[Qemu-devel] [PATCH v4 24/28] qapi: Add JSON output visitor

[Eric Blake]

We have several places that want to go from qapi to JSON; right now,
they have to create an intermediate QObject to do the work.  That
also has the drawback that the JSON formatting of a QDict will
rearrange keys (according to a deterministic, but unpredictable,
hash), when humans have an easier time if dicts are produced in
the same order as the qapi type.

For these reasons, it is time to add a new JSON output visitor.
This patch just adds the basic visitor and tests that it works;
later patches will add pretty-printing, support for visit_type_any(),
and conversion of clients to use the visitor.

Design choices: Unlike the QMP output visitor, the JSON visitor
refuses to visit a required string with a NULL value, via abort().
Reusing QString to grow the contents means that we easily share
code with both qobject-json.c and qjson.c; although it might be
nice to enhance things to take an optional output callback
function so that the output can truly be streamed instead of
collected in memory.

Signed-off-by: Eric Blake <address@hidden>

---
v4: retitle, split off inf/NaN handling, rebase to visit_complete
and visit_free changes, defer type_any, address other findings from
Markus
v3: retitle, rebase to master, minor cleanups
v2: rebase to qapi subset E v8; add test of error outputting
infinity; use unsigned depth
---
 include/qapi/visitor.h             |  33 ++--
 include/qapi/json-output-visitor.h |  28 +++
 qapi/json-output-visitor.c         | 193 +++++++++++++++++++
 tests/test-json-output-visitor.c   | 383 +++++++++++++++++++++++++++++++++++++
 tests/test-qmp-output-visitor.c    |   5 +
 qapi/Makefile.objs                 |   2 +-
 tests/.gitignore                   |   1 +
 tests/Makefile                     |   5 +-
 8 files changed, 632 insertions(+), 18 deletions(-)
 create mode 100644 include/qapi/json-output-visitor.h
 create mode 100644 qapi/json-output-visitor.c
 create mode 100644 tests/test-json-output-visitor.c

diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
index 3f46921..c097507 100644
--- a/include/qapi/visitor.h
+++ b/include/qapi/visitor.h
@@ -26,17 +26,17 @@
  *
  * There are four kinds of visitor classes: input visitors (QMP,
  * string, and QemuOpts) parse an external representation and build
- * the corresponding QAPI graph, output visitors (QMP and string) take
- * a completed QAPI graph and generate an external representation, the
- * dealloc visitor can take a QAPI graph (possibly partially
- * constructed) and recursively free its resources, and the clone
- * visitor performs a deep clone of one QAPI object to another.  While
- * the dealloc and QMP input/output visitors are general, the string,
- * QemuOpts, and clone visitors have some implementation limitations;
- * see the documentation for each visitor for more details on what it
- * supports.  Also, see visitor-impl.h for the callback contracts
- * implemented by each visitor, and docs/qapi-code-gen.txt for more
- * about the QAPI code generator.
+ * the corresponding QAPI graph, output visitors (QMP, string, and
+ * JSON) take a completed QAPI graph and generate an external
+ * representation, the dealloc visitor can take a QAPI graph (possibly
+ * partially constructed) and recursively free its resources, and the
+ * clone visitor performs a deep clone of one QAPI object to another.
+ * While the dealloc, JSON output, and QMP input/output visitors are
+ * general, the string, QemuOpts, and clone visitors have some
+ * implementation limitations; see the documentation for each visitor
+ * for more details on what it supports.  Also, see visitor-impl.h for
+ * the callback contracts implemented by each visitor, and
+ * docs/qapi-code-gen.txt for more about the QAPI code generator.
  *
  * All of the visitors are created via:
  *
@@ -59,11 +59,12 @@
  * visitors are declared here; the remaining visitors are generated in
  * qapi-visit.h.
