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, 7 insertions, 4 deletions

diff --git a/backends/rng.c b/backends/rng.c
index 8b8d5a4973..0f2fc11dd8 100644
--- a/backends/rng.c
+++ b/backends/rng.c
@@ -50,6 +50,7 @@ static void rng_backend_prop_set_opened(Object *obj, bool value, Error **errp)
 {
     RngBackend *s = RNG_BACKEND(obj);
     RngBackendClass *k = RNG_BACKEND_GET_CLASS(s);
+    Error *local_err = NULL;
 
     if (value == s->opened) {
         return;
@@ -61,12 +62,14 @@ static void rng_backend_prop_set_opened(Object *obj, bool value, Error **errp)
     }
 
     if (k->opened) {
-        k->opened(s, errp);
+        k->opened(s, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            return;
+        }
     }
 
-    if (!error_is_set(errp)) {
-        s->opened = value;
-    }
+    s->opened = true;
 }
 
 static void rng_backend_init(Object *obj)