qmp: Fix reference-counting of qnull on empty output visit

Commit 6c2f9a1 ensured that we would not return NULL when the
caller used an output visitor but had nothing to visit. But
in doing so, it added a FIXME about a reference count leak
that could abort qemu in the (unlikely) case of SIZE_MAX such
visits (more plausible on 32-bit).  (Although that commit
suggested we might fix it in time for 2.5, we ran out of time;
fortunately, it is unlikely enough to bite that it was not
worth worrying about during the 2.5 release.)

This fixes things by documenting the internal contracts, and
explaining why the internal function can return NULL and only
the public facing interface needs to worry about qnull(),
thus avoiding over-referencing the qnull_ global object.

It does not, however, fix the stupidity of the stack mixing
up two separate pieces of information; add a FIXME to explain
that issue, which will be fixed shortly in a future patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
Cc: qemu-stable@nongnu.org
Message-Id: <1454075341-13658-25-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
(cherry picked from commit a861564)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qapi/qmp-output-visitor.c

@@ -29,6 +29,15 @@ typedef QTAILQ_HEAD(QStack, QStackEntry) QStack;
 struct QmpOutputVisitor
 {
     Visitor visitor;
+    /* FIXME: we are abusing stack to hold two separate pieces of
+     * information: the current root object in slot 0, and the stack
+     * of N objects still being built in slots 1 through N (for N+1
+     * slots in use).  Worse, our behavior is inconsistent:
+     * qmp_output_add_obj() visiting two top-level scalars in a row
+     * discards the first in favor of the second, but visiting two
+     * top-level objects in a row tries to append the second object
+     * into the first (since the first object was placed in the stack
+     * in both slot 0 and 1, but only popped from slot 1).  */
     QStack stack;
 };
 
@@ -41,55 +50,66 @@ static QmpOutputVisitor *to_qov(Visitor *v)
     return container_of(v, QmpOutputVisitor, visitor);
 }
 
+/* Push @value onto the stack of current QObjects being built */
 static void qmp_output_push_obj(QmpOutputVisitor *qov, QObject *value)
 {
     QStackEntry *e = g_malloc0(sizeof(*e));
 
+    assert(value);
     e->value = value;
     if (qobject_type(e->value) == QTYPE_QLIST) {
         e->is_list_head = true;
     }
     QTAILQ_INSERT_HEAD(&qov->stack, e, node);
 }
 
+/* Pop a value off the stack of QObjects being built, and return it. */
 static QObject *qmp_output_pop(QmpOutputVisitor *qov)
 {
     QStackEntry *e = QTAILQ_FIRST(&qov->stack);
     QObject *value;
+
+    assert(e);
     QTAILQ_REMOVE(&qov->stack, e, node);
     value = e->value;
+    assert(value);
     g_free(e);
     return value;
 }
 
+/* Grab the root QObject, if any */
 static QObject *qmp_output_first(QmpOutputVisitor *qov)
 {
     QStackEntry *e = QTAILQ_LAST(&qov->stack, QStack);
 
-    /*
-     * FIXME Wrong, because qmp_output_get_qobject() will increment
-     * the refcnt *again*.  We need to think through how visitors
-     * handle null.
-     */
     if (!e) {
-        return qnull();
+        /* No root */
+        return NULL;
     }
-
+    assert(e->value);
     return e->value;
 }
 
+/* Peek at the top of the stack of QObjects being built.
+ * The stack must not be empty. */
 static QObject *qmp_output_last(QmpOutputVisitor *qov)
 {
     QStackEntry *e = QTAILQ_FIRST(&qov->stack);
+
+    assert(e && e->value);
     return e->value;
 }
 
+/* Add @value to the current QObject being built.
+ * If the stack is visiting a dictionary or list, @value is now owned
+ * by that container. Otherwise, @value is now the root.  */
 static void qmp_output_add_obj(QmpOutputVisitor *qov, const char *name,
                                QObject *value)
 {
     QObject *cur;
 
     if (QTAILQ_EMPTY(&qov->stack)) {
+        /* Stack was empty, track this object as root */
         qmp_output_push_obj(qov, value);
         return;
     }
@@ -98,13 +118,17 @@ static void qmp_output_add_obj(QmpOutputVisitor *qov, const char *name,
 
     switch (qobject_type(cur)) {
     case QTYPE_QDICT:
+        assert(name);
         qdict_put_obj(qobject_to_qdict(cur), name, value);
         break;
     case QTYPE_QLIST:
         qlist_append_obj(qobject_to_qlist(cur), value);
         break;
     default:
+        /* The previous root was a scalar, replace it with a new root */
+        /* FIXME this is abusing the stack; see comment above */
         qobject_decref(qmp_output_pop(qov));
+        assert(QTAILQ_EMPTY(&qov->stack));
         qmp_output_push_obj(qov, value);
         break;
     }
@@ -198,11 +222,14 @@ static void qmp_output_type_any(Visitor *v, QObject **obj, const char *name,
     qmp_output_add_obj(qov, name, *obj);
 }
 
+/* Finish building, and return the root object. Will not be NULL. */
 QObject *qmp_output_get_qobject(QmpOutputVisitor *qov)
 {
     QObject *obj = qmp_output_first(qov);
     if (obj) {
         qobject_incref(obj);
+    } else {
+        obj = qnull();
     }
     return obj;
 }

tests/test-qmp-output-visitor.c

@@ -461,6 +461,8 @@ static void test_visitor_out_empty(TestOutputVisitorData *data,
 
     arg = qmp_output_get_qobject(data->qov);
     g_assert(qobject_type(arg) == QTYPE_QNULL);
+    /* Check that qnull reference counting is sane */
+    g_assert(arg->refcnt == 2);
     qobject_decref(arg);
 }