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>

1 files changed, 2 insertions, 6 deletions

diff --git a/tests/test-string-output-visitor.c b/tests/test-string-output-visitor.c
index e17035be5e..444844a15a 100644
--- a/tests/test-string-output-visitor.c
+++ b/tests/test-string-output-visitor.c
@@ -20,7 +20,6 @@
 #include "qapi/qmp/types.h"
 
 typedef struct TestOutputVisitorData {
-    StringOutputVisitor *sov;
     Visitor *ov;
     char *str;
     bool human;
@@ -30,9 +29,7 @@ static void visitor_output_setup_internal(TestOutputVisitorData *data,
                                           bool human)
 {
     data->human = human;
-    data->sov = string_output_visitor_new(human);
-    g_assert(data->sov);
-    data->ov = string_output_get_visitor(data->sov);
+    data->ov = string_output_visitor_new(human, &data->str);
     g_assert(data->ov);
 }
 
@@ -52,7 +49,6 @@ static void visitor_output_teardown(TestOutputVisitorData *data,
                                     const void *unused)
 {
     visit_free(data->ov);
-    data->sov = NULL;
     data->ov = NULL;
     g_free(data->str);
     data->str = NULL;
@@ -60,7 +56,7 @@ static void visitor_output_teardown(TestOutputVisitorData *data,
 
 static char *visitor_get(TestOutputVisitorData *data)
 {
-    data->str = string_output_get_string(data->sov);
+    visit_complete(data->ov, &data->str);
     g_assert(data->str);
     return data->str;
 }