pallet-mmr: handle forks without collisions in offchain storage

frame/beefy-mmr/src/tests.rs

@@ -44,16 +44,12 @@ pub fn beefy_log(log: ConsensusLog<BeefyId>) -> DigestItem {
 	DigestItem::Consensus(BEEFY_ENGINE_ID, log.encode())
 }
 
-fn offchain_key(pos: usize) -> Vec<u8> {
-	(<Test as pallet_mmr::Config>::INDEXING_PREFIX, pos as u64).encode()
-}
-
-fn read_mmr_leaf(ext: &mut TestExternalities, index: usize) -> MmrLeaf {
+fn read_mmr_leaf(ext: &mut TestExternalities, key: Vec<u8>) -> MmrLeaf {
 	type Node = pallet_mmr::primitives::DataOrHash<Keccak256, MmrLeaf>;
 	ext.persist_offchain_overlay();
 	let offchain_db = ext.offchain_db();
 	offchain_db
-		.get(&offchain_key(index))
+		.get(&key)
 		.map(|d| Node::decode(&mut &*d).unwrap())
 		.map(|n| match n {
 			Node::Data(d) => d,
@@ -105,12 +101,17 @@ fn should_contain_mmr_digest() {
 
 #[test]
 fn should_contain_valid_leaf_data() {
+	fn node_offchain_key(parent_hash: H256, pos: usize) -> Vec<u8> {
+		(<Test as pallet_mmr::Config>::INDEXING_PREFIX, parent_hash, pos as u64).encode()
+	}
+
 	let mut ext = new_test_ext(vec![1, 2, 3, 4]);
-	ext.execute_with(|| {
+	let parent_hash = ext.execute_with(|| {
 		init_block(1);
+		<frame_system::Pallet<Test>>::parent_hash()
 	});
 
-	let mmr_leaf = read_mmr_leaf(&mut ext, 0);
+	let mmr_leaf = read_mmr_leaf(&mut ext, node_offchain_key(parent_hash, 0));
 	assert_eq!(
 		mmr_leaf,
 		MmrLeaf {
@@ -128,11 +129,12 @@ fn should_contain_valid_leaf_data() {
 	);
 
 	// build second block on top
-	ext.execute_with(|| {
+	let parent_hash = ext.execute_with(|| {
 		init_block(2);
+		<frame_system::Pallet<Test>>::parent_hash()
 	});
 
-	let mmr_leaf = read_mmr_leaf(&mut ext, 1);
+	let mmr_leaf = read_mmr_leaf(&mut ext, node_offchain_key(parent_hash, 1));
 	assert_eq!(
 		mmr_leaf,
 		MmrLeaf {

frame/merkle-mountain-range/src/lib.rs

@@ -58,7 +58,10 @@
 
 use codec::Encode;
 use frame_support::weights::Weight;
-use sp_runtime::traits::{self, One, Saturating};
+use sp_runtime::{
+	traits::{self, One, Saturating},
+	SaturatedConversion,
+};
 
 #[cfg(any(feature = "runtime-benchmarks", test))]
 mod benchmarking;
@@ -116,12 +119,12 @@ pub mod pallet {
 		/// Prefix for elements stored in the Off-chain DB via Indexing API.
 		///
 		/// Each node of the MMR is inserted both on-chain and off-chain via Indexing API.
-		/// The former does not store full leaf content, just it's compact version (hash),
+		/// The former does not store full leaf content, just its compact version (hash),
 		/// and some of the inner mmr nodes might be pruned from on-chain storage.
 		/// The latter will contain all the entries in their full form.
 		///
 		/// Each node is stored in the Off-chain DB under key derived from the
-		/// [`Self::INDEXING_PREFIX`] and it's in-tree index (MMR position).
+		/// [`Self::INDEXING_PREFIX`] and its in-tree index (MMR position).
 		const INDEXING_PREFIX: &'static [u8];
 
