QJSON: Add JSON writer

To support programmatic JSON assembly while keeping the code that generates it
readable, this patch introduces a simple JSON writer. It emits JSON serially
into a buffer in memory.

The nice thing about this writer is its simplicity and low memory overhead.
Unlike the QMP JSON writer, this one does not need to spawn QObjects for every
element it wants to represent.

This is a prerequisite for the migration stream format description generator.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

Makefile.objs

@@ -51,6 +51,7 @@ common-obj-$(CONFIG_LINUX) += fsdev/
 common-obj-y += migration/
 common-obj-y += qemu-char.o #aio.o
 common-obj-y += page_cache.o
+common-obj-y += qjson.o
 
 common-obj-$(CONFIG_SPICE) += spice-qemu-char.o
 

include/qjson.h

@@ -0,0 +1,29 @@
+/*
+ * QEMU JSON writer
+ *
+ * Copyright Alexander Graf
+ *
+ * Authors:
+ *  Alexander Graf <agraf@suse.de>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+#ifndef QEMU_QJSON_H
+#define QEMU_QJSON_H
+
+#define TYPE_QJSON "QJSON"
+typedef struct QJSON QJSON;
+
+QJSON *qjson_new(void);
+void json_prop_str(QJSON *json, const char *name, const char *str);
+void json_prop_int(QJSON *json, const char *name, int64_t val);
+void json_end_array(QJSON *json);
+void json_start_array(QJSON *json, const char *name);
+void json_end_object(QJSON *json);
+void json_start_object(QJSON *json, const char *name);
+const char *qjson_get_str(QJSON *json);
+void qjson_finish(QJSON *json);
+
+#endif /* QEMU_QJSON_H */

qjson.c

@@ -0,0 +1,129 @@
+/*
+ * QEMU JSON writer
+ *
+ * Copyright Alexander Graf
+ *
+ * Authors:
+ *  Alexander Graf <agraf@suse.de
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#include <qapi/qmp/qstring.h>
+#include <stdbool.h>
+#include <glib.h>
+#include <qjson.h>
+#include <qemu/module.h>
+#include <qom/object.h>
+
+struct QJSON {
+    Object obj;
+    QString *str;
+    bool omit_comma;
+};
+
+static void json_emit_element(QJSON *json, const char *name)
+{
+    /* Check whether we need to print a , before an element */
+    if (json->omit_comma) {
+        json->omit_comma = false;
+    } else {
+        qstring_append(json->str, ", ");
+    }
+
+    if (name) {
+        qstring_append(json->str, "\"");
+        qstring_append(json->str, name);
+        qstring_append(json->str, "\" : ");
+    }
+}
+
+void json_start_object(QJSON *json, const char *name)
+{
+    json_emit_element(json, name);
+    qstring_append(json->str, "{ ");
+    json->omit_comma = true;
+}
+
+void json_end_object(QJSON *json)
+{
+    qstring_append(json->str, " }");
+    json->omit_comma = false;
+}
+
+void json_start_array(QJSON *json, const char *name)
+{
+    json_emit_element(json, name);
+    qstring_append(json->str, "[ ");
+    json->omit_comma = true;
+}
+
+void json_end_array(QJSON *json)
+{
+    qstring_append(json->str, " ]");
+    json->omit_comma = false;
+}
+
+void json_prop_int(QJSON *json, const char *name, int64_t val)
+{
+    json_emit_element(json, name);
+    qstring_append_int(json->str, val);
+}
+
+void json_prop_str(QJSON *json, const char *name, const char *str)
+{
+    json_emit_element(json, name);
+    qstring_append_chr(json->str, '"');
+    qstring_append(json->str, str);
+    qstring_append_chr(json->str, '"');
+}
+
+const char *qjson_get_str(QJSON *json)
+{
+    return qstring_get_str(json->str);
+}
+
+QJSON *qjson_new(void)
+{
+    QJSON *json = (QJSON *)object_new(TYPE_QJSON);
+    return json;
+}
+
+void qjson_finish(QJSON *json)
+{
+    json_end_object(json);
+}
+
+static void qjson_initfn(Object *obj)
+{
+    QJSON *json = (QJSON *)object_dynamic_cast(obj, TYPE_QJSON);
+    assert(json);
+
+    json->str = qstring_from_str("{ ");
+    json->omit_comma = true;
+}
+
+static void qjson_finalizefn(Object *obj)
+{
+    QJSON *json = (QJSON *)object_dynamic_cast(obj, TYPE_QJSON);
+
+    assert(json);
+    qobject_decref(QOBJECT(json->str));
+}
+
+static const TypeInfo qjson_type_info = {
+    .name = TYPE_QJSON,
+    .parent = TYPE_OBJECT,
+    .instance_size = sizeof(QJSON),
+    .instance_init = qjson_initfn,
+    .instance_finalize = qjson_finalizefn,
+};
+
+static void qjson_register_types(void)
+{
+    type_register_static(&qjson_type_info);
+}
+
+type_init(qjson_register_types)