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Re: [Qemu-devel] [PATCH v1 3/6] qapi: assert that visitor impls have req
From: |
Daniel P. Berrange |
Subject: |
Re: [Qemu-devel] [PATCH v1 3/6] qapi: assert that visitor impls have required callbacks |
Date: |
Tue, 7 Jun 2016 16:46:53 +0100 |
User-agent: |
Mutt/1.6.0 (2016-04-01) |
On Tue, Jun 07, 2016 at 09:40:25AM -0600, Eric Blake wrote:
> On 06/07/2016 04:11 AM, Daniel P. Berrange wrote:
> > Not all visitor implementations supply the full set of
> > visitor callback functions. For example, the string
> > output visitor does not provide 'start_struct' and
> > friends. If you don't know this and feed it an object
> > that uses structs, you'll get a crash:
> >
> > Segmentation fault (core dumped)
> >
> > Crashing is fine, because this is a programmer mistake,
> > but we can improve the error message upon crash to make
> > it obvious what failed by adding assert()s:
> >
> > qapi/qapi-visit-core.c:32: visit_start_struct: Assertion `v->start_struct
> > != ((void *)0)' failed.
> >
> > Signed-off-by: Daniel P. Berrange <address@hidden>
> > ---
> > qapi/qapi-visit-core.c | 15 +++++++++++++++
> > 1 file changed, 15 insertions(+)
>
> Up to Markus if he likes this (I think I've proposed the idea, but never
> actually written it as a patch, because he implied that the core dump
> still points people in the right direction).
I think from the error message I show in the description above
that the new crash is much more "user friendly" - if I saw a
bug report with that assertion message I'd be more likely to
dive in an fix it becasue it's obvious where the issue is,
while a report with just a "Segmentation fault" could be anything.
> > diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
> > index eada467..3b5efbe 100644
> > --- a/qapi/qapi-visit-core.c
> > +++ b/qapi/qapi-visit-core.c
> > @@ -29,6 +29,7 @@ void visit_start_struct(Visitor *v, const char *name,
> > void **obj,
> > assert(size);
> > assert(v->type != VISITOR_OUTPUT || *obj);
> > }
> > + assert(v->start_struct != NULL);
>
> I'd have written it assert(v->start_struct) (explicit comparison against
> NULL matters in Java, but is just line noise in C).
The only reason I included the != NULL, is that it makes
the corresponding assertion message printed a little
more obvious
eg
Assertion `v->start_struct != ((void *)0)' failed.
vs
Assertion `v->start_struct' failed.
Sure, seasoned C programmers will know they're the same, but the less
experianced would likely find the former more obvious at first glance
Regards,
Daniel
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