qobject: Factor JSON writer out of qobject_to_json()

We have two JSON writers written in C: qobject/qjson.c provides
qobject_to_json(), and migration/qjson.c provides a more low level
imperative interface.  They don't share code.  The latter tacitly
limits numbers to int64_t, and strings contents to characters that
don't need escaping.

Factor out qobject_to_json()'s JSON writer as qobject/json-writer.c.
Straightforward, except for numbers: since the writer is to be
independent of QObject, it can't use qnum_to_string().  Open-code it
instead.  This is actually an improvement of sorts, because it
liberates qnum_to_string() from JSON's needs: its JSON-related FIXMEs
move to the JSON writer, where they belong.

The next commit will replace migration/qjson.c.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-16-armbru@redhat.com>

1 files changed, 37 insertions, 0 deletions

diff --git a/include/qapi/qmp/json-writer.h b/include/qapi/qmp/json-writer.h
new file mode 100644
index 0000000..708d129
--- /dev/null
+++ b/include/qapi/qmp/json-writer.h
@@ -0,0 +1,37 @@
+/*
+ * JSON Writer
+ *
+ * Copyright (c) 2020 Red Hat Inc.
+ *
+ * Authors:
+ *  Markus Armbruster <armbru@redhat.com>
+ *
+ * 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 JSON_WRITER_H
+#define JSON_WRITER_H
+
+typedef struct JSONWriter JSONWriter;
+
+JSONWriter *json_writer_new(bool pretty);
+const char *json_writer_get(JSONWriter *);
+GString *json_writer_get_and_free(JSONWriter *);
+void json_writer_free(JSONWriter *);
+
+G_DEFINE_AUTOPTR_CLEANUP_FUNC(JSONWriter, json_writer_free)
+
+void json_writer_start_object(JSONWriter *, const char *name);
+void json_writer_end_object(JSONWriter *);
+void json_writer_start_array(JSONWriter *, const char *name);
+void json_writer_end_array(JSONWriter *);
+void json_writer_bool(JSONWriter *, const char *name, bool val);
+void json_writer_null(JSONWriter *, const char *name);
+void json_writer_int64(JSONWriter *, const char *name, int64_t val);
+void json_writer_uint64(JSONWriter *, const char *name, uint64_t val);
+void json_writer_double(JSONWriter *, const char *name, double val);
+void json_writer_str(JSONWriter *, const char *name, const char *str);
+
+#endif