json: add channel_id helper

common/json_helpers.c

@@ -142,6 +142,13 @@ void json_add_node_id(struct json_stream *response,
 	json_add_hex(response, fieldname, id->k, sizeof(id->k));
 }
 
+void json_add_channel_id(struct json_stream *response,
+			 const char *fieldname,
+			 const struct channel_id *cid)
+{
+	json_add_hex(response, fieldname, cid->id, sizeof(cid->id));
+}
+
 void json_add_pubkey(struct json_stream *response,
 		     const char *fieldname,
 		     const struct pubkey *key)

common/json_helpers.h

@@ -89,6 +89,11 @@ void json_add_node_id(struct json_stream *response,
 				const char *fieldname,
 				const struct node_id *id);
 
+/* '"fieldname" : "0289abcdef..."' or "0289abcdef..." if fieldname is NULL */
+void json_add_channel_id(struct json_stream *response,
+			 const char *fieldname,
+			 const struct channel_id *cid);
+
 /* '"fieldname" : <hexrev>' or "<hexrev>" if fieldname is NULL */
 void json_add_txid(struct json_stream *result, const char *fieldname,
 		   const struct bitcoin_txid *txid);

lightningd/notification.c

@@ -219,8 +219,7 @@ static void channel_state_changed_notification_serialize(struct json_stream *str
 {
 	json_object_start(stream, "channel_state_changed");
 	json_add_node_id(stream, "peer_id", peer_id);
-	json_add_string(stream, "channel_id",
+	json_add_channel_id(stream, "channel_id", cid);
-			type_to_string(tmpctx, struct channel_id, cid));
 	if (scid)
 		json_add_short_channel_id(stream, "short_channel_id", scid);
 	else