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, 17 insertions, 21 deletions

diff --git a/qom/object.c b/qom/object.c
index 2407b667d9..8166b7dace 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -1221,7 +1221,6 @@ int object_property_get_enum(Object *obj, const char *name,
                              const char *typename, Error **errp)
 {
     Error *err = NULL;
-    StringOutputVisitor *sov;
     Visitor *v;
     char *str;
     int ret;
@@ -1241,15 +1240,14 @@ int object_property_get_enum(Object *obj, const char *name,
 
     enumprop = prop->opaque;
 
-    sov = string_output_visitor_new(false);
-    v = string_output_get_visitor(sov);
+    v = string_output_visitor_new(false, &str);
     object_property_get(obj, v, name, &err);
     if (err) {
         error_propagate(errp, err);
         visit_free(v);
         return 0;
     }
-    str = string_output_get_string(sov);
+    visit_complete(v, &str);
     visit_free(v);
     v = string_input_visitor_new(str);
     visit_type_enum(v, name, &ret, enumprop->strings, errp);
@@ -1264,25 +1262,23 @@ void object_property_get_uint16List(Object *obj, const char *name,
                                     uint16List **list, Error **errp)
 {
     Error *err = NULL;
-    StringOutputVisitor *ov;
     Visitor *v;
     char *str;
 
-    ov = string_output_visitor_new(false);
-    object_property_get(obj, string_output_get_visitor(ov),
-                        name, &err);
+    v = string_output_visitor_new(false, &str);
+    object_property_get(obj, v, name, &err);
     if (err) {
         error_propagate(errp, err);
         goto out;
     }
-    str = string_output_get_string(ov);
+    visit_complete(v, &str);
+    visit_free(v);
     v = string_input_visitor_new(str);
     visit_type_uint16List(v, NULL, list, errp);
 
     g_free(str);
-    visit_free(v);
 out:
-    visit_free(string_output_get_visitor(ov));
+    visit_free(v);
 }
 
 void object_property_parse(Object *obj, const char *string,
@@ -1296,21 +1292,21 @@ void object_property_parse(Object *obj, const char *string,
 char *object_property_print(Object *obj, const char *name, bool human,
                             Error **errp)
 {
-    StringOutputVisitor *sov;
+    Visitor *v;
     char *string = NULL;
     Error *local_err = NULL;
 
-    sov = string_output_visitor_new(human);
-    object_property_get(obj, string_output_get_visitor(sov), name, &local_err);
+    v = string_output_visitor_new(human, &string);
+    object_property_get(obj, v, name, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         goto out;
     }
 
-    string = string_output_get_string(sov);
+    visit_complete(v, &string);
 
 out:
-    visit_free(string_output_get_visitor(sov));
+    visit_free(v);
     return string;
 }
 
diff --git a/qom/qom-qobject.c b/qom/qom-qobject.c
index 6714565f13..c225abcbad 100644
--- a/qom/qom-qobject.c
+++ b/qom/qom-qobject.c
@@ -33,14 +33,14 @@ QObject *object_property_get_qobject(Object *obj, const char *name,
 {
     QObject *ret = NULL;
     Error *local_err = NULL;
-    QmpOutputVisitor *qov;
+    Visitor *v;
 
-    qov = qmp_output_visitor_new();
-    object_property_get(obj, qmp_output_get_visitor(qov), name, &local_err);
+    v = qmp_output_visitor_new(&ret);
+    object_property_get(obj, v, name, &local_err);
     if (!local_err) {
-        ret = qmp_output_get_qobject(qov);
+        visit_complete(v, &ret);
     }
     error_propagate(errp, local_err);
-    visit_free(qmp_output_get_visitor(qov));
+    visit_free(v);
     return ret;
 }