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>

diff --git a/include/qapi/opts-visitor.h b/include/qapi/opts-visitor.h
index 633aa7170ffc75e317bf9c884b9bb92aea0c5ac6..fe37ed9a36ed5714fcf01e09ecbf5a6567f898ba 100644 (file)
--- 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 fdf33ae2b4b5f235d234c78233a4970df1440f6b..a8d8f6723be43f6bcd5dc3cf7f7d42aa6f63d703 100644 (file)
--- 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 3bb09aff2ff4a0c5acc287571b130ae861367e98..89b7e4bc9b60a48e68e1fbf26dd1be5b9f696e79 100644 (file)
--- 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/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
index 796d1800d42b37fac9ce55c2a65563af257d4724..88d27d58debeebfcf929eb08cc1a13dc196188fd 100644 (file)
--- 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/visitor.h b/include/qapi/visitor.h
index 221163742c585995c1976d08cf3e4f6d269386ac..709cca077482a0b425101d2826a596e99b251d01 100644 (file)
--- a/include/qapi/visitor.h
+++ b/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

diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
index c19a459391b18b684feec10b88e04c0559aa0ca6..413d525ad85cd1be6125d67c8a77f0d68c785386 100644 (file)
--- 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/qapi-visit-core.c b/qapi/qapi-visit-core.c
index 0f59a1d8e54dbeb059d85a28b8d4525448506405..fe6afe8ca8b2b6d515f7b1592a5c2743000058e9 100644 (file)
--- 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/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
index 515983357d31dafc55783eee0f3ff17a36865ee2..fa460a3c379127efe40e0746b5f93d923a52294e 100644 (file)
--- 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 1f2a7ba646975d3b579970b12232f5934104a527..adf7731e3fcdba4551e978a724b1d317a20986f6 100644 (file)
--- 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);