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>

qapi/qmp-input-visitor.c

@@ -342,7 +342,13 @@ static void qmp_input_type_any(Visitor *v, const char *name, QObject **obj,
 
 static void qmp_input_type_null(Visitor *v, const char *name, Error **errp)
 {
-    abort();
+    QmpInputVisitor *qiv = to_qiv(v);
+    QObject *qobj = qmp_input_get_object(qiv, name, true);
+
+    if (qobject_type(qobj) != QTYPE_QNULL) {
+        error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
+                   "null");
+    }
 }
 
 static void qmp_input_optional(Visitor *v, const char *name, bool *present)

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. */

tests/check-qnull.c

@@ -11,7 +11,9 @@
 
 #include "qapi/qmp/qobject.h"
 #include "qemu-common.h"
+#include "qapi/qmp-input-visitor.h"
 #include "qapi/qmp-output-visitor.h"
+#include "qapi/error.h"
 
 /*
  * Public Interface test-cases
@@ -37,6 +39,7 @@ static void qnull_visit_test(void)
 {
     QObject *obj;
     QmpOutputVisitor *qov;
+    QmpInputVisitor *qiv;
 
     /*
      * Most tests of interactions between QObject and visitors are in
@@ -45,13 +48,19 @@ static void qnull_visit_test(void)
      */
 
     g_assert(qnull_.refcnt == 1);
+    obj = qnull();
+    qiv = qmp_input_visitor_new(obj, true);
+    qobject_decref(obj);
+    visit_type_null(qmp_input_get_visitor(qiv), NULL, &error_abort);
+    qmp_input_visitor_cleanup(qiv);
+
     qov = qmp_output_visitor_new();
-    /* FIXME: Empty visits are ugly, we should have a visit_type_null(). */
+    visit_type_null(qmp_output_get_visitor(qov), NULL, &error_abort);
     obj = qmp_output_get_qobject(qov);
     g_assert(obj == &qnull_);
     qobject_decref(obj);
-
     qmp_output_visitor_cleanup(qov);
+
     g_assert(qnull_.refcnt == 1);
 }
 

tests/test-qmp-input-visitor.c

@@ -279,6 +279,33 @@ static void test_visitor_in_any(TestInputVisitorData *data,
     qobject_decref(res);
 }
 
+static void test_visitor_in_null(TestInputVisitorData *data,
+                                 const void *unused)
+{
+    Visitor *v;
+    Error *err = NULL;
+    char *tmp;
+
+    /*
+     * FIXME: Since QAPI doesn't know the 'null' type yet, we can't
+     * test visit_type_null() by reading into a QAPI struct then
+     * checking that it was populated correctly.  The best we can do
+     * for now is ensure that we consumed null from the input, proven
+     * by the fact that we can't re-read the key; and that we detect
+     * when input is not null.
+     */
+
+    v = visitor_input_test_init(data, "{ 'a': null, 'b': '' }");
+    visit_start_struct(v, NULL, NULL, 0, &error_abort);
+    visit_type_null(v, "a", &error_abort);
+    visit_type_str(v, "a", &tmp, &err);
+    g_assert(!tmp);
+    error_free_or_abort(&err);
+    visit_type_null(v, "b", &err);
+    error_free_or_abort(&err);
+    visit_end_struct(v, &error_abort);
+}
+
 static void test_visitor_in_union_flat(TestInputVisitorData *data,
                                        const void *unused)
 {
@@ -829,6 +856,8 @@ int main(int argc, char **argv)
                            &in_visitor_data, test_visitor_in_list);
     input_visitor_test_add("/visitor/input/any",
                            &in_visitor_data, test_visitor_in_any);
+    input_visitor_test_add("/visitor/input/null",
+                           &in_visitor_data, test_visitor_in_null);
     input_visitor_test_add("/visitor/input/union-flat",
                            &in_visitor_data, test_visitor_in_union_flat);
     input_visitor_test_add("/visitor/input/alternate",

tests/test-qmp-output-visitor.c

@@ -477,13 +477,23 @@ static void test_visitor_out_alternate(TestOutputVisitorData *data,
     qobject_decref(arg);
 }
 
-static void test_visitor_out_empty(TestOutputVisitorData *data,
-                                   const void *unused)
+static void test_visitor_out_null(TestOutputVisitorData *data,
+                                  const void *unused)
 {
     QObject *arg;
+    QDict *qdict;
+    QObject *nil;
 
+    visit_start_struct(data->ov, NULL, NULL, 0, &error_abort);
+    visit_type_null(data->ov, "a", &error_abort);
+    visit_end_struct(data->ov, &error_abort);
     arg = qmp_output_get_qobject(data->qov);
-    g_assert(qobject_type(arg) == QTYPE_QNULL);
+    g_assert(qobject_type(arg) == QTYPE_QDICT);
+    qdict = qobject_to_qdict(arg);
+    g_assert_cmpint(qdict_size(qdict), ==, 1);
+    nil = qdict_get(qdict, "a");
+    g_assert(nil);
+    g_assert(qobject_type(nil) == QTYPE_QNULL);
     qobject_decref(arg);
 }
 
@@ -837,8 +847,8 @@ int main(int argc, char **argv)
                             &out_visitor_data, test_visitor_out_union_flat);
     output_visitor_test_add("/visitor/output/alternate",
                             &out_visitor_data, test_visitor_out_alternate);
-    output_visitor_test_add("/visitor/output/empty",
-                            &out_visitor_data, test_visitor_out_empty);
+    output_visitor_test_add("/visitor/output/null",
+                            &out_visitor_data, test_visitor_out_null);
     output_visitor_test_add("/visitor/output/native_list/int",
                             &out_visitor_data,
                             test_visitor_out_native_list_int);