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Date: Thu, 1 Dec 2011 08:33:16 +0000
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <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: Anthony Liguori <aliguori@us.ibm.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 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.