Re: [Qemu-devel] [PATCH 1/2] qapi: Stub out StringOutputVisitor struct support

[Dr. David Alan Gilbert]

* Markus Armbruster (address@hidden) wrote:
> "Dr. David Alan Gilbert" <address@hidden> writes:
> 
> > * Markus Armbruster (address@hidden) wrote:
> >> "Dr. David Alan Gilbert" <address@hidden> writes:
> >> 
> >> > * Markus Armbruster (address@hidden) wrote:
> >> >> "Dr. David Alan Gilbert (git)" <address@hidden> writes:
> >> >> 
> >> >> > From: "Dr. David Alan Gilbert" <address@hidden>
> >> >> >
> >> >> > Avoid a segfault when visiting, e.g., the QOM rtc-time property,
> >> >> > by implementing the struct callbacks and raising an Error.
> >> >> >
> >> >> > Signed-off-by: Andreas Färber <address@hidden>
> >> >> >
> >> >> > Updated for changed interface:
> >> >> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> >> >> > ---
> >> >> >  qapi/string-output-visitor.c | 13 +++++++++++++
> >> >> >  1 file changed, 13 insertions(+)
> >> >> >
> >> >> > diff --git a/qapi/string-output-visitor.c 
> >> >> > b/qapi/string-output-visitor.c
> >> >> > index 94ac821..4e7e97f 100644
> >> >> > --- a/qapi/string-output-visitor.c
> >> >> > +++ b/qapi/string-output-visitor.c
> >> >> > @@ -12,6 +12,7 @@
> >> >> >  
> >> >> >  #include "qemu/osdep.h"
> >> >> >  #include "qemu-common.h"
> >> >> > +#include "qapi/error.h"
> >> >> >  #include "qapi/string-output-visitor.h"
> >> >> >  #include "qapi/visitor-impl.h"
> >> >> >  #include "qemu/host-utils.h"
> >> >> > @@ -266,6 +267,16 @@ static void print_type_number(Visitor *v, const 
> >> >> > char *name, double *obj,
> >> >> >      string_output_set(sov, g_strdup_printf("%f", *obj));
> >> >> >  }
> >> >> >  
> >> >> > +static void start_struct(Visitor *v, const char *name, void **obj, 
> >> >> > size_t size,
> >> >> > +           Error **errp)
> >> >> > +{
> >> >> > +    error_setg(errp, "struct type not implemented");
> >> >> > +}
> >> >> > +
> >> >> > +static void end_struct(Visitor *v, void **obj)
> >> >> > +{
> >> >> > +}
> >> >> > +
> >> >> 
> >> >> This is just one of the several things this visitor doesn't implement.
> >> >> See the comment in string-output-visitor.h.
> >> >> 
> >> >> String input visitor and options visitor have similar holes; see the
> >> >> comments in string-input-visitor.h and opts-visitor.h.
> >> >> 
> >> >> Should we change all of them together to report errors instead of crash?
> >> >> With common "error out because this isn't implemented" methods?
> >> >
> >> > In that case wouldn't it be best to change 
> >> > visit_start_struct/visit_end_struct
> >> > to do the check (Like visit_check_struct does).
> >> 
> >> In my opinion.
> >> 
> >>     if (v->foo) {
> >>         v->foo(...);
> >>     } else {
> >>         ... default action ...
> >>     }
> >> 
> >> is an anti-pattern.  Wrap the default action in a default method, and
> >> put that in the function pointer.
> >
> > I've got some sympathy to that, but with the way our visitors are
> > built that's a pain.
> >
> > Lets say you add a new eat_struct method, and a eat_struct_default 
> > implementation,
> > now you have to go around and fix all the visitor implementations to 
> > initialise
> > their eat_struct member to eat_struct_default.   Of course you'll forget 
> > some
> > and then we'll end up segging when you fall down the NULL pointer.
> >
> > Now, if our visitors had nice shared constructor functions that wouldn't
> > be a problem, and you wouldn't need most of the visit_ wrapper functions;
> > but they don't, so the if (v->foo) { ... } else { error; }   is the
> > current cleanest we can do.
> 
> Well, it's the cleanest we can do as long as we constrain ourselves not
> to do much :)

Yes, although I hate to turn a patchset for a tiny feature into a
fix-all-the-broken-stuff set!

> We currently have seven visitors.  Every single one defines a
> FOO_visitor_new() function that basically looks like this:
> 
>     Visitor *FOO_visitor_new(... whatever ...)
>     {
>         FOOVisitor v = g_malloc0(sizeof(*v));
> 
>         v->visitor.type = ...
>         ... initialize more of v->visitor ...
>         ... initialize other members of *v, if any ...
> 
>         return &v->visitor;
>     }
> 
> I grant you that putting sensible defaults into v->visitor by
> initializing them correctly in all the FOO_visitor_new() functions is a
> bit of pain.  Not much pain; there are only seven.  Anyway, there are
> several obvious ways to do this without pain:
> 
> (1) Have a visitor core function to set the defaults, call it first.
> 
> (2) Replace g_malloc0() by a visitor core function that additionally
>     sets the defaults.  Basically fusing g_malloc0() into (1)'s
>     function.

That's my preferred way of doing it, chaining constructors.

Dave

> (3) Have a visitor core function that replaces null methods by defaults,
>     and call it last.  This function can also check you filled out in
>     the mandatory bits.  Have it return the visitor, so you can make it
>     a tail call: return visitor_check(&v->visitor).
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK