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, 12 insertions, 9 deletions

diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
index 3d12623cf9..0452056d42 100644
--- a/qapi/qmp-output-visitor.c
+++ b/qapi/qmp-output-visitor.c
@@ -33,6 +33,7 @@ struct QmpOutputVisitor
     Visitor visitor;
     QStack stack; /* Stack of containers that haven't yet been finished */
     QObject *root; /* Root of the output visit */
+    QObject **result; /* User's storage location for result */
 };
 
 #define qmp_output_add(qov, name, value) \
@@ -200,18 +201,17 @@ static void qmp_output_type_null(Visitor *v, const char *name, Error **errp)
 /* Finish building, and return the root object.
  * The root object is never null. The caller becomes the object's
  * owner, and should use qobject_decref() when done with it.  */
-QObject *qmp_output_get_qobject(QmpOutputVisitor *qov)
+static void qmp_output_complete(Visitor *v, void *opaque)
 {
+    QmpOutputVisitor *qov = to_qov(v);
+
     /* A visit must have occurred, with each start paired with end.  */
     assert(qov->root && QTAILQ_EMPTY(&qov->stack));
+    assert(opaque == qov->result);
 
     qobject_incref(qov->root);
-    return qov->root;
-}
-
-Visitor *qmp_output_get_visitor(QmpOutputVisitor *v)
-{
-    return &v->visitor;
+    *qov->result = qov->root;
+    qov->result = NULL;
 }
 
 static void qmp_output_free(Visitor *v)
@@ -228,7 +228,7 @@ static void qmp_output_free(Visitor *v)
     g_free(qov);
 }
 
-QmpOutputVisitor *qmp_output_visitor_new(void)
+Visitor *qmp_output_visitor_new(QObject **result)
 {
     QmpOutputVisitor *v;
 
@@ -247,9 +247,12 @@ QmpOutputVisitor *qmp_output_visitor_new(void)
     v->visitor.type_number = qmp_output_type_number;
     v->visitor.type_any = qmp_output_type_any;
     v->visitor.type_null = qmp_output_type_null;
+    v->visitor.complete = qmp_output_complete;
     v->visitor.free = qmp_output_free;
 
     QTAILQ_INIT(&v->stack);
+    *result = NULL;
+    v->result = result;
 
-    return v;
+    return &v->visitor;
 }