Re: [Qemu-devel] [RFC PATCH v2 0/7] qapi: Specify default value for optional argument in schema json

[Fam Zheng]

On Wed, 05/21 10:23, Kevin Wolf wrote:
> Am 21.05.2014 um 09:46 hat Markus Armbruster geschrieben:
> > Fam Zheng <address@hidden> writes:
> > 
> > > On Wed, 05/21 07:54, Markus Armbruster wrote:
> > >> Fam Zheng <address@hidden> writes:
> > >> 
> > >> > On Tue, 05/20 13:13, Eric Blake wrote:
> > >> >> On 05/20/2014 03:07 AM, Fam Zheng wrote:
> > >> >> > Please first take a look at patch 7 to see what is supported by 
> > >> >> > this series.
> > >> >> > 
> > >> >> > Patch 1 ~ 3 allows some useful basic types in schema.
> > >> >> > 
> > >> >> > Patch 4 ~ 6 implements the new syntax.
> > >> >> > 
> > >> >> > Note: The introduced '@arg' sigil, just like the preexisting 
> > >> >> > '*arg', is
> > >> >> > reducing the cleanness of the syntax. We should get rid of both of 
> > >> >> > them in long
> > >> >> > term. Here, this series compromises on this and introduces '@arg' 
> > >> >> > because:
> > >> >> > 
> > >> >> >   - We have to distinguish the argument property dictionary from 
> > >> >> > nested struct:
> > >> >> > 
> > >> >> >     I.e.:
> > >> >> > 
> > >> >> >         'data': {
> > >> >> >             'arg1': { 'member1': 'int', 'member2': 'str' }
> > >> >> >             '@arg2': { 'type': 'int', 'default': 100 }
> > >> >> >          }
> > >> >> > 
> > >> >> >     Until we completely drop and forbid the 'arg1' nested struct 
> > >> >> > use case.
> > >> >> > 
> > >> >> >   - Forbidding 'arg1' it's doable, but doing it now means we pull 
> > >> >> > in many
> > >> >> >     distractive patches to this series.
> > >> >> 
> > >> >> Question - since we WANT to get rid of nested struct, why not reverse
> > >> >> the sense?  Mark all existing nested structs (weren't there just three
> > >> >> that we found?) with the '@' sigil, and let the new syntax be
> > >> >> sigil-free.  Then when we clean up the nesting, we are also getting 
> > >> >> rid
> > >> >> of the bad syntax, plus the sigil gives us something to search for in
> > >> >> knowing how much to clean up.  But if you stick the sigil on the new
> > >> >> code, instead of the obsolete code, then as more and more places in 
> > >> >> the
> > >> >> schema use defaults, it gets harder and harder to remove the use of 
> > >> >> the
> > >> >> sigil even if the nested structs are eventually removed.
> > >> >> 
> > >> >
> > >> > It makes not much difference I can see. The hard part is actaully 
> > >> > dropping
> > >> > nested, converting from sigil <-> non-sigil is easy. Of course, 
> > >> > nothing is
> > >> > seriously hard, there are only three nested structs plus some more
> > >> > qapi-schema
> > >> > test code.
> > >> 
> > >> Adding three ugly sigils and making everybody include one when they add
> > >> a nested struct feels much better to me than ugly sigils all over the
> > >> place.
> > >
> > > Well, I could use some background here. Why did we introduce nested 
> > > structure
> > > in the first place?
> > 
> > Because we could?
> > 
> > Felt like a good idea at the time?
> > 
> > I quick glance at commit 0f923be and fb3182c suggests they have been
> > supported since the beginning.  There is no design rationale.
> 
> Let me extend Fam's question: Why don't we simply remove them right
> now? If it's really only three instances, converting them to full
> types should be a matter of five minutes.
> 

Actually, my question is: do we want it independently, or do we want to include
the removal of nested as the first part of this series?

I would prefer the former because I feel uncomfortable with making more changes
in this series, since there are already many things to do: adding qapi types,
adding argument property dict, adding all test cases for all of them, updating
documentation, and apply the new syntax in qapi-schema.json. A non-RFC revision
could be long and hard to review.

Fam