qom: Clean up fragile use of error_is_set() in set() methods

Using error_is_set(ERRP) to find out whether a function failed is
either wrong, fragile, or unnecessarily opaque.  It's wrong when ERRP
may be null, because errors go undetected when it is.  It's fragile
when proving ERRP non-null involves a non-local argument.  Else, it's
unnecessarily opaque (see commit 84d18f0).

I guess the error_is_set(errp) in the ObjectProperty set() methods are
merely fragile right now, because I can't find a call chain that
passes a null errp argument.

Make the code more robust and more obviously correct: receive the
error in a local variable, then propagate it through the parameter.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

1 files changed, 4 insertions, 2 deletions

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index a470a0b..971a921 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -142,10 +142,12 @@ static void balloon_stats_set_poll_interval(Object *obj, struct Visitor *v,
                                             Error **errp)
 {
     VirtIOBalloon *s = opaque;
+    Error *local_err = NULL;
     int64_t value;
 
-    visit_type_int(v, &value, name, errp);
-    if (error_is_set(errp)) {
+    visit_type_int(v, &value, name, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
         return;
     }