  *
- * The @name parameter of visit_type_FOO() describes the relation
- * between this QAPI value and its parent container.  When visiting
- * the root of a tree, @name is ignored; when visiting a member of an
- * object, @name is the key associated with the value; and when
- * visiting a member of a list, @name is NULL.
+ * The @name parameter of visit_type_FOO() and visit_start_OBJECT()
+ * describes the relation between this QAPI value and its parent
+ * container.  When visiting the root of a tree, @name is ignored;
+ * when visiting a member of an object, @name is the key associated
+ * with the value; and when visiting a member of a list, @name is
+ * NULL.
  *
  * FIXME: Clients must pass NULL for @name when visiting a member of a
  * list, but this leads to poor error messages; it might be nicer to
diff --git a/include/qapi/json-output-visitor.h 
b/include/qapi/json-output-visitor.h
new file mode 100644
index 0000000..41c79f4
--- /dev/null
+++ b/include/qapi/json-output-visitor.h
@@ -0,0 +1,28 @@
+/*
+ * JSON Output Visitor
+ *
+ * Copyright (C) 2015-2016 Red Hat, Inc.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#ifndef JSON_OUTPUT_VISITOR_H
+#define JSON_OUTPUT_VISITOR_H
+
+#include "qapi/visitor.h"
+
+typedef struct JsonOutputVisitor JsonOutputVisitor;
+
+/*
+ * Create a new JSON output visitor.
+ *
+ * If everything else succeeds, pass @result to visit_complete() to
+ * collect the result of the visit.
+ *
+ * For now, this cannot be used to visit the 'any' type.
+ */
+Visitor *json_output_visitor_new(char **result);
+
+#endif
diff --git a/qapi/json-output-visitor.c b/qapi/json-output-visitor.c
new file mode 100644
index 0000000..7010ff8
--- /dev/null
+++ b/qapi/json-output-visitor.c
@@ -0,0 +1,193 @@
+/*
+ * Convert QAPI to JSON
+ *
+ * Copyright (C) 2015-2016 Red Hat, Inc.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "qapi/json-output-visitor.h"
+#include "qapi/visitor-impl.h"
+#include "qapi/qmp/qstring.h"
+#include "qapi/qmp/qobject-json.h"
+
+struct JsonOutputVisitor {
+    Visitor visitor;
+    QString *str;
+    bool comma;
+    unsigned int depth;
+    char **result;
+};
+
+static JsonOutputVisitor *to_jov(Visitor *v)
+{
+    return container_of(v, JsonOutputVisitor, visitor);
+}
+
+static void json_output_name(JsonOutputVisitor *jov, const char *name)
+{
+    if (jov->comma) {
+        qstring_append(jov->str, ", ");
+    } else {
+        jov->comma = true;
+    }
+    if (name && jov->depth) {
+        qstring_append_json_string(jov->str, name);
+        qstring_append(jov->str, ": ");
+    }
+}
+
+static void json_output_start_struct(Visitor *v, const char *name, void **obj,
+                                     size_t unused, Error **errp)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    json_output_name(jov, name);
+    qstring_append(jov->str, "{");
+    jov->comma = false;
+    jov->depth++;
+}
+
+static void json_output_end_struct(Visitor *v, void **obj)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    assert(jov->depth);
+    jov->depth--;
+    qstring_append(jov->str, "}");
+    jov->comma = true;
+}
+
+static void json_output_start_list(Visitor *v, const char *name,
+                                   GenericList **listp, size_t size,
+                                   Error **errp)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    json_output_name(jov, name);
+    qstring_append(jov->str, "[");
+    jov->comma = false;
+    jov->depth++;
+}
+
+static GenericList *json_output_next_list(Visitor *v, GenericList *tail,
+                                          size_t size)
+{
+    return tail->next;
+}
+
