Re: [Qemu-devel] [PATCH] qom: abort on error in property setter if caller passed errp == NULL

[Peter Crosthwaite]

Hi,

On Thu, Nov 28, 2013 at 11:24 AM, Igor Mammedov <address@hidden> wrote:
> in case if caller setting property doesn't care about error and
> passes in NULL as errp argument but error occurs in property setter,
> it is silently discarded leaving object in undefined state.
>
> As result it leads to hard to find bugs, so if caller doesn't
> care about error it must be sure that property exists and
> accepts provided value, otherwise it's better to abort early
> since error case couldn't be handled gracefully and find
> invalid usecase early.
>
> In addition multitude of property setters will be always
> guarantied to have error object present and won't be required
> to handle this condition individually.
>

So there seems to be contention between different APIs and use cases
on what to do in the NULL case. Personally, I'm of the opinion that
NULL should be a silent don't care policy. But you are right in that
checking every API call at call site is cumbersome, and its better to
have some sort of collective mechanism to implement different policys.
I think this can be done caller side efficiently by having a special
Error * that can be passed in that tells the error API to assert or
error raise.

extern error *abort_on_err;

And update your call sites to do this:

object_property_set(Foo, bar, "baz", &abort_on_err);

Error_set and friends are then taught to recognise abort_on_err and
abort accordingly and theres no need for this form of change pattern -
its all solved in the error API.

You can also implement a range of automatic error handling policies e.g:

extern Error *report_err; /* report any errors to monitor/stdout */

To report an error without the assertion.

Regards,
Peter

> Signed-off-by: Igor Mammedov <address@hidden>
> ---
>  qom/object.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/qom/object.c b/qom/object.c
> index fc19cf6..2c0bb64 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -792,16 +792,25 @@ void object_property_get(Object *obj, Visitor *v, const 
> char *name,
>  void object_property_set(Object *obj, Visitor *v, const char *name,
>                           Error **errp)
>  {
> -    ObjectProperty *prop = object_property_find(obj, name, errp);
> -    if (prop == NULL) {
> -        return;
> +    Error *local_error = NULL;
> +    ObjectProperty *prop = object_property_find(obj, name, &local_error);
> +    if (local_error) {
> +        goto out;
>      }
>
>      if (!prop->set) {
> -        error_set(errp, QERR_PERMISSION_DENIED);
> +        error_set(&local_error, QERR_PERMISSION_DENIED);
>      } else {
> -        prop->set(obj, v, prop->opaque, name, errp);
> +        prop->set(obj, v, prop->opaque, name, &local_error);
>      }
> +out:
> +    if (local_error) {
> +        if (!errp) {
> +            assert_no_error(local_error);
> +        }
> +        error_propagate(errp, local_error);
> +    }
> +
>  }
>
>  void object_property_set_str(Object *obj, const char *value,
> --
> 1.8.3.1
>
>