varlink: improve compat with varlink C reference implementation

The reference implementation seems to set the 'parameters' field for
method calls to 'null' if nothing is specified on its command line. We
so far only could deal if the parameters field was unset or set to the
empty object. Let's also accept the 'null' type.

(cherry picked from commit f2ad892)
(cherry picked from commit 2dab8bc)
(cherry picked from commit 8760f4c)

Author: poettering

src/shared/varlink.c

@@ -678,12 +678,25 @@ static int varlink_dispatch_disconnect(Varlink *v) {
 }
 
 static int varlink_sanitize_parameters(JsonVariant **v) {
+        int r;
+
         assert(v);
 
         /* Varlink always wants a parameters list, hence make one if the caller doesn't want any */
         if (!*v)
                 return json_variant_new_object(v, NULL, 0);
-        else if (!json_variant_is_object(*v))
+        if (json_variant_is_null(*v)) {
+                JsonVariant *empty;
+
+                r = json_variant_new_object(&empty, NULL, 0);
+                if (r < 0)
+                        return r;
+
+                json_variant_unref(*v);
+                *v = empty;
+                return 0;
+        }
+        if (!json_variant_is_object(*v))
                 return -EINVAL;
 
         return 0;
@@ -723,7 +736,7 @@ static int varlink_dispatch_reply(Varlink *v) {
                 } else if (streq(k, "parameters")) {
                         if (parameters)
                                 goto invalid;
-                        if (!json_variant_is_object(e))
+                        if (!json_variant_is_object(e) && !json_variant_is_null(e))
                                 goto invalid;
 
                         parameters = json_variant_ref(e);
@@ -822,7 +835,7 @@ static int varlink_dispatch_method(Varlink *v) {
                 } else if (streq(k, "parameters")) {
                         if (parameters)
                                 goto invalid;
-                        if (!json_variant_is_object(e))
+                        if (!json_variant_is_object(e) && !json_variant_is_null(e))
                                 goto invalid;
 
                         parameters = json_variant_ref(e);