+static void json_output_end_list(Visitor *v, void **obj)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    assert(jov->depth);
+    jov->depth--;
+    qstring_append(jov->str, "]");
+    jov->comma = true;
+}
+
+static void json_output_type_int64(Visitor *v, const char *name, int64_t *obj,
+                                   Error **errp)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    json_output_name(jov, name);
+    qstring_append_printf(jov->str, "%" PRId64, *obj);
+}
+
+static void json_output_type_uint64(Visitor *v, const char *name,
+                                    uint64_t *obj, Error **errp)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    json_output_name(jov, name);
+    qstring_append_printf(jov->str, "%" PRIu64, *obj);
+}
+
+static void json_output_type_bool(Visitor *v, const char *name, bool *obj,
+                                  Error **errp)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    json_output_name(jov, name);
+    qstring_append(jov->str, *obj ? "true" : "false");
+}
+
+static void json_output_type_str(Visitor *v, const char *name, char **obj,
+                                 Error **errp)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    assert(*obj);
+    json_output_name(jov, name);
+    /* FIXME: report invalid UTF-8 encoding */
+    qstring_append_json_string(jov->str, *obj);
+}
+
+static void json_output_type_number(Visitor *v, const char *name, double *obj,
+                                    Error **errp)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    json_output_name(jov, name);
+    /* FIXME: report Inf/NaN problems */
+    qstring_append_json_number(jov->str, *obj);
+}
+
+static void json_output_type_null(Visitor *v, const char *name, Error **errp)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    json_output_name(jov, name);
+    qstring_append(jov->str, "null");
+}
+
+static void json_output_complete(Visitor *v, void *result)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    assert(!jov->depth);
+    assert(qstring_get_length(jov->str));
+    assert(jov->result == result);
+    *jov->result = qstring_consume_str(jov->str);
+    jov->str = NULL;
+    jov->result = NULL;
+}
+
+static void json_output_free(Visitor *v)
+{
+    JsonOutputVisitor *jov = to_jov(v);
+
+    QDECREF(jov->str);
+    g_free(jov);
+}
+
+Visitor *json_output_visitor_new(char **result)
+{
+    JsonOutputVisitor *v;
+
+    v = g_malloc0(sizeof(*v));
+    v->result = result;
+    *result = NULL;
+    v->str = qstring_new();
+
+    v->visitor.type = VISITOR_OUTPUT;
+    v->visitor.start_struct = json_output_start_struct;
+    v->visitor.end_struct = json_output_end_struct;
+    v->visitor.start_list = json_output_start_list;
+    v->visitor.next_list = json_output_next_list;
+    v->visitor.end_list = json_output_end_list;
+    v->visitor.type_int64 = json_output_type_int64;
+    v->visitor.type_uint64 = json_output_type_uint64;
+    v->visitor.type_bool = json_output_type_bool;
+    v->visitor.type_str = json_output_type_str;
+    v->visitor.type_number = json_output_type_number;
+    v->visitor.type_null = json_output_type_null;
+    v->visitor.complete = json_output_complete;
+    v->visitor.free = json_output_free;
+
+    return &v->visitor;
+}
diff --git a/tests/test-json-output-visitor.c b/tests/test-json-output-visitor.c
new file mode 100644
index 0000000..3c77a61
--- /dev/null
+++ b/tests/test-json-output-visitor.c
@@ -0,0 +1,383 @@
+/*
+ * JSON Output Visitor unit-tests.
+ *
+ * Copyright (C) 2015-2016 Red Hat Inc.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+/*
+ * Similar in design to test-qmp-output-visitor; if you add tests
+ * here, consider adding tests there as well.
