qmp-input: Require struct push to visit members of top dict

Don't embed the root of the visit into the stack of current
containers being visited. That way, we no longer get confused
on whether the first visit of a dictionary is to the dictionary
itself or to one of the members of the dictionary, based on
whether the caller passed name=NULL; and makes the QMP Input
visitor like other visitors where the value of 'name' is now
ignored on the root visit. (We may someday want to revisit
the rules on what 'name' should be on a top-level visit,
rather than just ignoring it; but that would be the topic of
another patch).

An audit of all qmp_input_visitor_new() call sites shows that
there were only two places where callers had previously been
visiting to a QDict with a non-NULL name to bypass a call to
visit_start_struct(), and those were fixed in prior patches.

Backports commit ce140b176920b5b65184020735a3c65ed3e9aeda from qemu

qemu/qapi/qmp-input-visitor.c

@@ -34,9 +34,11 @@ struct QmpInputVisitor
 {
     Visitor visitor;
 
-    /* Stack of objects being visited.  stack[0] is root of visit,
-     * stack[1..] records the nesting of start_struct()/end_struct()
-     * and start_list()/end_list() pairs. */
+    /* Root of visit at visitor creation. */
+    QObject *root;
+
+    /* Stack of objects being visited (all entries will be either
+     * QDict or QList). */
     StackObject stack[QIV_STACK_SIZE];
     int nb_stack;
 
@@ -53,33 +55,34 @@ static QObject *qmp_input_get_object(QmpInputVisitor *qiv,
                                      const char *name,
                                      bool consume)
 {
-    StackObject *tos = &qiv->stack[qiv->nb_stack - 1];
-    QObject *qobj = tos->obj;
+    StackObject *tos;
+    QObject *qobj;
     QObject *ret;
 
+    if (!qiv->nb_stack) {
+        /* Starting at root, name is ignored. */
+        return qiv->root;
+    }
+
+    /* We are in a container; find the next element. */
+    tos = &qiv->stack[qiv->nb_stack - 1];
+    qobj = tos->obj;
     assert(qobj);
 
-    /* If we have a name, and we're in a dictionary, then return that
-     * value. */
-    if (name && qobject_type(qobj) == QTYPE_QDICT) {
+    if (qobject_type(qobj) == QTYPE_QDICT) {
+        assert(name);
         ret = qdict_get(qobject_to_qdict(qobj), name);
         if (tos->h && consume && ret) {
             bool removed = g_hash_table_remove(tos->h, name);
             assert(removed);
         }
-        return ret;
-    }
-
-    /* If we are in the middle of a list, then return the next element
-     * of the list. */
-    if (tos->entry) {
+    } else {
         assert(qobject_type(qobj) == QTYPE_QLIST);
-        return qlist_entry_obj(tos->entry);
+        assert(!name);
+        ret = qlist_entry_obj(tos->entry);
     }
 
-    /* Otherwise, we are at the root of the visit or the start of a
-     * list, and return the object as-is. */
-    return qobj;
+    return ret;
 }
 
 static void qdict_add_key(const char *key, QObject *obj, void *opaque)
@@ -367,7 +370,7 @@ Visitor *qmp_input_get_visitor(QmpInputVisitor *v)
 
 void qmp_input_visitor_cleanup(QmpInputVisitor *v)
 {
-    qobject_decref(v->stack[0].obj);
+    qobject_decref(v->root);
     g_free(v);
 }
 
@@ -395,7 +398,7 @@ QmpInputVisitor *qmp_input_visitor_new(QObject *obj, bool strict)
     v->strict = strict;
     v->visitor.get_next_type = qmp_input_get_next_type;
 
-    qmp_input_push(v, obj, NULL);
+    v->root = obj;
     qobject_incref(obj);
 
     return v;