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/api/Cargo.toml

@@ -35,6 +35,7 @@ sp-runtime = { path = "../../primitives/runtime", default-features = false}
 sp-state-machine = { path = "../../primitives/state-machine" }
 sp-statement-store = { path = "../../primitives/statement-store" }
 sp-storage = { path = "../../primitives/storage" }
+sp-trie = { path = "../../primitives/trie" }
 
 [dev-dependencies]
 thiserror = "1.0.48"

substrate/client/api/src/backend.rs

@@ -33,6 +33,7 @@ use sp_state_machine::{
 	OffchainChangesCollection, StorageCollection, StorageIterator,
 };
 use sp_storage::{ChildInfo, StorageData, StorageKey};
+pub use sp_trie::MerkleValue;
 
 use crate::{blockchain::Backend as BlockchainBackend, UsageInfo};
 
@@ -470,6 +471,21 @@ pub trait StorageProvider<Block: BlockT, B: Backend<Block>> {
 		child_info: &ChildInfo,
 		key: &StorageKey,
 	) -> sp_blockchain::Result<Option<Block::Hash>>;
+
+	/// Given a block's `Hash` and a key, return the closest merkle value.
+	fn closest_merkle_value(
+		&self,
+		hash: Block::Hash,
+		key: &StorageKey,
+	) -> sp_blockchain::Result<Option<MerkleValue<Block::Hash>>>;
+
+	/// Given a block's `Hash`, a key and a child storage key, return the closest merkle value.
+	fn child_closest_merkle_value(
+		&self,
+		hash: Block::Hash,
+		child_info: &ChildInfo,
+		key: &StorageKey,
+	) -> sp_blockchain::Result<Option<MerkleValue<Block::Hash>>>;
 }
 
 /// Client backend.