+ */
+
+#include "qemu/osdep.h"
+#include <glib.h>
+
+#include "qemu-common.h"
+#include "qapi/json-output-visitor.h"
+#include "test-qapi-types.h"
+#include "test-qapi-visit.h"
+#include "qapi/qmp/types.h"
+#include "qapi/error.h"
+
+typedef struct TestOutputVisitorData {
+    Visitor *ov;
+    char *str;
+} TestOutputVisitorData;
+
+static void visitor_output_setup(TestOutputVisitorData *data,
+                                 const void *unused)
+{
+    data->ov = json_output_visitor_new(&data->str);
+    g_assert(data->ov);
+}
+
+static void visitor_output_teardown(TestOutputVisitorData *data,
+                                    const void *unused)
+{
+    visit_free(data->ov);
+    data->ov = NULL;
+    g_free(data->str);
+    data->str = NULL;
+}
+
+static const char *visitor_get(TestOutputVisitorData *data)
+{
+    visit_complete(data->ov, &data->str);
+    g_assert(data->str);
+    return data->str;
+}
+
+static void visitor_reset(TestOutputVisitorData *data)
+{
+    visitor_output_teardown(data, NULL);
+    visitor_output_setup(data, NULL);
+}
+
+static void test_visitor_out_int(TestOutputVisitorData *data,
+                                 const void *unused)
+{
+    int64_t value = -42;
+    const char *out;
+
+    visit_type_int(data->ov, NULL, &value, &error_abort);
+
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==, "-42");
+}
+
+static void test_visitor_out_bool(TestOutputVisitorData *data,
+                                  const void *unused)
+{
+    bool value = true;
+    const char *out;
+
+    visit_type_bool(data->ov, NULL, &value, &error_abort);
+
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==, "true");
+}
+
+static void test_visitor_out_number(TestOutputVisitorData *data,
+                                    const void *unused)
+{
+    double value = 3.14;
+    const char *out;
+
+    visit_type_number(data->ov, NULL, &value, &error_abort);
+
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==, "3.14");
+
+    /* FIXME: JSON requires finite numbers */
+}
+
+static void test_visitor_out_string(TestOutputVisitorData *data,
+                                    const void *unused)
+{
+    char *string = (char *) "Q E M U";
+    const char *out;
+
+    visit_type_str(data->ov, NULL, &string, &error_abort);
+
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==, "\"Q E M U\"");
+}
+
+static void test_visitor_out_enum(TestOutputVisitorData *data,
+                                  const void *unused)
+{
+    const char *out;
+    EnumOne i;
+    size_t len;
+
+    for (i = 0; i < ENUM_ONE__MAX; i++) {
+        visit_type_EnumOne(data->ov, "unused", &i, &error_abort);
+
+        out = visitor_get(data);
+        g_assert(*out == '"');
+        len = strlen(out);
+        g_assert_cmpint(len, >, 2);
+        g_assert(out[len - 1] == '"');
+        g_assert_cmpint(memcmp(out + 1, EnumOne_lookup[i], len - 2), ==, 0);
+        visitor_reset(data);
+    }
+}
+
+static void test_visitor_out_enum_errors(TestOutputVisitorData *data,
+                                         const void *unused)
+{
+    EnumOne i, bad_values[] = { ENUM_ONE__MAX, -1 };
+    Error *err;
+
+    for (i = 0; i < ARRAY_SIZE(bad_values) ; i++) {
+        err = NULL;
+        visit_type_EnumOne(data->ov, "unused", &bad_values[i], &err);
+        error_free_or_abort(&err);
+    }
+}
+
+
+static void test_visitor_out_struct(TestOutputVisitorData *data,
+                                    const void *unused)
+{
+    TestStruct test_struct = { .integer = 42,
+                               .boolean = false,
+                               .string = (char *) "foo"};
+    TestStruct *p = &test_struct;
+    const char *out;
+
+    visit_type_TestStruct(data->ov, NULL, &p, &error_abort);
+
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==,
+                    "{"
+                     "\"integer\": 42, "
+                     "\"boolean\": false, "
+                     "\"string\": \"foo\""
+                    "}");
+}
+
