[Qemu-devel] [PATCH v12 09/18] 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.  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 the next patch
will then wire it up into the qmp visitors.  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.

If QAPI had a 'null' type, we'd also have to use visit_type_null()
in the generated visitor functions (most likely, we'd use it via
an alternate type that permits 'null' or an object); we'll create
that usage when we need it.

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

---
v12: rebase to earlier changes, drop R-b due to better documentation
[no v10, v11]
v9: no change
v8: rebase to 'name' motion
v7: new patch, based on discussion about spapr_drc.c
---
 include/qapi/visitor.h               | 12 ++++++++++++
 include/qapi/visitor-impl.h          |  3 +++
 include/qapi/opts-visitor.h          |  2 +-
 include/qapi/string-input-visitor.h  |  2 +-
 include/qapi/string-output-visitor.h |  2 +-
 qapi/qapi-visit-core.c               |  5 +++++
 qapi/qapi-dealloc-visitor.c          |  5 +++++
 7 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 33ae28a..2b85c6f 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -507,4 +507,16 @@ 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
diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index d967b18..90bcaec 100644
--- a/include/qapi/visitor-impl.h
+++ b/include/qapi/visitor-impl.h
@@ -86,6 +86,9 @@ 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);
diff --git a/include/qapi/opts-visitor.h b/include/qapi/opts-visitor.h
index 2002e37..3fcd327 100644
--- a/include/qapi/opts-visitor.h
+++ b/include/qapi/opts-visitor.h
@@ -31,7 +31,7 @@ typedef struct OptsVisitor OptsVisitor;
  * - values above INT64_MAX or LLONG_MAX are rejected.
  *
  * The Opts input visitor does not yet implement support for visiting
- * QAPI alternates, numbers (other than integers), or arbitrary
+ * QAPI alternates, numbers (other than integers), null, or arbitrary
  * QTypes.
  */
 OptsVisitor *opts_visitor_new(const QemuOpts *opts);
diff --git a/include/qapi/string-input-visitor.h b/include/qapi/string-input-visitor.h
index 4c8d1ea..1a34c52 100644
--- a/include/qapi/string-input-visitor.h
+++ b/include/qapi/string-input-visitor.h
@@ -19,7 +19,7 @@ typedef struct StringInputVisitor StringInputVisitor;

 /*
  * The string input visitor does not yet implement support for
- * visiting QAPI structs, alternates, or arbitrary QTypes.
+ * visiting QAPI structs, alternates, null, or arbitrary QTypes.
  */
 StringInputVisitor *string_input_visitor_new(const char *str);
 void string_input_visitor_cleanup(StringInputVisitor *v);
diff --git a/include/qapi/string-output-visitor.h b/include/qapi/string-output-visitor.h
index 094a11e..2564833 100644
--- a/include/qapi/string-output-visitor.h
+++ b/include/qapi/string-output-visitor.h
@@ -19,7 +19,7 @@ typedef struct StringOutputVisitor StringOutputVisitor;

 /*
  * The string output visitor does not yet implement support for
- * visiting QAPI structs, alternates, or arbitrary QTypes.
+ * visiting QAPI structs, alternates, null, or arbitrary QTypes.
  */
 StringOutputVisitor *string_output_visitor_new(bool human);
 void string_output_visitor_cleanup(StringOutputVisitor *v);
diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 1388462..116c627 100644
--- a/qapi/qapi-visit-core.c
+++ b/qapi/qapi-visit-core.c
@@ -218,6 +218,11 @@ 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);
+}
+
 static void output_type_enum(Visitor *v, const char *name, int *obj,
                              const char *const strings[], Error **errp)
 {
diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
index c19a459..413d525 100644
--- a/qapi/qapi-dealloc-visitor.c
+++ b/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);

-- 
2.5.0