Re: [PATCH v2 12/13] block/qcow2: simplify qcow2_co_invalidate_cache()

[Greg Kurz]

On Fri, 18 Sep 2020 19:01:34 +0300
Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> wrote:

> 18.09.2020 18:51, Alberto Garcia wrote:
> > On Fri 18 Sep 2020 05:30:06 PM CEST, Greg Kurz wrote:
> >>> qcow2_do_open correctly sets errp on each failure path. So, we can
> >>> simplify code in qcow2_co_invalidate_cache() and drop explicit error
> >>> propagation. We should use ERRP_GUARD() (accordingly to comment in
> >>> include/qapi/error.h) together with error_append() call which we add to
> >>> avoid problems with error_fatal.
> >>>
> >>
> >> The wording gives the impression that we add error_append() to avoid 
> >> problems
> >> with error_fatal which is certainly not true. Also it isn't _append() but
> >> _prepend() :)
> >>
> >> What about ?
> >>
> >> "Add ERRP_GUARD() as mandated by the documentation in include/qapi/error.h
> >>   to avoid problems with the error_prepend() call if errp is
> >>   &error_fatal."
> 
> OK for me.
> 
> > 
> > I had to go to the individual error functions to see what "it doesn't
> > work with &error_fatal" actually means.
> > 
> > So in a case like qcow2_do_open() which has:
> > 
> >     error_setg(errp, ...)
> >     error_append_hint(errp, ...)
> > 
> > As far as I can see this works just fine without ERRP_GUARD() and with
> > error_fatal, the difference is that if we don't use the guard then the
> > process exists during error_setg(), and if we use the guard it exists
> > during the implicit error_propagate() call triggered by its destruction
> > at the end of the function. In this latter case the printed error
> > message would include the hint.
> > 
> 
> Yes the only problem is that without ERRP_GUARD we lose the hint in case of 
> error_fatal.
> 

Yeah, so rather:

"Add ERRP_GUARD() as mandated by the documentation in include/qapi/error.h
 so that error_prepend() is actually called even if errp is &error_fatal."

Cheers,

--
Greg