Re: [Qemu-devel] [PATCH] qom: avoid reporting errors for NULL error object

[Daniel P . Berrangé]

On Tue, Nov 20, 2018 at 07:16:31PM +0100, Markus Armbruster wrote:
> Daniel P. Berrangé <address@hidden> writes:
> 
> > When debugging QEMU it is often useful to put a breakpoint on the
> > error_setg_internal method impl.
> >
> > Unfortunately the object_property_add / object_class_property_add
> > methods call object_property_find / object_class_property_find methods
> > to check if a property exists already before adding the new property.
> >
> > As a result there are a huge number of calls to error_setg_internal
> > on startup of most QEMU commands, making it very painful to set a
> > breakpoint on this method.
> >
> > This puts a minor optimization on the code so that we avoid calling
> > error_setg() when errp is NULL. Functionally there's no difference
> > since error_setg() is a no-op when errp is NULL, but this lets us
> > use breakpoints in GDB in a practical way.
> >
> > Signed-off-by: Daniel P. Berrangé <address@hidden>
> > ---
> >  qom/object.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/qom/object.c b/qom/object.c
> > index 547dcf97c3..ddd5e7a30e 100644
> > --- a/qom/object.c
> > +++ b/qom/object.c
> > @@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj, 
> > const char *name,
> >          return prop;
> >      }
> >  
> > -    error_setg(errp, "Property '.%s' not found", name);
> > +    /* Optimized to avoid calling error_setg if errp == NULL
> > +     * otherwise every property add call hits error_setg
> > +     * making it impratical to set breakpoints in GDB */
> > +    if (errp) {
> > +        error_setg(errp, "Property '.%s' not found", name);
> > +    }
> >      return NULL;
> >  }
> >  
> 
> In my opinion, this function's design is awkward.  Stress on *opinion*.
> 
> On success, it returns a (non-null) pointer.
> 
> On failure, it sets an error and returns null.  Note that it has just
> one failure mode: "Property '.%s' not found".  Setting an error is just
> a convenience for those callers that want to propagate exactly this
> error to their callers.
> 
> I count 30 callers.  Only six pass a non-NULL argument to @errp.
> 
> I'd rather have a pair of functions similar to how Python has both
> .get() and .__getitem__(): the former doesn't fail, but returns None
> instead, and the latter does fail, throwing KeyError.  In QEMU, we can't
> throw, so we set an error.  Here's the obvious code:
> 
>     ObjectProperty *object_property_find(Object *obj, const char *name)
>     {
>         ObjectProperty *prop;
>         ObjectClass *klass = object_get_class(obj);
> 
>         prop = object_class_property_find(klass, name, NULL);
>         if (prop) {
>             return prop;
>         }
> 
>         prop = g_hash_table_lookup(obj->properties, name);
>         if (prop) {
>             return prop;
>         }
> 
>         return NULL;
>     }
> 
>     ObjectProperty *object_property_find_err(Object *obj, const char *name,
>                                              Error **errp)
>     {
>         ObjectProperty *prop = object_property_find(obj, name);
> 
>         if (!prop) {
>             error_setg(errp, "Property '.%s' not found", name);
>         }
>         return prop;
>     }

Yes, I'd be happy with this approach as I think it is more useful in
general


Regards,
Daniel
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