Complex RialtoParachain <> Millau relay (#1405)

* complex parachain relay

* fix spelling

bridges/relays/lib-substrate-relay/src/on_demand/mod.rs

@@ -0,0 +1,35 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Types and functions intended to ease adding of new Substrate -> Substrate
+//! on-demand pipelines.
+
+use async_trait::async_trait;
+
+pub mod headers;
+pub mod parachains;
+
+/// On-demand headers relay that is relaying finalizing headers only when requested.
+#[async_trait]
+pub trait OnDemandRelay<SourceHeaderNumber>: Send + Sync {
+	/// Ask relay to relay source header with given number  to the target chain.
+	///
+	/// Depending on implementation, on-demand relay may also relay `required_header` ancestors
+	/// (e.g. if they're mandatory), or its descendants. The request is considered complete if
+	/// the best avbailable header at the target chain has number that is larger than or equal
+	/// to the `required_header`.
+	async fn require_more_headers(&self, required_header: SourceHeaderNumber);
+}