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/primitives/state-machine/src/trie_backend.rs

@@ -30,7 +30,6 @@ use codec::Codec;
 use hash_db::HashDB;
 use hash_db::Hasher;
 use sp_core::storage::{ChildInfo, StateVersion};
-use sp_trie::PrefixedMemoryDB;
 #[cfg(feature = "std")]
 use sp_trie::{
 	cache::{LocalTrieCache, TrieCache},
@@ -39,6 +38,7 @@ use sp_trie::{
 };
 #[cfg(not(feature = "std"))]
 use sp_trie::{Error, NodeCodec};
+use sp_trie::{MerkleValue, PrefixedMemoryDB};
 use trie_db::TrieCache as TrieCacheT;
 #[cfg(not(feature = "std"))]
 use trie_db::{node::NodeOwned, CachedValue};
@@ -405,6 +405,18 @@ where
 		self.essence.child_storage(child_info, key)
 	}
 
+	fn closest_merkle_value(&self, key: &[u8]) -> Result<Option<MerkleValue<H::Out>>, Self::Error> {
+		self.essence.closest_merkle_value(key)
+	}
+
+	fn child_closest_merkle_value(
+		&self,
+		child_info: &ChildInfo,
+		key: &[u8],
+	) -> Result<Option<MerkleValue<H::Out>>, Self::Error> {
+		self.essence.child_closest_merkle_value(child_info, key)
+	}
+
 	fn next_storage_key(&self, key: &[u8]) -> Result<Option<StorageKey>, Self::Error> {
 		let (is_cached, mut cache) = access_cache(&self.next_storage_key_cache, Option::take)
 			.map(|cache| (cache.last_key == key, cache))