[PATCH v3 00/30] qapi/qom: QAPIfy --object and object-add

[Kevin Wolf]

Am 15.03.2021 um 16:26 hat Markus Armbruster geschrieben:
> Kevin Wolf <kwolf at redhat.com> writes:
> 
> > Am 13.03.2021 um 14:40 hat Markus Armbruster geschrieben:
> >> Markus Armbruster <armbru at redhat.com> writes:
> >> 
> >> > Paolo Bonzini <pbonzini at redhat.com> writes:
> >> >
> >> >> On 11/03/21 15:08, Markus Armbruster wrote:
> >> >>>> I would rather keep the OptsVisitor here.  Do the same check for JSON
> >> >>>> syntax that you have in qobject_input_visitor_new_str, and whenever
> >> >>>> you need to walk all -object arguments, use something like this:
> >> >>>>
> >> >>>>      typedef struct ObjectArgument {
> >> >>>>          const char *id;
> >> >>>>          QDict *json;    /* or NULL for QemuOpts */
> >> >>>>          QSIMPLEQ_ENTRY(ObjectArgument) next;
> >> >>>>      }
> >> >>>>
> >> >>>> I already had patches in my queue to store -object in a GSList of
> >> >>>> dictionaries, changing it to use the above is easy enough.
> >> >>> 
> >> >>> I think I'd prefer following -display's precedence.  See my reply to
> >> >>> Kevin for details.
> >> >>
> >> >> Yeah, I got independently to the same conclusion and posted patches
> >> >> for that.  I was scared that visit_type_ObjectOptions was too much for 
> >> >> OptsVisitor but it seems to work...
> >> >
> >> > We have reason to be scared.  I'll try to cover this in my review.
> >> 
> >> The opts visitor has serious limitations.  From its header:
> >> 
> >>  * The Opts input visitor does not implement support for visiting QAPI
> >>  * alternates, numbers (other than integers), null, or arbitrary
> >>  * QTypes.  It also requires a non-null list argument to
> >>  * visit_start_list().
> >> 
> >> This is retro-documentation for hairy code.  I don't trust it.  Commit
> >> eb7ee2cbeb "qapi: introduce OptsVisitor" hints at additional
> >> restrictions:
> >> 
> >>     The type tree in the schema, corresponding to an option with a
> >>     discriminator, must have the following structure:
> >>     
> >>       struct
> >>         scalar member for non-discriminated optarg 1 [*]
> >>         list for repeating non-discriminated optarg 2 [*]
> >>           wrapper struct
> >>             single scalar member
> >>         union
> >>           struct for discriminator case 1
> >>             scalar member for optarg 3 [*]
> >>             list for repeating optarg 4 [*]
> >>               wrapper struct
> >>                 single scalar member
> >>             scalar member for optarg 5 [*]
> >>           struct for discriminator case 2
> >>             ...
> >
> > Is this a long-winded way of saying that it has to be flat, except that
> > it allows lists, i.e. there must be no nested objects on the "wire"?
> 
> I think so.
> 
> > The difference between structs and unions, and different branches inside
> > the union isn't visible for the visitor anyway.
> 
> Yes, only the code using the visitor deals with that.
> 
> >>     The "type" optarg name is fixed for the discriminator role. Its schema
> >>     representation is "union of structures", and each discriminator value must
> >>     correspond to a member name in the union.
> >>     
> >>     If the option takes no "type" descriminator, then the type subtree rooted
> >>     at the union must be absent from the schema (including the union itself).
> >>     
> >>     Optarg values can be of scalar types str / bool / integers / size.
> >> 
> >> Unsupported visits are treated as programming error.  Which is a nice
> >> way to say "they crash".
> >
> > The OptsVisitor never seems to crash explicitly by calling something
> > like abort().
> >
> > It may crash because of missing callbacks that are called without a NULL
> > check, like v->type_null.
> 
> Correct.
> 
> >                           This case should probably be fixed in
> > qapi/qapi-visit-core.c to do the check and simply return an error.
> 
> I retro-documented what I inherited: qapi-visit-core.c code expects the
> visitors to implement the full visitor-impl.h interface, but some
> visitors don't.  So I documented "method must be set to visit FOOs" in
> visitor-impl.h, and for the visitors that don't, I documented "can't
> visit FOOs".
> 
> If the crashing behavior we've always had gets in the way, there are two
> ways to change it:
> 
> 1. Complicate qapi-visit-core.c slightly to cope with incomplete visitor
>    implementations.
> 
> 2. Complete the visitor implementations: add dummy callbacks that fail.
> 
> I prefer 2., because I feel it keeps the visitor-impl.h interface
> simpler, and puts the extra complications where they belong.

I suggested making the callbacks optional because I expected that there
might be more than one visitor that doesn't support a callback and I
wouldn't like duplicating dummy callbacks in multiple places. But if
it's only the OptsVisitor, then we wouldn't get any duplication either
way and it becomes a matter of taste.

Kevin