Daniel P. Berrangé <berra...@redhat.com> writes:
> When debugging QEMU it is often useful to put a breakpoint on the
> error_setg_internal method impl.
>
> Unfortunately the object_property_add / object_class_property_add
> methods call object_property_find / object_class_property_find methods
> to check if a property exists already before adding the new property.
>
> As a result there are a huge number of calls to error_setg_internal
> on startup of most QEMU commands, making it very painful to set a
> breakpoint on this method.
>
> This puts a minor optimization on the code so that we avoid calling
> error_setg() when errp is NULL. Functionally there's no difference
> since error_setg() is a no-op when errp is NULL, but this lets us
> use breakpoints in GDB in a practical way.
>
> Signed-off-by: Daniel P. Berrangé <berra...@redhat.com>
> ---
> qom/object.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/qom/object.c b/qom/object.c
> index 547dcf97c3..ddd5e7a30e 100644
> --- a/qom/object.c
> +++ b/qom/object.c
> @@ -1087,7 +1087,12 @@ ObjectProperty *object_property_find(Object *obj,
> const char *name,
> return prop;
> }
>
> - error_setg(errp, "Property '.%s' not found", name);
> + /* Optimized to avoid calling error_setg if errp == NULL
> + * otherwise every property add call hits error_setg
> + * making it impratical to set breakpoints in GDB */
> + if (errp) {
> + error_setg(errp, "Property '.%s' not found", name);
> + }
> return NULL;
> }
>
In my opinion, this function's design is awkward. Stress on *opinion*.
On success, it returns a (non-null) pointer.
On failure, it sets an error and returns null. Note that it has just
one failure mode: "Property '.%s' not found". Setting an error is just
a convenience for those callers that want to propagate exactly this
error to their callers.
I count 30 callers. Only six pass a non-NULL argument to @errp.
I'd rather have a pair of functions similar to how Python has both
.get() and .__getitem__(): the former doesn't fail, but returns None
instead, and the latter does fail, throwing KeyError. In QEMU, we can't
throw, so we set an error. Here's the obvious code:
ObjectProperty *object_property_find(Object *obj, const char *name)
{
ObjectProperty *prop;
ObjectClass *klass = object_get_class(obj);
prop = object_class_property_find(klass, name, NULL);
if (prop) {
return prop;
}
prop = g_hash_table_lookup(obj->properties, name);
if (prop) {
return prop;
}
return NULL;
}
ObjectProperty *object_property_find_err(Object *obj, const char *name,
Error **errp)
{
ObjectProperty *prop = object_property_find(obj, name);
if (!prop) {
error_setg(errp, "Property '.%s' not found", name);
}
return prop;
}
> @@ -1133,7 +1138,10 @@ ObjectProperty *object_class_property_find(ObjectClass
> *klass, const char *name,
> }
>
> prop = g_hash_table_lookup(klass->properties, name);
> - if (!prop) {
> + /* Optimized to avoid calling error_setg if errp == NULL
> + * otherwise every property add call hits error_setg
> + * making it impratical to set breakpoints in GDB */
> + if (!prop && errp) {
> error_setg(errp, "Property '.%s' not found", name);
> }
> return prop;
Likewise, just more so: callers passing non-NULL do not exist.
