Bridge: added free headers submission support to the substrate-relay (p…

…aritytech#4157)

Original PR:
paritytech/parity-bridges-common#2884. Since
chain-specific code lives in the `parity-bridges-common` repo, some
parts of original PR will require another PR

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>

bridges/chains/chain-kusama/src/lib.rs

@@ -67,6 +67,8 @@ pub const PARAS_PALLET_NAME: &str = "Paras";
 
 /// Name of the With-Kusama GRANDPA pallet instance that is deployed at bridged chains.
 pub const WITH_KUSAMA_GRANDPA_PALLET_NAME: &str = "BridgeKusamaGrandpa";
+/// Name of the With-Kusama parachains pallet instance that is deployed at bridged chains.
+pub const WITH_KUSAMA_BRIDGE_PARACHAINS_PALLET_NAME: &str = "BridgeKusamaParachains";
 
 /// Maximal size of encoded `bp_parachains::ParaStoredHeaderData` structure among all Polkadot
 /// parachains.

bridges/chains/chain-polkadot/src/lib.rs

@@ -69,6 +69,8 @@ pub const PARAS_PALLET_NAME: &str = "Paras";
 
 /// Name of the With-Polkadot GRANDPA pallet instance that is deployed at bridged chains.
 pub const WITH_POLKADOT_GRANDPA_PALLET_NAME: &str = "BridgePolkadotGrandpa";
+/// Name of the With-Polkadot parachains pallet instance that is deployed at bridged chains.
+pub const WITH_POLKADOT_BRIDGE_PARACHAINS_PALLET_NAME: &str = "BridgePolkadotParachains";
 
 /// Maximal size of encoded `bp_parachains::ParaStoredHeaderData` structure among all Polkadot
 /// parachains.

bridges/chains/chain-rococo/src/lib.rs

@@ -67,6 +67,8 @@ pub const PARAS_PALLET_NAME: &str = "Paras";
 
 /// Name of the With-Rococo GRANDPA pallet instance that is deployed at bridged chains.
 pub const WITH_ROCOCO_GRANDPA_PALLET_NAME: &str = "BridgeRococoGrandpa";
+/// Name of the With-Rococo parachains pallet instance that is deployed at bridged chains.
+pub const WITH_ROCOCO_BRIDGE_PARACHAINS_PALLET_NAME: &str = "BridgeRococoParachains";
 
 /// Maximal size of encoded `bp_parachains::ParaStoredHeaderData` structure among all Rococo
 /// parachains.

bridges/chains/chain-westend/src/lib.rs

@@ -67,6 +67,8 @@ pub const PARAS_PALLET_NAME: &str = "Paras";
 
 /// Name of the With-Westend GRANDPA pallet instance that is deployed at bridged chains.
 pub const WITH_WESTEND_GRANDPA_PALLET_NAME: &str = "BridgeWestendGrandpa";
+/// Name of the With-Westend parachains pallet instance that is deployed at bridged chains.
+pub const WITH_WESTEND_BRIDGE_PARACHAINS_PALLET_NAME: &str = "BridgeWestendParachains";
 
 /// Maximal size of encoded `bp_parachains::ParaStoredHeaderData` structure among all Westend
 /// parachains.

bridges/relays/client-substrate/src/chain.rs

@@ -46,6 +46,12 @@ pub trait Chain: ChainBase + Clone {
 	/// Keep in mind that this method is normally provided by the other chain, which is
 	/// bridged with this chain.
 	const BEST_FINALIZED_HEADER_ID_METHOD: &'static str;
+	/// Name of the runtime API method that is returning interval between source chain
+	/// headers that may be submitted for free to the target chain.
+	///
+	/// Keep in mind that this method is normally provided by the other chain, which is
+	/// bridged with this chain.
+	const FREE_HEADERS_INTERVAL_METHOD: &'static str;
 
 	/// Average block interval.
 	///
@@ -75,6 +81,9 @@ pub trait ChainWithRuntimeVersion: Chain {
 pub trait RelayChain: Chain {
 	/// Name of the `runtime_parachains::paras` pallet in the runtime of this chain.
 	const PARAS_PALLET_NAME: &'static str;
+	/// Name of the `pallet-bridge-parachains`, deployed at the **bridged** chain to sync
+	/// parachains of **this** chain.
+	const WITH_CHAIN_BRIDGE_PARACHAINS_PALLET_NAME: &'static str;
 }
 
 /// Substrate-based chain that is using direct GRANDPA finality from minimal relay-client point of

bridges/relays/client-substrate/src/test_chain.rs

@@ -56,6 +56,7 @@ impl bp_runtime::Chain for TestChain {
 impl Chain for TestChain {
 	const NAME: &'static str = "Test";
 	const BEST_FINALIZED_HEADER_ID_METHOD: &'static str = "TestMethod";
+	const FREE_HEADERS_INTERVAL_METHOD: &'static str = "TestMethod";
 	const AVERAGE_BLOCK_INTERVAL: Duration = Duration::from_millis(0);
 
 	type SignedBlock = sp_runtime::generic::SignedBlock<
@@ -124,6 +125,7 @@ impl bp_runtime::UnderlyingChainProvider for TestParachain {
 impl Chain for TestParachain {
 	const NAME: &'static str = "TestParachain";
 	const BEST_FINALIZED_HEADER_ID_METHOD: &'static str = "TestParachainMethod";
+	const FREE_HEADERS_INTERVAL_METHOD: &'static str = "TestParachainMethod";
 	const AVERAGE_BLOCK_INTERVAL: Duration = Duration::from_millis(0);
 
 	type SignedBlock = sp_runtime::generic::SignedBlock<

bridges/relays/finality/README.md

@@ -33,7 +33,9 @@ node. The transaction is then tracked by the relay until it is mined and finaliz
 The main entrypoint for the crate is the [`run` function](./src/finality_loop.rs), which takes source and target
 clients and [`FinalitySyncParams`](./src/finality_loop.rs) parameters. The most important parameter is the
 `only_mandatory_headers` - it is set to `true`, the relay will only submit mandatory headers. Since transactions
-with mandatory headers are fee-free, the cost of running such relay is zero (in terms of fees).
+with mandatory headers are fee-free, the cost of running such relay is zero (in terms of fees). If a similar,
+`only_free_headers` parameter, is set to `true`, then free headers (if configured in the runtime) are also
+relayed.
 
 ## Finality Relay Metrics