From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49145) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bm0ea-0001u8-PX for qemu-devel@nongnu.org; Mon, 19 Sep 2016 11:38:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bm0eV-0000wQ-LY for qemu-devel@nongnu.org; Mon, 19 Sep 2016 11:38:43 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55132) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bm0eV-0000wM-BZ for qemu-devel@nongnu.org; Mon, 19 Sep 2016 11:38:39 -0400 Date: Mon, 19 Sep 2016 16:38:32 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20160919153831.GE2041@work-vm> References: <1472117833-10236-1-git-send-email-dgilbert@redhat.com> <1472117833-10236-2-git-send-email-dgilbert@redhat.com> <87wpi8f310.fsf@dusky.pond.sub.org> <20160919134736.GC2041@work-vm> <871t0gdlj0.fsf@dusky.pond.sub.org> <20160919142133.GD2041@work-vm> <8737kvc3lk.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <8737kvc3lk.fsf@dusky.pond.sub.org> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 1/2] qapi: Stub out StringOutputVisitor struct support List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: kwolf@redhat.com, qemu-devel@nongnu.org, lcapitulino@redhat.com, arei.gonglei@huawei.com, pbonzini@redhat.com, afaerber@suse.de * Markus Armbruster (armbru@redhat.com) wrote: > "Dr. David Alan Gilbert" writes: >=20 > > * Markus Armbruster (armbru@redhat.com) wrote: > >> "Dr. David Alan Gilbert" writes: > >>=20 > >> > * Markus Armbruster (armbru@redhat.com) wrote: > >> >> "Dr. David Alan Gilbert (git)" writes: > >> >>=20 > >> >> > From: "Dr. David Alan Gilbert" > >> >> > > >> >> > 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 F=E4rber > >> >> > > >> >> > Updated for changed interface: > >> >> > Signed-off-by: Dr. David Alan Gilbert > >> >> > --- > >> >> > 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 @@ > >> >> > =20 > >> >> > #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)); > >> >> > } > >> >> > =20 > >> >> > +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) > >> >> > +{ > >> >> > +} > >> >> > + > >> >>=20 > >> >> This is just one of the several things this visitor doesn't imple= ment. > >> >> See the comment in string-output-visitor.h. > >> >>=20 > >> >> String input visitor and options visitor have similar holes; see = the > >> >> comments in string-input-visitor.h and opts-visitor.h. > >> >>=20 > >> >> 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/visi= t_end_struct > >> > to do the check (Like visit_check_struct does). > >>=20 > >> In my opinion. > >>=20 > >> if (v->foo) { > >> v->foo(...); > >> } else { > >> ... default action ... > >> } > >>=20 > >> is an anti-pattern. Wrap the default action in a default method, an= d > >> 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 im= plementation, > > 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 for= get 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 would= n't > > be a problem, and you wouldn't need most of the visit_ wrapper functi= ons; > > but they don't, so the if (v->foo) { ... } else { error; } is the > > current cleanest we can do. >=20 > 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: >=20 > Visitor *FOO_visitor_new(... whatever ...) > { > FOOVisitor v =3D g_malloc0(sizeof(*v)); >=20 > v->visitor.type =3D ... > ... initialize more of v->visitor ... > ... initialize other members of *v, if any ... >=20 > return &v->visitor; > } >=20 > 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: >=20 > (1) Have a visitor core function to set the defaults, call it first. >=20 > (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 / dgilbert@redhat.com / Manchester, UK