Companion for: pallet-mmr: move offchain logic to client-side gadget (#6321)

* Spawn MMR gadget when offchain indexing is enabled

* companion PR code review changes: 1st iteration

* Code review changes: 2nd iteration

* update lockfile for {"substrate"}

Co-authored-by: acatangiu <adrian@parity.io>
Co-authored-by: parity-processbot <>

polkadot/node/service/src/lib.rs

@@ -95,6 +95,7 @@ pub use polkadot_client::PolkadotExecutorDispatch;
 
 pub use chain_spec::{KusamaChainSpec, PolkadotChainSpec, RococoChainSpec, WestendChainSpec};
 pub use consensus_common::{block_validation::Chain, Proposal, SelectChain};
+use mmr_gadget::MmrGadget;
 #[cfg(feature = "full-node")]
 pub use polkadot_client::{
 	AbstractClient, Client, ClientHandle, ExecuteWithClient, FullBackend, FullClient,
@@ -758,6 +759,7 @@ where
 {
 	use polkadot_node_network_protocol::request_response::IncomingRequest;
 
+	let is_offchain_indexing_enabled = config.offchain_worker.indexing_enabled;
 	let role = config.role.clone();
 	let force_authoring = config.force_authoring;
 	let backoff_authoring_blocks = {
@@ -1219,6 +1221,18 @@ where
 		} else {
 			task_manager.spawn_handle().spawn_blocking("beefy-gadget", None, gadget);
 		}
+
+		if is_offchain_indexing_enabled {
+			task_manager.spawn_handle().spawn_blocking(
+				"mmr-gadget",
+				None,
+				MmrGadget::start(
+					client.clone(),
+					backend.clone(),
+					sp_mmr_primitives::INDEXING_PREFIX.to_vec(),
+				),
+			);
+		}
 	}
 
 	let config = grandpa::Config {