Re: [Qemu-devel] [Qemu-block] [PATCH v2 2/3] error: Remove unnecessary local_err variables

[Eric Blake <eblake@redhat.com>]

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On 06/13/2016 09:52 AM, Eduardo Habkost wrote:

>>
>> There is an (ugly) difference between
>>
>>     error_setg(&local_err, ...);
>>     error_propagate(errp, &local_err);
>>
>> and
>>
>>     error_setg(errp, ...);
>>
>> The latter aborts when @errp already contains an error, the former does
>> nothing.
> 
> Why the difference? Couldn't we change that so that both are equivalent?

Maybe, but I think it weakens our position. An abort() on an attempt to
incorrectly set an error twice helps catch errors where we are throwing
away a more useful first error message.  The documentation for
error_propagate() already mentioned that it was an exception to the rule.

> 
>>
>> Your transformation has the error_setg() or similar hidden in F2().  It
>> can add aborts.
>>
>> I think it can be salvaged: we know that @errp must not contain an error
>> on function entry.  If @errp doesn't occur elsewhere in this function,
>> it cannot pick up an error on the way to the transformed spot.  Can you
>> add that to your when constraints?
> 
> Do we really know that *errp is NULL on entry? Aren't we allowed to call
> functions with a non-NULL *errp?

Except for error_propagate(), no.

> 
> See, e.g.:
> 
> void qmp_guest_suspend_disk(Error **errp)
> {
>     Error *local_err = NULL;
>     GuestSuspendMode *mode = g_new(GuestSuspendMode, 1);
> 
>     *mode = GUEST_SUSPEND_MODE_DISK;
>     check_suspend_mode(*mode, &local_err);
>     acquire_privilege(SE_SHUTDOWN_NAME, &local_err);
>     execute_async(do_suspend, mode, &local_err);

That usage is a bug.  A Coccinelle script to root out such buggy
instances would be nice.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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