qapi: Simplify visits of optional fields

None of the visitor callbacks would set an error when testing
if an optional field was present; make this part of the interface
contract by eliminating the errp argument.

The resulting generated code has a nice diff:

|-    visit_optional(v, &has_fdset_id, "fdset-id", &err);
|-    if (err) {
|-        goto out;
|-    }
|+    visit_optional(v, &has_fdset_id, "fdset-id");
|     if (has_fdset_id) {
|         visit_type_int(v, &fdset_id, "fdset-id", &err);
|         if (err) {
|             goto out;
|         }
|     }

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1449033659-25497-9-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 1414de1..9be60d4 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -36,8 +36,14 @@ void visit_end_implicit_struct(Visitor *v, Error **errp);
 void visit_start_list(Visitor *v, const char *name, Error **errp);
 GenericList *visit_next_list(Visitor *v, GenericList **list, Error **errp);
 void visit_end_list(Visitor *v, Error **errp);
-void visit_optional(Visitor *v, bool *present, const char *name,
-                    Error **errp);
+
+/**
+ * Check if an optional member @name of an object needs visiting.
+ * For input visitors, set *@present according to whether the
+ * corresponding visit_type_*() needs calling; for other visitors,
+ * leave *@present unchanged.
+ */
+void visit_optional(Visitor *v, bool *present, const char *name);
 
 /**
  * Determine the qtype of the item @name in the current object visit.