[Qemu-devel] [PATCH v13 10/18] qmp: Support explicit null during visits

[Eric Blake <eblake@redhat.com>]

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), 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).  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().  Repurpose the
test_visitor_out_empty test, particularly since a future patch
will tighten semantics to forbid immediate use of
qmp_output_get_qobject() without at least one visit_type_*.

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

---
v13: no change
v12: retitle and merge in output visitor support, move refcount
tests to separate file
[no v10, v11]
v9: new patch
---
 qapi/qmp-input-visitor.c        | 12 ++++++++++++
 qapi/qmp-output-visitor.c       |  7 +++++++
 tests/check-qnull.c             | 10 +++++++++-
 tests/test-qmp-input-visitor.c  | 16 ++++++++++++++++
 tests/test-qmp-output-visitor.c |  9 +++++----
 5 files changed, 49 insertions(+), 5 deletions(-)

diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
index ed54d4c..faa40e7 100644
--- a/qapi/qmp-input-visitor.c
+++ b/qapi/qmp-input-visitor.c
@@ -338,6 +338,17 @@ 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) {
+        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);
@@ -381,6 +392,7 @@ 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;

     qmp_input_push(v, obj, NULL, NULL);
diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
index 1f2a7ba..5681ad3 100644
--- a/qapi/qmp-output-visitor.c
+++ b/qapi/qmp-output-visitor.c
@@ -196,6 +196,12 @@ static void qmp_output_type_any(Visitor *v, const char *name, QObject **obj,
     qmp_output_add_obj(qov, name, *obj);
 }

+static void qmp_output_type_null(Visitor *v, const char *name, Error **errp)
+{
+    QmpOutputVisitor *qov = to_qov(v);
+    qmp_output_add_obj(qov, name, qnull());
+}
+
 /* Finish building, and return the root object. Will not be NULL. */
 QObject *qmp_output_get_qobject(QmpOutputVisitor *qov)
 {
@@ -246,6 +252,7 @@ QmpOutputVisitor *qmp_output_visitor_new(void)
     v->visitor.type_str = qmp_output_type_str;
     v->visitor.type_number = qmp_output_type_number;
     v->visitor.type_any = qmp_output_type_any;
+    v->visitor.type_null = qmp_output_type_null;

     QTAILQ_INIT(&v->stack);

diff --git a/tests/check-qnull.c b/tests/check-qnull.c
index b0fb1e6..e16c783 100644
--- a/tests/check-qnull.c
+++ b/tests/check-qnull.c
@@ -11,6 +11,7 @@

 #include "qapi/qmp/qobject.h"
 #include "qemu-common.h"
+#include "qapi/qmp-input-visitor.h"
 #include "qapi/qmp-output-visitor.h"

 /*
@@ -37,15 +38,22 @@ static void qnull_visit_test(void)
 {
     QObject *obj;
     QmpOutputVisitor *qov;
+    QmpInputVisitor *qiv;

     g_assert(qnull_.refcnt == 1);
+    obj = qnull();
+    qiv = qmp_input_visitor_new(obj);
+    qobject_decref(obj);
+    visit_type_null(qmp_input_get_visitor(qiv), NULL, &error_abort);
+
     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);
+    qmp_input_visitor_cleanup(qiv);
     g_assert(qnull_.refcnt == 1);
 }

diff --git a/tests/test-qmp-input-visitor.c b/tests/test-qmp-input-visitor.c
index 5941e90..46fa1b9 100644
--- a/tests/test-qmp-input-visitor.c
+++ b/tests/test-qmp-input-visitor.c
@@ -278,6 +278,20 @@ static void test_visitor_in_any(TestInputVisitorData *data,
     qobject_decref(res);
 }

+static void test_visitor_in_null(TestInputVisitorData *data,
+                                 const void *unused)
+{
+    Visitor *v;
+
+    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);
+}
+
 static void test_visitor_in_union_flat(TestInputVisitorData *data,
                                        const void *unused)
 {
@@ -828,6 +842,8 @@ 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",
diff --git a/tests/test-qmp-output-visitor.c b/tests/test-qmp-output-visitor.c
index 81c90bc..4f7692b 100644
--- a/tests/test-qmp-output-visitor.c
+++ b/tests/test-qmp-output-visitor.c
@@ -476,11 +476,12 @@ 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;

+    visit_type_null(data->ov, NULL, &error_abort);
     arg = qmp_output_get_qobject(data->qov);
     g_assert(qobject_type(arg) == QTYPE_QNULL);
     qobject_decref(arg);
@@ -836,8 +837,8 @@ 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);
-- 
2.5.0