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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Nov 20, 2018 at 09:45:23AM +0400, Marc-André Lureau wrote:
> Hi
> 
> On Mon, Nov 19, 2018 at 5:59 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > 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 <berrange@redhat.com>
> 
> Or maybe change the error_setg* macros? After all, if errp is NULL,
> it's an error qemu can handle.

That would mean that all callers would get short circuited which
I don't think it is desirable in general. 

This QOM usage is special in that the calling code is using the
error scenario as a means to trigger different code flow. IOW
it doesn't consider it an error scenario at all.

> > ---
> >  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;
> >  }
> >
> > @@ -1133,7 +1138,10 @@ ObjectProperty *object_class_property_find(ObjectClass *klass, const char *name,
> >      }
> >
> >      prop = g_hash_table_lookup(klass->properties, name);
> > -    if (!prop) {
> > +    /* 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 (!prop && errp) {
> >          error_setg(errp, "Property '.%s' not found", name);
> >      }
> >      return prop;
> > --
> > 2.19.1
> >
> >
> 
> 
> -- 
> Marc-André Lureau

Regards,
Daniel
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