qobject: Modify qobject_ref() to return obj

For convenience and clarity, make it possible to call qobject_ref() at
the time when the reference is associated with a variable, or
argument, by making qobject_ref() return the same pointer as given.
Use that to simplify the callers.

Backports commit f5a74a5a50387c6f980b2e2f94f062487a1826da from qemu

qemu/include/qapi/qmp/qnull.h

@@ -23,8 +23,7 @@ extern QNull qnull_;
 
 static inline QNull *qnull(void)
 {
-    qobject_ref(&qnull_);
-    return &qnull_;
+    return qobject_ref(&qnull_);
 }
 
 bool qnull_is_equal(const QObject *x, const QObject *y);

qemu/include/qapi/qmp/qobject.h

@@ -103,8 +103,15 @@ static inline void qobject_unref_impl(QObject *obj)
 
 /**
  * qobject_ref(): Increment QObject's reference count
+ *
+ * Returns: the same @obj. The type of @obj will be propagated to the
+ * return type.
  */
-#define qobject_ref(obj) qobject_ref_impl(QOBJECT(obj))
+#define qobject_ref(obj) ({                     \
+    typeof(obj) _o = (obj);                     \
+    qobject_ref_impl(QOBJECT(_o));              \
+    _o;                                         \
+})
 
 /**
  * qobject_unref(): Decrement QObject's reference count, deallocate

qemu/qapi/qobject-input-visitor.c

@@ -463,8 +463,7 @@ static void qobject_input_type_any(Visitor *v, const char *name, QObject **obj,
         return;
     }
 
-    qobject_ref(qobj);
-    *obj = qobj;
+    *obj = qobject_ref(qobj);
 }
 
 static void qobject_input_type_null(Visitor *v, const char *name,
@@ -536,8 +535,7 @@ static QObjectInputVisitor *qobject_input_visitor_base_new(QObject *obj)
     v->visitor.optional = qobject_input_optional;
     v->visitor.free = qobject_input_free;
 
-    v->root = obj;
-    qobject_ref(obj);
+    v->root = qobject_ref(obj);
 
     return v;
 }

qemu/qapi/qobject-output-visitor.c

@@ -191,8 +191,7 @@ static void qobject_output_type_any(Visitor *v, const char *name, QObject **obj,
                                     Error **errp)
 {
     QObjectOutputVisitor *qov = to_qov(v);
-    qobject_ref(*obj);
-    qobject_output_add_obj(qov, name, *obj);
+    qobject_output_add_obj(qov, name, qobject_ref(*obj));
 }
 
 static void qobject_output_type_null(Visitor *v, const char *name,

qemu/qobject/qdict.c

@@ -493,8 +493,7 @@ static void qdict_flatten_qlist(QList *qlist, QDict *target, const char *prefix)
             qdict_flatten_qlist(qobject_to(QList, value), target, new_key);
         } else {
             /* All other types are moved to the target unchanged. */
-            qobject_ref(value);
-            qdict_put_obj(target, new_key, value);
+            qdict_put_obj(target, new_key, qobject_ref(value));
         }
 
         g_free(new_key);
@@ -533,8 +532,7 @@ static void qdict_flatten_qdict(QDict *qdict, QDict *target, const char *prefix)
             delete = true;
         } else if (prefix) {
             /* All other objects are moved to the target unchanged. */
-            qobject_ref(value);
-            qdict_put_obj(target, new_key, value);
+            qdict_put_obj(target, new_key, qobject_ref(value));
             delete = true;
         }
 
@@ -576,8 +574,7 @@ bool qdict_rename_keys(QDict *qdict, const QDictRenames *renames, Error **errp)
             }
 
             qobj = qdict_get(qdict, renames->from);
-            qobject_ref(qobj);
-            qdict_put_obj(qdict, renames->to, qobj);
+            qdict_put_obj(qdict, renames->to, qobject_ref(qobj));
             qdict_del(qdict, renames->from);
         }
 
@@ -611,8 +608,7 @@ void qdict_extract_subqdict(QDict *src, QDict **dst, const char *start)
     while (entry != NULL) {
         next = qdict_next(src, entry);
         if (strstart(entry->key, start, &p)) {
-            qobject_ref(entry->value);
-            qdict_put_obj(*dst, p, entry->value);
+            qdict_put_obj(*dst, p, qobject_ref(entry->value));
             qdict_del(src, entry->key);
         }
         entry = next;
@@ -890,16 +886,14 @@ QObject *qdict_crumple(const QDict *src, Error **errp)
                 qdict_put_obj(two_level, prefix, QOBJECT(child_dict));
             }
 
-            qobject_ref(ent->value);
-            qdict_put_obj(child_dict, suffix, ent->value);
+            qdict_put_obj(child_dict, suffix, qobject_ref(ent->value));
         } else {
             if (child) {
                 error_setg(errp, "Key %s prefix is already set as a dict",
                            prefix);
                 goto error;
             }
-            qobject_ref(ent->value);
-            qdict_put_obj(two_level, prefix, ent->value);
+            qdict_put_obj(two_level, prefix, qobject_ref(ent->value));
         }
 
         g_free(prefix);
@@ -921,8 +915,7 @@ QObject *qdict_crumple(const QDict *src, Error **errp)
 
             qdict_put_obj(multi_level, ent->key, child);
         } else {
-            qobject_ref(ent->value);
-            qdict_put_obj(multi_level, ent->key, ent->value);
+            qdict_put_obj(multi_level, ent->key, qobject_ref(ent->value));
         }
     }
     qobject_unref(two_level);
@@ -948,8 +941,7 @@ QObject *qdict_crumple(const QDict *src, Error **errp)
                 goto error;
             }
 
-            qobject_ref(child);
-            qlist_append_obj(qobject_to(QList, dst), child);
+            qlist_append_obj(qobject_to(QList, dst), qobject_ref(child));
         }
         qobject_unref(multi_level);
         multi_level = NULL;
@@ -990,8 +982,7 @@ void qdict_join(QDict *dest, QDict *src, bool overwrite)
         next = qdict_next(src, entry);
 
         if (overwrite || !qdict_haskey(dest, entry->key)) {
-            qobject_ref(entry->value);
-            qdict_put_obj(dest, entry->key, entry->value);
+            qdict_put_obj(dest, entry->key, qobject_ref(entry->value));
             qdict_del(src, entry->key);
         }