qapi: Add visit_type_null() visitor

Right now, qmp-output-visitor happens to produce a QNull result
if nothing is actually visited between the creation of the visitor
and the request for the resulting QObject.  A stronger protocol
would require that a QMP output visit MUST visit something.  But
to still be able to produce a JSON 'null' output, we need a new
visitor function that states our intentions.  Yes, we could say
that such a visit must go through visit_type_any(), but that
feels clunky.

So this patch introduces the new visit_type_null() interface and
its no-op interface in the dealloc visitor, and stubs in the
qmp visitors (the next patch will finish the implementation).
For the visitors that will not implement the callback, document
the situation. The code in qapi-visit-core unconditionally
dereferences the callback pointer, so that a segfault will inform
a developer if they need to implement the callback for their
choice of visitor.

Note that JSON has a primitive null type, with the single value
null; likewise with the QNull type for QObject; but for QAPI,
we just have the 'null' value without a null type.  We may
eventually want to add more support in QAPI for null (most likely,
we'd use it via an alternate type that permits 'null' or an
object); but we'll create that usage when we need it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-15-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

include/qapi/opts-visitor.h

@@ -31,7 +31,8 @@ typedef struct OptsVisitor OptsVisitor;
  * - values above INT64_MAX or LLONG_MAX are rejected.
  *
  * The Opts input visitor does not implement support for visiting QAPI
- * alternates, numbers (other than integers), or arbitrary QTypes.
+ * alternates, numbers (other than integers), null, or arbitrary
+ * QTypes.
  */
 OptsVisitor *opts_visitor_new(const QemuOpts *opts);
 void opts_visitor_cleanup(OptsVisitor *nv);

include/qapi/string-input-visitor.h

@@ -19,7 +19,7 @@ typedef struct StringInputVisitor StringInputVisitor;
 
 /*
  * The string input visitor does not implement support for visiting
- * QAPI structs, alternates, or arbitrary QTypes.
+ * QAPI structs, alternates, null, or arbitrary QTypes.
  */
 StringInputVisitor *string_input_visitor_new(const char *str);
 void string_input_visitor_cleanup(StringInputVisitor *v);

include/qapi/string-output-visitor.h

@@ -19,7 +19,7 @@ typedef struct StringOutputVisitor StringOutputVisitor;
 
 /*
  * The string output visitor does not implement support for visiting
- * QAPI structs, alternates, or arbitrary QTypes.
+ * QAPI structs, alternates, null, or arbitrary QTypes.
  */
 StringOutputVisitor *string_output_visitor_new(bool human);
 void string_output_visitor_cleanup(StringOutputVisitor *v);

include/qapi/visitor-impl.h

@@ -90,6 +90,9 @@ struct Visitor
     void (*type_any)(Visitor *v, const char *name, QObject **obj,
                      Error **errp);
 
+    /* Must be set to visit explicit null values.  */
+    void (*type_null)(Visitor *v, const char *name, Error **errp);
+
     /* Must be set for input visitors, optional otherwise.  The core
      * takes care of the return type in the public interface. */
     void (*optional)(Visitor *v, const char *name, bool *present);

include/qapi/visitor.h

@@ -522,4 +522,16 @@ void visit_type_number(Visitor *v, const char *name, double *obj,
  */
 void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp);
 
+/*
+ * Visit a JSON null value.
+ *
+ * @name expresses the relationship of the null value to its parent
+ * container; see the general description of @name above.
+ *
+ * Unlike all other visit_type_* functions, no obj parameter is
+ * needed; rather, this is a witness that an explicit null value is
+ * expected rather than any other type.
+ */
+void visit_type_null(Visitor *v, const char *name, Error **errp);
+
 #endif

qapi/qapi-dealloc-visitor.c

@@ -163,6 +163,10 @@ static void qapi_dealloc_type_anything(Visitor *v, const char *name,
     }
 }
 
+static void qapi_dealloc_type_null(Visitor *v, const char *name, Error **errp)
+{
+}
+
 Visitor *qapi_dealloc_get_visitor(QapiDeallocVisitor *v)
 {
     return &v->visitor;
@@ -193,6 +197,7 @@ QapiDeallocVisitor *qapi_dealloc_visitor_new(void)
     v->visitor.type_str = qapi_dealloc_type_str;
     v->visitor.type_number = qapi_dealloc_type_number;
     v->visitor.type_any = qapi_dealloc_type_anything;
+    v->visitor.type_null = qapi_dealloc_type_null;
 
     QTAILQ_INIT(&v->stack);
 

qapi/qapi-visit-core.c

@@ -244,6 +244,11 @@ void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp)
     error_propagate(errp, err);
 }
 
+void visit_type_null(Visitor *v, const char *name, Error **errp)
+{
+    v->type_null(v, name, errp);
+}
+
 static void output_type_enum(Visitor *v, const char *name, int *obj,
                              const char *const strings[], Error **errp)
 {

qapi/qmp-input-visitor.c

@@ -340,6 +340,11 @@ static void qmp_input_type_any(Visitor *v, const char *name, QObject **obj,
     *obj = qobj;
 }
 
+static void qmp_input_type_null(Visitor *v, const char *name, Error **errp)
+{
+    abort();
+}
+
 static void qmp_input_optional(Visitor *v, const char *name, bool *present)
 {
     QmpInputVisitor *qiv = to_qiv(v);
@@ -383,6 +388,7 @@ QmpInputVisitor *qmp_input_visitor_new(QObject *obj, bool strict)
     v->visitor.type_str = qmp_input_type_str;
     v->visitor.type_number = qmp_input_type_number;
     v->visitor.type_any = qmp_input_type_any;
+    v->visitor.type_null = qmp_input_type_null;
     v->visitor.optional = qmp_input_optional;
     v->strict = strict;
 

qapi/qmp-output-visitor.c

@@ -196,6 +196,11 @@ static void qmp_output_type_any(Visitor *v, const char *name, QObject **obj,
     qmp_output_add_obj(qov, name, *obj);
 }
 
+static void qmp_output_type_null(Visitor *v, const char *name, Error **errp)
+{
+    abort();
+}
+
 /* Finish building, and return the root object. Will not be NULL. */
 QObject *qmp_output_get_qobject(QmpOutputVisitor *qov)
 {
@@ -246,6 +251,7 @@ QmpOutputVisitor *qmp_output_visitor_new(void)
     v->visitor.type_str = qmp_output_type_str;
     v->visitor.type_number = qmp_output_type_number;
     v->visitor.type_any = qmp_output_type_any;
+    v->visitor.type_null = qmp_output_type_null;
 
     QTAILQ_INIT(&v->stack);