[Qemu-devel] [PATCH v8 22/35] qapi: Add visit_type_null() visitor

[Eric Blake <eblake@redhat.com>]

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.

This patch introduces the new visit_type_null() interface, and
a later patch will then wire it up into the qmp output visitor.

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v8: rebase to 'name' motion
v7: new patch, based on discussion about spapr_drc.c
---
 include/qapi/visitor-impl.h | 3 +++
 include/qapi/visitor.h      | 8 ++++++++
 qapi/qapi-dealloc-visitor.c | 5 +++++
 qapi/qapi-visit-core.c      | 5 +++++
 4 files changed, 21 insertions(+)

diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index 8110ebd..d301901 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -75,6 +75,9 @@ struct Visitor
      * visitors do not currently visit arbitrary types).  */
     void (*type_any)(Visitor *v, const char *name, QObject **obj,
                      Error **errp);
+    /* Must be provided to visit explicit null values (right now, only the
+     * dealloc visitor supports this).  */
+    void (*type_null)(Visitor *v, const char *name, Error **errp);

     /* May be NULL; most useful for input visitors. */
     void (*optional)(Visitor *v, const char *name, bool *present);
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 5349a64..6e49b51 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -267,6 +267,14 @@ 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 tied to @name in the current object visit.
+ * @name will be NULL if this is visited as part of a list.
+ * 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);
+
+/**
  * Mark the start of visiting the branches of a union. Return true if
  * @data_present.
  * FIXME: Should not be needed
diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
index 560feb3..ede1703 100644
--- a/qapi/qapi-dealloc-visitor.c
+++ b/qapi/qapi-dealloc-visitor.c
@@ -162,6 +162,10 @@ static void qapi_dealloc_type_anything(Visitor *v, const char *name,
     }
 }

+static void qapi_dealloc_type_null(Visitor *v, const char *name, Error **errp)
+{
+}
+
 static void qapi_dealloc_type_enum(Visitor *v, const char *name, int *obj,
                                    const char * const strings[], Error **errp)
 {
@@ -222,6 +226,7 @@ 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;
     v->visitor.start_union = qapi_dealloc_start_union;

     QTAILQ_INIT(&v->stack);
diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 1612d0d..399256b 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -262,6 +262,11 @@ void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp)
     v->type_any(v, name, obj, errp);
 }

+void visit_type_null(Visitor *v, const char *name, Error **errp)
+{
+    v->type_null(v, name, errp);
+}
+
 void output_type_enum(Visitor *v, const char *name, int *obj,
                       const char * const strings[], Error **errp)
 {
-- 
2.4.3