+static void test_visitor_out_struct_nested(TestOutputVisitorData *data,
+                                           const void *unused)
+{
+    int64_t value = 42;
+    UserDefTwo *ud2;
+    const char *string = "user def string";
+    const char *strings[] = { "forty two", "forty three", "forty four",
+                              "forty five" };
+    const char *out;
+
+    ud2 = g_malloc0(sizeof(*ud2));
+    ud2->string0 = g_strdup(strings[0]);
+
+    ud2->dict1 = g_malloc0(sizeof(*ud2->dict1));
+    ud2->dict1->string1 = g_strdup(strings[1]);
+
+    ud2->dict1->dict2 = g_malloc0(sizeof(*ud2->dict1->dict2));
+    ud2->dict1->dict2->userdef = g_new0(UserDefOne, 1);
+    ud2->dict1->dict2->userdef->string = g_strdup(string);
+    ud2->dict1->dict2->userdef->integer = value;
+    ud2->dict1->dict2->string = g_strdup(strings[2]);
+
+    ud2->dict1->dict3 = g_malloc0(sizeof(*ud2->dict1->dict3));
+    ud2->dict1->has_dict3 = true;
+    ud2->dict1->dict3->userdef = g_new0(UserDefOne, 1);
+    ud2->dict1->dict3->userdef->string = g_strdup(string);
+    ud2->dict1->dict3->userdef->integer = value;
+    ud2->dict1->dict3->string = g_strdup(strings[3]);
+
+    visit_type_UserDefTwo(data->ov, "unused", &ud2, &error_abort);
+
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==,
+                    "{"
+                     "\"string0\": \"forty two\", "
+                     "\"dict1\": {"
+                      "\"string1\": \"forty three\", "
+                      "\"dict2\": {"
+                       "\"userdef\": {"
+                        "\"integer\": 42, "
+                        "\"string\": \"user def string\""
+                        "}, "
+                       "\"string\": \"forty four\""
+                       "}, "
+                      "\"dict3\": {"
+                       "\"userdef\": {"
+                        "\"integer\": 42, "
+                        "\"string\": \"user def string\""
+                        "}, "
+                      "\"string\": \"forty five\""
+                      "}"
+                     "}"
+                    "}");
+    qapi_free_UserDefTwo(ud2);
+}
+
+static void test_visitor_out_struct_errors(TestOutputVisitorData *data,
+                                           const void *unused)
+{
+    EnumOne bad_values[] = { ENUM_ONE__MAX, -1 };
+    UserDefOne u = { .string = (char *)"" };
+    UserDefOne *pu = &u;
+    Error *err;
+    int i;
+
+    for (i = 0; i < ARRAY_SIZE(bad_values); i++) {
+        err = NULL;
+        u.has_enum1 = true;
+        u.enum1 = bad_values[i];
+        visit_type_UserDefOne(data->ov, "unused", &pu, &err);
+        error_free_or_abort(&err);
+        visitor_reset(data);
+    }
+}
+
+
+static void test_visitor_out_list(TestOutputVisitorData *data,
+                                  const void *unused)
+{
+    const char *value_str = "list value";
+    TestStructList *p, *head = NULL;
+    const int max_items = 10;
+    bool value_bool = true;
+    int value_int = 10;
+    int i;
+    const char *out;
+
+    for (i = 0; i < max_items; i++) {
+        p = g_malloc0(sizeof(*p));
+        p->value = g_malloc0(sizeof(*p->value));
+        p->value->integer = value_int + (max_items - i - 1);
+        p->value->boolean = value_bool;
+        p->value->string = g_strdup(value_str);
+
+        p->next = head;
+        head = p;
+    }
+
+    visit_type_TestStructList(data->ov, NULL, &head, &error_abort);
+
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==,
+                    "["
+                     "{\"integer\": 10, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 11, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 12, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 13, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 14, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 15, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 16, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 17, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 18, \"boolean\": true, "
