qapi: Add new visit_complete() function

Making each output visitor provide its own output collection
function was the only remaining reason for exposing visitor
sub-types to the rest of the code base.  Add a polymorphic
visit_complete() function which is a no-op for input visitors,
and which populates an opaque pointer for output visitors.  For
maximum type-safety, also add a parameter to the output visitor
constructors with a type-correct version of the output pointer,
and assert that the two uses match.

This approach was considered superior to either passing the
output parameter only during construction (action at a distance
during visit_free() feels awkward) or only during visit_complete()
(defeating type safety makes it easier to use incorrectly).

Most callers were function-local, and therefore a mechanical
conversion; the testsuite was a bit trickier, but the previous
cleanup patch minimized the churn here.

The visit_complete() function may be called at most once; doing
so lets us use transfer semantics rather than duplication or
ref-count semantics to get the just-built output back to the
caller, even though it means our behavior is not idempotent.

Generated code is simplified as follows for events:

|@@ -26,7 +26,7 @@ void qapi_event_send_acpi_device_ost(ACP
|     QDict *qmp;
|     Error *err = NULL;
|     QMPEventFuncEmit emit;
|-    QmpOutputVisitor *qov;
|+    QObject *obj;
|     Visitor *v;
|     q_obj_ACPI_DEVICE_OST_arg param = {
|         info
|@@ -39,8 +39,7 @@ void qapi_event_send_acpi_device_ost(ACP
|
|     qmp = qmp_event_build_dict("ACPI_DEVICE_OST");
|
|-    qov = qmp_output_visitor_new();
|-    v = qmp_output_get_visitor(qov);
|+    v = qmp_output_visitor_new(&obj);
|
|     visit_start_struct(v, "ACPI_DEVICE_OST", NULL, 0, &err);
|     if (err) {
|@@ -55,7 +54,8 @@ void qapi_event_send_acpi_device_ost(ACP
|         goto out;
|     }
|
|-    qdict_put_obj(qmp, "data", qmp_output_get_qobject(qov));
|+    visit_complete(v, &obj);
|+    qdict_put_obj(qmp, "data", obj);
|     emit(QAPI_EVENT_ACPI_DEVICE_OST, qmp, &err);

and for commands:

| {
|     Error *err = NULL;
|-    QmpOutputVisitor *qov = qmp_output_visitor_new();
|     Visitor *v;
|
|-    v = qmp_output_get_visitor(qov);
|+    v = qmp_output_visitor_new(ret_out);
|     visit_type_AddfdInfo(v, "unused", &ret_in, &err);
|-    if (err) {
|-        goto out;
|+    if (!err) {
|+        visit_complete(v, ret_out);
|     }
|-    *ret_out = qmp_output_get_qobject(qov);
|-
|-out:
|     error_propagate(errp, err);

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1465490926-28625-13-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

2 files changed, 7 insertions, 11 deletions

diff --git a/scripts/qapi-commands.py b/scripts/qapi-commands.py
index 49d4ce2590..34b6a3a07f 100644
--- a/scripts/qapi-commands.py
+++ b/scripts/qapi-commands.py
@@ -61,17 +61,13 @@ def gen_marshal_output(ret_type):
 static void qmp_marshal_output_%(c_name)s(%(c_type)s ret_in, QObject **ret_out, Error **errp)
 {
     Error *err = NULL;
-    QmpOutputVisitor *qov = qmp_output_visitor_new();
     Visitor *v;
 
-    v = qmp_output_get_visitor(qov);
+    v = qmp_output_visitor_new(ret_out);
     visit_type_%(c_name)s(v, "unused", &ret_in, &err);
-    if (err) {
-        goto out;
+    if (!err) {
+        visit_complete(v, ret_out);
     }
-    *ret_out = qmp_output_get_qobject(qov);
-
-out:
     error_propagate(errp, err);
     visit_free(v);
     v = qapi_dealloc_visitor_new();
diff --git a/scripts/qapi-event.py b/scripts/qapi-event.py
index 909e8d6b8f..9c88627c9f 100644
--- a/scripts/qapi-event.py
+++ b/scripts/qapi-event.py
@@ -71,7 +71,7 @@ def gen_event_send(name, arg_type):
 
     if arg_type and arg_type.members:
         ret += mcgen('''
-    QmpOutputVisitor *qov;
+    QObject *obj;
     Visitor *v;
 ''')
         ret += gen_param_var(arg_type)
@@ -90,8 +90,7 @@ def gen_event_send(name, arg_type):
 
     if arg_type and arg_type.members:
         ret += mcgen('''
-    qov = qmp_output_visitor_new();
-    v = qmp_output_get_visitor(qov);
+    v = qmp_output_visitor_new(&obj);
 
     visit_start_struct(v, "%(name)s", NULL, 0, &err);
     if (err) {
@@ -106,7 +105,8 @@ def gen_event_send(name, arg_type):
         goto out;
     }
 
-    qdict_put_obj(qmp, "data", qmp_output_get_qobject(qov));
+    visit_complete(v, &obj);
+    qdict_put_obj(qmp, "data", obj);
 ''',
                      name=name, c_name=arg_type.c_name())