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

From: Alberto Garcia
Subject: Re: [PATCH v2 12/13] block/qcow2: simplify qcow2_co_invalidate_cache()
Date: Fri, 18 Sep 2020 17:51:30 +0200
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

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.


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