Re: [RFC PATCH 0/4] qom: Introduce object_class_property_deprecate()

[Daniel P . Berrangé]

On Wed, Jan 11, 2023 at 10:55:47AM +0100, Philippe Mathieu-Daudé wrote:
> On 10/1/23 14:02, Kevin Wolf wrote:
> > Am 09.01.2023 um 23:54 hat Philippe Mathieu-Daudé geschrieben:
> > > Hi,
> > > 
> > > There will always be a need to deprecate things. Here I'm
> > > tackling the QOM (class) properties, since they can be set
> > > from some CLI options (-object -device -global ...).
> > > 
> > > As an experiment, we add object_class_property_deprecate()
> > > to register a class property as deprecated (since some version),
> > > then we deprecate the TYPE_PFLASH_CFI02 'width' property, and
> > > finally as a bonus we emit a warning when the deprecation period
> > > is over, as a reminder. (For that we introduce few 'versions'
> > > helpers).
> > 
> > The last part means that increasing the version number (i.e. the commit
> > that opens the development tree for the next release) can change the
> > output, and this is turn can break test cases.
> > 
> > If we are happy to introduce breakage with a version number change that
> > will require future commits to open the development tree less trivial
> > than they are today because they need to fix the breakage, too, why not
> > make it a build error instead of a different warning message at runtime?
> 
> To avoid build breakages, maybe it is clever is to store the deprecation
> version in ObjectPropertyInfo and let QAPI inspection scripts enumerate
> / report deprecated features?

I don't think we want the version information in the code nor
introspectable at all.

We want applications to only apply logic based off features that are
actually available, not predicted future versions where something may
or may not be removed. This is why we exposed only a plain 'deprecated'
boolean field in QAPI schema for other deprecations.  This is just a
warning to be ready for something to change in future. If an application
has not been updated they are fine to carry on using the deprecated
feature. If an application has been updated, they should probe for
existance of the new feature and use that if available, in preference
to the deprecated feature. There's no reason for an application to
consider version numbers.

With regards,
Daniel
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