define block hash provider and default impl using frame_system (#4080)

This PR introduces `BlockHashProvider` into `pallet_mmr::Config`
This type is used to get `block_hash` for a given `block_number` rather
than directly using `frame_system::Pallet::block_hash`

The `DefaultBlockHashProvider` uses `frame_system::Pallet::block_hash`
to get the `block_hash`

Closes: #4062

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

@@ -103,6 +103,24 @@ impl<T: frame_system::Config> LeafDataProvider for ParentNumberAndHash<T> {
 	}
 }
 
+/// Block hash provider for a given block number.
+pub trait BlockHashProvider<BlockNumber, BlockHash> {
+	fn block_hash(block_number: BlockNumber) -> BlockHash;
+}
+
+/// Default implementation of BlockHashProvider using frame_system.
+pub struct DefaultBlockHashProvider<T: frame_system::Config> {
+	_phantom: sp_std::marker::PhantomData<T>,
+}
+
+impl<T: frame_system::Config> BlockHashProvider<BlockNumberFor<T>, T::Hash>
+	for DefaultBlockHashProvider<T>
+{
+	fn block_hash(block_number: BlockNumberFor<T>) -> T::Hash {
+		frame_system::Pallet::<T>::block_hash(block_number)
+	}
+}
+
 pub trait WeightInfo {
 	fn on_initialize(peaks: NodeIndex) -> Weight;
 }
@@ -177,6 +195,12 @@ pub mod pallet {
 		/// Clients. Hook complexity should be `O(1)`.
 		type OnNewRoot: primitives::OnNewRoot<HashOf<Self, I>>;
 
+		/// Block hash provider for a given block number.
+		type BlockHashProvider: BlockHashProvider<
+			BlockNumberFor<Self>,
+			<Self as frame_system::Config>::Hash,
+		>;
+
 		/// Weights for this pallet.
 		type WeightInfo: WeightInfo;
 	}

substrate/frame/merkle-mountain-range/src/mmr/storage.rs

@@ -29,7 +29,7 @@ use sp_std::prelude::*;
 use crate::{
 	mmr::{Node, NodeOf},
 	primitives::{self, NodeIndex},
-	Config, Nodes, NumberOfLeaves, Pallet,
+	BlockHashProvider, Config, Nodes, NumberOfLeaves, Pallet,
 };
 
 /// A marker type for runtime-specific storage implementation.
@@ -87,7 +87,7 @@ where
 		// Fall through to searching node using fork-specific key.
 		let ancestor_parent_block_num =
 			Pallet::<T, I>::leaf_index_to_parent_block_num(ancestor_leaf_idx, leaves);
-		let ancestor_parent_hash = <frame_system::Pallet<T>>::block_hash(ancestor_parent_block_num);
+		let ancestor_parent_hash = T::BlockHashProvider::block_hash(ancestor_parent_block_num);
 		let temp_key = Pallet::<T, I>::node_temp_offchain_key(pos, ancestor_parent_hash);
 		debug!(
 			target: "runtime::mmr::offchain",

substrate/frame/merkle-mountain-range/src/mock.rs

@@ -44,6 +44,7 @@ impl Config for Test {
 	type Hashing = Keccak256;
 	type LeafData = Compact<Keccak256, (ParentNumberAndHash<Test>, LeafData)>;
 	type OnNewRoot = ();
+	type BlockHashProvider = DefaultBlockHashProvider<Test>;
 	type WeightInfo = ();
 }