+                      "\"string\": \"list value\"}, "
+                     "{\"integer\": 19, \"boolean\": true, "
+                      "\"string\": \"list value\"}"
+                    "]");
+    qapi_free_TestStructList(head);
+}
+
+static void test_visitor_out_union_flat(TestOutputVisitorData *data,
+                                        const void *unused)
+{
+    const char *out;
+    UserDefFlatUnion *tmp = g_malloc0(sizeof(UserDefFlatUnion));
+
+    tmp->enum1 = ENUM_ONE_VALUE1;
+    tmp->string = g_strdup("str");
+    tmp->integer = 41;
+    tmp->u.value1.boolean = true;
+
+    visit_type_UserDefFlatUnion(data->ov, NULL, &tmp, &error_abort);
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==,
+                    "{"
+                     "\"integer\": 41, "
+                     "\"string\": \"str\", "
+                     "\"enum1\": \"value1\", "
+                     "\"boolean\": true"
+                    "}");
+    qapi_free_UserDefFlatUnion(tmp);
+}
+
+static void test_visitor_out_alternate(TestOutputVisitorData *data,
+                                       const void *unused)
+{
+    UserDefAlternate *tmp;
+    const char *out;
+
+    tmp = g_new0(UserDefAlternate, 1);
+    tmp->type = QTYPE_QINT;
+    tmp->u.i = 42;
+
+    visit_type_UserDefAlternate(data->ov, NULL, &tmp, &error_abort);
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==, "42");
+    qapi_free_UserDefAlternate(tmp);
+
+    visitor_reset(data);
+    tmp = g_new0(UserDefAlternate, 1);
+    tmp->type = QTYPE_QSTRING;
+    tmp->u.s = g_strdup("hello");
+
+    visit_type_UserDefAlternate(data->ov, NULL, &tmp, &error_abort);
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==, "\"hello\"");
+    qapi_free_UserDefAlternate(tmp);
+}
+
+static void test_visitor_out_null(TestOutputVisitorData *data,
+                                  const void *unused)
+{
+    const char *out;
+
+    visit_type_null(data->ov, NULL, &error_abort);
+    out = visitor_get(data);
+    g_assert_cmpstr(out, ==, "null");
+}
+
+static void output_visitor_test_add(const char *testpath,
+                                    void (*test_func)(TestOutputVisitorData *,
+                                                      const void *))
+{
+    g_test_add(testpath, TestOutputVisitorData, NULL, visitor_output_setup,
+               test_func, visitor_output_teardown);
+}
+
+int main(int argc, char **argv)
+{
+    g_test_init(&argc, &argv, NULL);
+
+    output_visitor_test_add("/visitor/json/int", test_visitor_out_int);
+    output_visitor_test_add("/visitor/json/bool", test_visitor_out_bool);
+    output_visitor_test_add("/visitor/json/number", test_visitor_out_number);
+    output_visitor_test_add("/visitor/json/string", test_visitor_out_string);
+    output_visitor_test_add("/visitor/json/enum", test_visitor_out_enum);
+    output_visitor_test_add("/visitor/json/enum-errors",
+                            test_visitor_out_enum_errors);
+    output_visitor_test_add("/visitor/json/struct", test_visitor_out_struct);
+    output_visitor_test_add("/visitor/json/struct-nested",
+                            test_visitor_out_struct_nested);
+    output_visitor_test_add("/visitor/json/struct-errors",
+                            test_visitor_out_struct_errors);
+    output_visitor_test_add("/visitor/json/list", test_visitor_out_list);
+    output_visitor_test_add("/visitor/json/union-flat",
+                            test_visitor_out_union_flat);
+    output_visitor_test_add("/visitor/json/alternate",
+                            test_visitor_out_alternate);
+    output_visitor_test_add("/visitor/json/null", test_visitor_out_null);
+
+    g_test_run();
+
+    return 0;
+}
diff --git a/tests/test-qmp-output-visitor.c b/tests/test-qmp-output-visitor.c
index df8a495..0db0b19 100644
--- a/tests/test-qmp-output-visitor.c
+++ b/tests/test-qmp-output-visitor.c
@@ -10,6 +10,11 @@
  * See the COPYING file in the top-level directory.
