chainHead: Add support for storage closest merkle descendant #14818 (#1153)

This PR adds support for fetching the closest merkle value of some key.


Builds on top of
- https://github.com/paritytech/trie/pull/199

Migrates https://github.com/paritytech/substrate/pull/14818 to the
monorepo.
Closes: https://github.com/paritytech/substrate/issues/14550
Closes: https://github.com/paritytech/polkadot-sdk/issues/1506

// @paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>

substrate/client/db/src/bench.rs

@@ -37,7 +37,7 @@ use sp_state_machine::{
 };
 use sp_trie::{
 	cache::{CacheSize, SharedTrieCache},
-	prefixed_key, MemoryDB,
+	prefixed_key, MemoryDB, MerkleValue,
 };
 use std::{
 	cell::{Cell, RefCell},
@@ -382,6 +382,27 @@ impl<B: BlockT> StateBackend<HashingFor<B>> for BenchmarkingState<B> {
 			.child_storage_hash(child_info, key)
 	}
 
+	fn closest_merkle_value(
+		&self,
+		key: &[u8],
+	) -> Result<Option<MerkleValue<B::Hash>>, Self::Error> {
+		self.add_read_key(None, key);
+		self.state.borrow().as_ref().ok_or_else(state_err)?.closest_merkle_value(key)
+	}
+
+	fn child_closest_merkle_value(
+		&self,
+		child_info: &ChildInfo,
+		key: &[u8],
+	) -> Result<Option<MerkleValue<B::Hash>>, Self::Error> {
+		self.add_read_key(None, key);
+		self.state
+			.borrow()
+			.as_ref()
+			.ok_or_else(state_err)?
+			.child_closest_merkle_value(child_info, key)
+	}
+
 	fn exists_storage(&self, key: &[u8]) -> Result<bool, Self::Error> {
 		self.add_read_key(None, key);
 		self.state.borrow().as_ref().ok_or_else(state_err)?.exists_storage(key)