[Qemu-devel] [PATCH v9 23/37] qmp: Support explicit null during input visit

[Eric Blake <eblake@redhat.com>]

Implement the new type_null() callback for the qmp input visitor.
While we don't yet have a use for this in qapi (the generator
will need some tweaks first), one usage is already envisioned:
when changing blockdev parameters, it would be nice to have a
difference between leaving a tuning parameter unchanged (omit
that parameter from the struct) and to explicitly reset the
parameter to its default without having to know what the default
value is (specify the parameter with an explicit null value,
which will require us to allow a qapi alternate that chooses
between the normal value and an explicit null).

At any rate, we can test this without the use of generated qapi
by manually using visit_start_struct()/visit_end_struct().

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v9: new patch
---
 include/qapi/visitor-impl.h    |  2 +-
 qapi/qmp-input-visitor.c       | 14 ++++++++++++++
 tests/test-qmp-input-visitor.c | 26 ++++++++++++++++++++++++++
 3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index 8705136..95408a5 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -76,7 +76,7 @@ struct Visitor
     void (*type_any)(Visitor *v, const char *name, QObject **obj,
                      Error **errp);
     /* Must be provided to visit explicit null values (right now, only the
-     * dealloc visitor supports this).  */
+     * dealloc and qmp-input visitors support this).  */
     void (*type_null)(Visitor *v, const char *name, Error **errp);

     /* May be NULL; most useful for input visitors. */
diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
index 597652c..ad23bec 100644
--- a/qapi/qmp-input-visitor.c
+++ b/qapi/qmp-input-visitor.c
@@ -315,6 +315,19 @@ static void qmp_input_type_any(Visitor *v, const char *name, QObject **obj,
     *obj = qobj;
 }

+static void qmp_input_type_null(Visitor *v, const char *name, Error **errp)
+{
+    QmpInputVisitor *qiv = to_qiv(v);
+    QObject *qobj = qmp_input_get_object(qiv, name, true);
+
+    if (qobject_type(qobj) == QTYPE_QNULL) {
+        return;
+    }
+
+    error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
+               "null");
+}
+
 static void qmp_input_optional(Visitor *v, const char *name, bool *present)
 {
     QmpInputVisitor *qiv = to_qiv(v);
@@ -358,6 +371,7 @@ QmpInputVisitor *qmp_input_visitor_new(QObject *obj)
     v->visitor.type_str = qmp_input_type_str;
     v->visitor.type_number = qmp_input_type_number;
     v->visitor.type_any = qmp_input_type_any;
+    v->visitor.type_null = qmp_input_type_null;
     v->visitor.optional = qmp_input_optional;
     v->visitor.get_next_type = qmp_input_get_next_type;

diff --git a/tests/test-qmp-input-visitor.c b/tests/test-qmp-input-visitor.c
index f6bd408..6489e4a 100644
--- a/tests/test-qmp-input-visitor.c
+++ b/tests/test-qmp-input-visitor.c
@@ -278,6 +278,30 @@ static void test_visitor_in_any(TestInputVisitorData *data,
     qobject_decref(res);
 }

+static void test_visitor_in_null(TestInputVisitorData *data,
+                                 const void *unused)
+{
+    Visitor *v;
+    QObject *null;
+
+    v = visitor_input_test_init(data, "null");
+    visit_type_null(v, NULL, &error_abort);
+
+    v = visitor_input_test_init(data, "{ 'a': null }");
+    visit_start_struct(v, NULL, NULL, 0, &error_abort);
+    visit_type_null(v, "a", &error_abort);
+    visit_end_struct(v, &error_abort);
+
+    /* Check that qnull reference counting is sane:
+     * 1 for global use, 1 for our qnull() use, and 1 still owned by 'v'
+     * until it is torn down */
+    null = qnull();
+    g_assert(null->refcnt == 3);
+    visitor_input_teardown(data, NULL);
+    g_assert(null->refcnt == 2);
+    qobject_decref(null);
+}
+
 static void test_visitor_in_union_flat(TestInputVisitorData *data,
                                        const void *unused)
 {
@@ -792,6 +816,8 @@ 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",
-- 
2.5.0