  */

+/*
+ * Similar in design to test-json-output-visitor; if you add tests
+ * here, consider adding tests there as well.
+ */
+
 #include "qemu/osdep.h"
 #include <glib.h>

diff --git a/qapi/Makefile.objs b/qapi/Makefile.objs
index 7ea4aeb..0a08492 100644
--- a/qapi/Makefile.objs
+++ b/qapi/Makefile.objs
@@ -1,6 +1,6 @@
 util-obj-y = qapi-visit-core.o qapi-dealloc-visitor.o qmp-input-visitor.o
 util-obj-y += qmp-output-visitor.o qmp-registry.o qmp-dispatch.o
 util-obj-y += string-input-visitor.o string-output-visitor.o
-util-obj-y += opts-visitor.o qapi-clone-visitor.o
+util-obj-y += opts-visitor.o qapi-clone-visitor.o json-output-visitor.o
 util-obj-y += qmp-event.o
 util-obj-y += qapi-util.o
diff --git a/tests/.gitignore b/tests/.gitignore
index 346d75d..60ff7cc 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -41,6 +41,7 @@ test-io-channel-file.txt
 test-io-channel-socket
 test-io-channel-tls
 test-io-task
+test-json-output-visitor
 test-logging
 test-mul64
 test-opts-visitor
diff --git a/tests/Makefile b/tests/Makefile
index d596493..0d06406 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -24,6 +24,8 @@ check-unit-y += tests/test-qmp-output-visitor$(EXESUF)
 gcov-files-test-qmp-output-visitor-y = qapi/qmp-output-visitor.c
 check-unit-y += tests/test-clone-visitor$(EXESUF)
 gcov-files-test-clone-visitor-y = qapi/qapi-clone-visitor.c
+check-unit-y += tests/test-json-output-visitor$(EXESUF)
+gcov-files-test-json-output-visitor-y = qapi/json-output-visitor.c
 check-unit-y += tests/test-qmp-input-visitor$(EXESUF)
 gcov-files-test-qmp-input-visitor-y = qapi/qmp-input-visitor.c
 check-unit-y += tests/test-qmp-input-strict$(EXESUF)
@@ -390,7 +392,7 @@ test-obj-y = tests/check-qint.o tests/check-qstring.o 
tests/check-qdict.o \
        tests/check-qobject-json.o \
        tests/test-coroutine.o tests/test-string-output-visitor.o \
        tests/test-string-input-visitor.o tests/test-qmp-output-visitor.o \
-       tests/test-clone-visitor.o \
+       tests/test-clone-visitor.o tests/test-json-output-visitor.o \
        tests/test-qmp-input-visitor.o tests/test-qmp-input-strict.o \
        tests/test-qmp-commands.o tests/test-visitor-serialization.o \
        tests/test-x86-cpuid.o tests/test-mul64.o tests/test-int128.o \
@@ -482,6 +484,7 @@ tests/test-string-input-visitor$(EXESUF): 
tests/test-string-input-visitor.o $(te
 tests/test-qmp-event$(EXESUF): tests/test-qmp-event.o $(test-qapi-obj-y)
 tests/test-qmp-output-visitor$(EXESUF): tests/test-qmp-output-visitor.o 
$(test-qapi-obj-y)
 tests/test-clone-visitor$(EXESUF): tests/test-clone-visitor.o 
$(test-qapi-obj-y)
+tests/test-json-output-visitor$(EXESUF): tests/test-json-output-visitor.o 
$(test-qapi-obj-y)
 tests/test-qmp-input-visitor$(EXESUF): tests/test-qmp-input-visitor.o 
$(test-qapi-obj-y)
 tests/test-qmp-input-strict$(EXESUF): tests/test-qmp-input-strict.o 
$(test-qapi-obj-y)
 tests/test-qmp-commands$(EXESUF): tests/test-qmp-commands.o 
tests/test-qmp-marshal.o $(test-qapi-obj-y)
-- 
2.5.5