[Qemu-devel] [PATCH v16 14/24] 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 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>

---
v16: make stubs simply abort()
v15: improve commit message, add stubs here
v14: no change
v13: no change
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          |  3 ++-
 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 +++++
 qapi/qmp-input-visitor.c             |  6 ++++++
 qapi/qmp-output-visitor.c            |  6 ++++++
 9 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 0bb2fe1..841a1bf 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -521,4 +521,16 @@ 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 796d180..88d27d5 100644
--- a/include/qapi/visitor-impl.h
+++ b/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);
diff --git a/include/qapi/opts-visitor.h b/include/qapi/opts-visitor.h
index 633aa71..fe37ed9 100644
--- a/include/qapi/opts-visitor.h
+++ b/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);
diff --git a/include/qapi/string-input-visitor.h b/include/qapi/string-input-visitor.h
index fdf33ae..a8d8f67 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 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);
diff --git a/include/qapi/string-output-visitor.h b/include/qapi/string-output-visitor.h
index 3bb09af..89b7e4b 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 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);
diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 0b7c04a..7ca8bf0 100644
--- a/qapi/qapi-visit-core.c
+++ b/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)
 {
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);

diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
index 5159833..fa460a3 100644
--- a/qapi/qmp-input-visitor.c
+++ b/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;

diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
index 1f2a7ba..adf7731 100644
--- a/qapi/qmp-output-visitor.c
+++ b/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);

-- 
2.5.5