 		/// A hasher type for MMR.
@@ -215,8 +218,31 @@ pub mod pallet {
 			<RootHash<T, I>>::put(root);
 
 			let peaks_after = mmr::utils::NodesUtils::new(leaves).number_of_peaks();
+
 			T::WeightInfo::on_initialize(peaks_before.max(peaks_after))
 		}
+
+		fn offchain_worker(n: T::BlockNumber) {
+			use mmr::storage::{OffchainStorage, Storage};
+			// MMR pallet uses offchain storage to hold full MMR and leaves.
+			// The leaves are saved under fork-unique keys `(parent_hash, pos)`.
+			// MMR Runtime depends on `frame_system::block_hash(block_num)` mappings to find
+			// parent hashes for particular nodes or leaves.
+			// This MMR offchain worker function moves a rolling window of the same size
+			// as `frame_system::block_hash` map, where nodes/leaves added by blocks that are just
+			// about to exit the window are "canonicalized" so that their offchain key no longer
+			// depends on `parent_hash` therefore on access to `frame_system::block_hash`.
+			//
+			// This approach works to eliminate fork-induced leaf collisions in offchain db,
+			// under the assumption that no fork will be deeper than `frame_system::BlockHashCount`
+			// blocks (2400 blocks on Polkadot, Kusama, Rococo, etc):
+			//   entries pertaining to block `N` where `N < current-2400` are moved to a key based
+			//   solely on block number. The only way to have collisions is if two competing forks
+			//   are deeper than 2400 blocks and they both "canonicalize" their view of block `N`.
+			// Once a block is canonicalized, all MMR entries pertaining to sibling blocks from
+			// other forks are pruned from offchain db.
+			Storage::<OffchainStorage, T, I, LeafOf<T, I>>::canonicalize_and_prune(n);
+		}
 	}
 }
 
@@ -254,17 +280,46 @@ where
 }
 
 impl<T: Config<I>, I: 'static> Pallet<T, I> {
-	fn offchain_key(pos: NodeIndex) -> sp_std::prelude::Vec<u8> {
+	/// Build offchain key from `parent_hash` of block that originally added node `pos` to MMR.
+	///
+	/// This combination makes the offchain (key,value) entry resilient to chain forks.
+	fn node_offchain_key(
+		parent_hash: <T as frame_system::Config>::Hash,
+		pos: NodeIndex,
+	) -> sp_std::prelude::Vec<u8> {
+		(T::INDEXING_PREFIX, parent_hash, pos).encode()
+	}
+
+	/// Build canonical offchain key for node `pos` in MMR.
+	///
+	/// Used for nodes added by now finalized blocks.
+	fn node_canon_offchain_key(pos: NodeIndex) -> sp_std::prelude::Vec<u8> {
 		(T::INDEXING_PREFIX, pos).encode()
 	}
+
+	/// Provide the parent number for the block that added `leaf_index` to the MMR.
+	fn leaf_index_to_parent_block_num(
+		leaf_index: LeafIndex,
+		leaves_count: LeafIndex,
+	) -> <T as frame_system::Config>::BlockNumber {
+		// leaves are zero-indexed and were added one per block since pallet activation,
+		// while block numbers are one-indexed, so block number that added `leaf_idx` is:
+		// `block_num = block_num_when_pallet_activated + leaf_idx + 1`
+		// `block_num = (current_block_num - leaves_count) + leaf_idx + 1`
+		// `parent_block_num = current_block_num - leaves_count + leaf_idx`.
+		<frame_system::Pallet<T>>::block_number()
+			.saturating_sub(leaves_count.saturated_into())
+			.saturating_add(leaf_index.saturated_into())
+	}
+
 	/// Generate a MMR proof for the given `leaf_indices`.
 	///
 	/// Note this method can only be used from an off-chain context
 	/// (Offchain Worker or Runtime API call), since it requires
 	/// all the leaves to be present.
 	/// It may return an error or panic if used incorrectly.
 	pub fn generate_batch_proof(
-		leaf_indices: Vec<NodeIndex>,
+		leaf_indices: Vec<LeafIndex>,
 	) -> Result<
 		(Vec<LeafOf<T, I>>, primitives::BatchProof<<T as Config<I>>::Hash>),
 		primitives::Error,