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

[Alberto Garcia]

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."

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.

Berto