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Date: Thu, 01 Dec 2011 07:31:56 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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	<20111201083316.GD29861@stefanha-thinkpad.localdomain>
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Subject: Re: [Qemu-devel] [PATCH 02/18] qom: register legacy properties as
 new style properties
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Peter Maydell <peter.maydell@linaro.org>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, Markus Armbruster <armbru@redhat.com>, Luiz Capitulino <lcapitulino@redhat.com>

On 12/01/2011 02:33 AM, Stefan Hajnoczi wrote:
> On Wed, Nov 30, 2011 at 03:03:32PM -0600, Anthony Liguori wrote:
>> +static void qdev_set_legacy_property(DeviceState *dev, Visitor *v, void *opaque,
>> +                                     const char *name, Error **errp)
>> +{
>> +    Property *prop = opaque;
>> +
>> +    if (dev->state != DEV_STATE_CREATED) {
>> +        error_set(errp, QERR_PERMISSION_DENIED);
>> +        return;
>> +    }
>> +
>> +    if (prop->info->parse) {
>> +        Error *local_err = NULL;
>> +        char *ptr = NULL;
>> +
>> +        visit_type_str(v,&ptr, name,&local_err);
>> +        if (!local_err) {
>> +            int ret;
>> +            ret = prop->info->parse(dev, prop, ptr);
>> +            if (ret != 0) {
>> +                error_set(errp, QERR_INVALID_PARAMETER_VALUE,
>> +                          name, prop->info->name);
>> +            }
>> +            g_free(ptr);
>> +        } else {
>> +            error_propagate(errp, local_err);
>> +        }
>
> I noticed something subtle about Error** here.  Your code is correct but
> I (incorrectly) wanted to eliminate local_err and use errp directly:
>
> if (prop->info->parse) {
>      char *ptr = NULL;
>
>      visit_type_str(v,&ptr, name, errp);
>      if (!error_is_set(errp)) {
>          int ret;
> 	ret = prop->info->parse(dev, prop, ptr);
> 	if (ret != 0) {
> 	    error_set(errp, QERR_INVALID_PARAMETER_VALUE,
> 	              name, prop->info->name);
> 	}
> 	g_free(ptr);
>      }
> } else {
> ...
>
> Turns out you cannot do this because error_is_set(errp) will be false if
> the caller passed in a NULL errp.  That means we wouldn't detect the
> error from visit_type_str()!
>
> So it's not okay to depend on the caller's errp.  We always need to
> juggle around a local_err with propagation :(.
>
> Just wanted to share.

Yes, you are correct.  You see this idiom a lot in QAPI an also in glib code 
that uses GError.

Regards,

Anthony Liguori

>