Re: [PATCH v4 04/31] error: auto propagated local_err

[Eric Blake <eblake@redhat.com>]

On 10/1/19 10:52 AM, Vladimir Sementsov-Ogievskiy wrote:
> Here is introduced ERRP_AUTO_PROPAGATE macro, to be used at start of
> functions with errp OUT parameter.
> 
> It has three goals:
> 
> 1. Fix issue with error_fatal & error_prepend/error_append_hint: user
> can't see this additional information, because exit() happens in
> error_setg earlier than information is added. [Reported by Greg Kurz]
> 
> 2. Fix issue with error_abort & error_propagate: when we wrap
> error_abort by local_err+error_propagate, resulting coredump will
> refer to error_propagate and not to the place where error happened.
> (the macro itself doesn't fix the issue, but it allows to [3.] drop all
> local_err+error_propagate pattern, which will definitely fix the issue)
> [Reported by Kevin Wolf]
> 
> 3. Drop local_err+error_propagate pattern, which is used to workaround
> void functions with errp parameter, when caller wants to know resulting
> status. (Note: actually these functions could be merely updated to
> return int error code).
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---

I'm still hoping Markus gets some review time for this series with a 
chance to speak up (and that we haven't been causing you a lot of work 
which would all have to change if Markus has a different viewpoint).

Reviewed-by: Eric Blake <eblake@redhat.com>

> +/*
> + * ERRP_AUTO_PROPAGATE
> + *
> + * This macro is created to be the first line of a function with Error **errp
> + * OUT parameter. It's needed only in cases where we want to use error_prepend,
> + * error_append_hint or dereference *errp. It's still safe (but useless) in
> + * other cases.
> + *
> + * If errp is NULL or points to error_fatal, it is rewritten to point to a
> + * local Error object, which will be automatically propagated to the original
> + * errp on function exit (see error_propagator_cleanup).
> + *
> + * After invocation of this macro it is always safe to dereference errp
> + * (as it's not NULL anymore) and to append hints (by error_append_hint)

Maybe: 'to append information (by error_prepend or error_append_hint)'

> + * (as, if it was error_fatal, we swapped it with a local_error to be
> + * propagated on cleanup).
> + *
> + * Note: we don't wrap the error_abort case, as we want resulting coredump
> + * to point to the place where the error happened, not to error_propagate.
> + */
> +#define ERRP_AUTO_PROPAGATE() \
> +g_auto(ErrorPropagator) __auto_errp_prop = {.errp = errp}; \
> +errp = ((errp == NULL || *errp == error_fatal) ? \
> +    &__auto_errp_prop.local_err : errp)
> +
>   /*
>    * Special error destination to abort on error.
>    * See error_setg() and error_propagate() for details.
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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