Introduce clear_gap API for Snap sync downstream

substrate/client/api/src/in_mem.rs

@@ -433,6 +433,10 @@ impl<Block: BlockT> blockchain::Backend<Block> for Blockchain<Block> {
 	) -> sp_blockchain::Result<Option<Vec<Vec<u8>>>> {
 		unimplemented!("Not supported by the in-mem backend.")
 	}
+
+	fn clear_block_gap(&self) -> sp_blockchain::Result<()> {
+		unimplemented!("Not supported by the in-mem backend.")
+	}
 }
 
 impl<Block: BlockT> backend::AuxStore for Blockchain<Block> {

substrate/client/db/src/lib.rs

@@ -790,6 +790,18 @@ impl<Block: BlockT> sc_client_api::blockchain::Backend<Block> for BlockchainDb<B
 				Err(sp_blockchain::Error::Backend(format!("Error decoding body list: {}", err))),
 		}
 	}
+
+	fn clear_block_gap(&self) -> ClientResult<()> {
+		debug!(target: "sync", "Clear block gap.");
+
+		let mut transaction = Transaction::new();
+		transaction.remove(columns::META, meta_keys::BLOCK_GAP);
+		self.db.commit(transaction)?;
+
+		self.update_block_gap(None);
+
+		Ok(())
+	}
 }
 
 impl<Block: BlockT> HeaderMetadata<Block> for BlockchainDb<Block> {
@@ -1768,6 +1780,7 @@ impl<Block: BlockT> Backend<Block> {
 		for m in meta_updates {
 			self.blockchain.update_meta(m);
 		}
+
 		self.blockchain.update_block_gap(block_gap);
 
 		Ok(())

substrate/client/network/sync/src/state_request_handler.rs

@@ -76,7 +76,10 @@ pub fn generate_protocol_config<
 }
 
 /// Generate the state protocol name from the genesis hash and fork id.
-fn generate_protocol_name<Hash: AsRef<[u8]>>(genesis_hash: Hash, fork_id: Option<&str>) -> String {
+pub fn generate_protocol_name<Hash: AsRef<[u8]>>(
+	genesis_hash: Hash,
+	fork_id: Option<&str>,
+) -> String {
 	let genesis_hash = genesis_hash.as_ref();
 	if let Some(fork_id) = fork_id {
 		format!("/{}/{}/state/2", array_bytes::bytes2hex("", genesis_hash), fork_id)

substrate/client/network/sync/src/strategy.rs

@@ -20,7 +20,7 @@
 //! and specific syncing algorithms.
 
 pub mod chain_sync;
-mod state;
+pub(crate) mod state;
 pub mod state_sync;
 pub mod warp;
 

substrate/client/service/src/client/client.rs

@@ -119,6 +119,18 @@ where
 	_phantom: PhantomData<RA>,
 }
 
+impl<B, E, Block, RA> crate::ClientExt<Block, B> for Client<B, E, Block, RA>
+where
+	B: backend::Backend<Block>,
+	E: CallExecutor<Block> + Send + Sync,
+	Block: BlockT,
+	RA: Sync + Send,
+{
+	fn clear_block_gap(&self) -> sp_blockchain::Result<()> {
+		self.backend.blockchain().clear_block_gap()
+	}
+}
+
 /// Used in importing a block, where additional changes are made after the runtime
 /// executed.
 enum PrePostHeader<H> {

substrate/client/service/src/lib.rs

@@ -40,7 +40,9 @@ use codec::{Decode, Encode};
 use futures::{pin_mut, FutureExt, StreamExt};
 use jsonrpsee::RpcModule;
 use log::{debug, error, warn};
-use sc_client_api::{blockchain::HeaderBackend, BlockBackend, BlockchainEvents, ProofProvider};
+use sc_client_api::{
+	backend, blockchain::HeaderBackend, BlockBackend, BlockchainEvents, ProofProvider,
+};
 use sc_network::{
 	config::MultiaddrWithPeerId, service::traits::NetworkService, NetworkBackend, NetworkBlock,
 	NetworkPeers, NetworkStateInfo,
@@ -100,6 +102,12 @@ const DEFAULT_PROTOCOL_ID: &str = "sup";
 #[derive(Clone)]
 pub struct RpcHandlers(Arc<RpcModule<()>>);
 
+/// Provides extended functions for `Client` to enable fast-sync.
+pub trait ClientExt<Block: BlockT, B: backend::Backend<Block>> {
+	/// Clear block gap after initial block insertion.
+	fn clear_block_gap(&self) -> sp_blockchain::Result<()>;
+}
+
 impl RpcHandlers {
 	/// Starts an RPC query.
 	///

substrate/primitives/blockchain/src/backend.rs

@@ -514,6 +514,9 @@ pub trait Backend<Block: BlockT>:
 
 		return Ok(result);
 	}
+
+    /// Clears the block gap from DB after the fast-sync.
+    fn clear_block_gap(&self) -> Result<()>;
 }
 
 /// Result of  [`Backend::displaced_leaves_after_finalizing`].