qobject: qobject_from_jsonv() is dangerous, hide it away

qobject_from_jsonv() takes ownership of %p arguments.  On failure, we
can't generally know whether we failed before or after %p, so
ownership becomes indeterminate.  To avoid leaks, callers passing %p
must terminate on error, e.g. by passing &error_abort.  Trap for the
unwary; document and give the function internal linkage.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-11-armbru@redhat.com>

include/qapi/qmp/qjson.h

@@ -15,8 +15,6 @@
 #define QJSON_H
 
 QObject *qobject_from_json(const char *string, Error **errp);
-QObject *qobject_from_jsonv(const char *string, va_list *ap, Error **errp)
-    GCC_FMT_ATTR(1, 0);
 
 QObject *qobject_from_vjsonf_nofail(const char *string, va_list ap)
     GCC_FMT_ATTR(1, 0);

qobject/qjson.c

@@ -39,7 +39,18 @@ static void parse_json(JSONMessageParser *parser, GQueue *tokens)
     s->result = json_parser_parse_err(tokens, s->ap, &s->err);
 }
 
-QObject *qobject_from_jsonv(const char *string, va_list *ap, Error **errp)
+/*
+ * Parse @string as JSON value.
+ * If @ap is non-null, interpolate %-escapes.
+ * Takes ownership of %p arguments.
+ * On success, return the JSON value.
+ * On failure, store an error through @errp and return NULL.
+ * Ownership of %p arguments becomes indeterminate then.  To avoid
+ * leaks, callers passing %p must terminate on error, e.g. by passing
+ * &error_abort.
+ */
+static QObject *qobject_from_jsonv(const char *string, va_list *ap,
+                                   Error **errp)
 {
     JSONParsingState state = {};