                |  1 +
>   24 files changed, 144 insertions(+), 55 deletions(-)


--
Best regards,
Vladimir