qmp: Support explicit null during visits

Implement the new type_null() callback for the qmp input and
output visitors. While we don't yet have a use for this in QAPI
input (the generator will need some tweaks first), some
potential usages have already been discussed on the list.
Meanwhile, the output visitor could already output explicit null
via type_any, but this gives us finer control.

At any rate, it's easy to test that we can round-trip an explicit
null through manual use of visit_type_null() wrapped by a virtual
visit_start_struct() walk, even if we can't do the visit in a
QAPI type.  Repurpose the test_visitor_out_empty test,
particularly since a future patch will tighten semantics to
forbid use of qmp_output_get_qobject() without at least one
intervening visit_type_*.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-16-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
index adf7731e3f..5681ad3471 100644
--- a/qapi/qmp-output-visitor.c
+++ b/qapi/qmp-output-visitor.c
@@ -198,7 +198,8 @@ static void qmp_output_type_any(Visitor *v, const char *name, QObject **obj,
 
 static void qmp_output_type_null(Visitor *v, const char *name, Error **errp)
 {
-    abort();
+    QmpOutputVisitor *qov = to_qov(v);
+    qmp_output_add_obj(qov, name, qnull());
 }
 
 /* Finish building, and return the root object. Will not be NULL. */