Child trie storage proof (#2433)

* proof on child trie

* higher level api for child storage proof

* boilerplate for proof from light fetch

* actually check proof on light fetch

* Do not break former encoding

* tabify

* tabify2

* Add child trie root tx to full_storage_root transaction.

* Shorten long lines.

* Temp rename for audit

* Make full_storage a trait method

* Name back and replace some code with full_storage where it looks fine.

* fix indentations, remove unused import

* flush child root to top when calculated

* impl +1

substrate/core/client/src/light/fetcher.rs

@@ -25,7 +25,8 @@ use hash_db::{HashDB, Hasher};
 use primitives::{ChangesTrieConfiguration, convert_hash};
 use runtime_primitives::traits::{As, Block as BlockT, Header as HeaderT, NumberFor};
 use state_machine::{CodeExecutor, ChangesTrieRootsStorage, ChangesTrieAnchorBlockId,
-	TrieBackend, read_proof_check, key_changes_proof_check, create_proof_check_backend_storage};
+	TrieBackend, read_proof_check, key_changes_proof_check,
+	create_proof_check_backend_storage, read_child_proof_check};
 
 use crate::cht;
 use crate::error::{Error as ClientError, Result as ClientResult};
@@ -71,6 +72,21 @@ pub struct RemoteReadRequest<Header: HeaderT> {
 	pub retry_count: Option<usize>,
 }
 
+/// Remote storage read child request.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct RemoteReadChildRequest<Header: HeaderT> {
+	/// Read at state of given block.
+	pub block: Header::Hash,
+	/// Header of block at which read is performed.
+	pub header: Header,
+	/// Storage key for child.
+	pub storage_key: Vec<u8>,
+	/// Child storage key to read.
+	pub key: Vec<u8>,
+	/// Number of times to retry request. None means that default RETRY_COUNT is used.
+	pub retry_count: Option<usize>,
+}
+
 /// Remote key changes read request.
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct RemoteChangesRequest<Header: HeaderT> {
@@ -123,7 +139,15 @@ pub trait Fetcher<Block: BlockT>: Send + Sync {
 	/// Fetch remote header.
 	fn remote_header(&self, request: RemoteHeaderRequest<Block::Header>) -> Self::RemoteHeaderResult;
 	/// Fetch remote storage value.
-	fn remote_read(&self, request: RemoteReadRequest<Block::Header>) -> Self::RemoteReadResult;
+	fn remote_read(
+		&self,
+		request: RemoteReadRequest<Block::Header>
+	) -> Self::RemoteReadResult;
+	/// Fetch remote storage child value.
+	fn remote_read_child(
+		&self,
+		request: RemoteReadChildRequest<Block::Header>
+	) -> Self::RemoteReadResult;
 	/// Fetch remote call result.
 	fn remote_call(&self, request: RemoteCallRequest<Block::Header>) -> Self::RemoteCallResult;
 	/// Fetch remote changes ((block number, extrinsic index)) where given key has been changed
@@ -149,6 +173,12 @@ pub trait FetchChecker<Block: BlockT>: Send + Sync {
 		request: &RemoteReadRequest<Block::Header>,
 		remote_proof: Vec<Vec<u8>>
 	) -> ClientResult<Option<Vec<u8>>>;
+	/// Check remote storage read proof.
+	fn check_read_child_proof(
+		&self,
+		request: &RemoteReadChildRequest<Block::Header>,
+		remote_proof: Vec<Vec<u8>>
+	) -> ClientResult<Option<Vec<u8>>>;
 	/// Check remote method execution proof.
 	fn check_execution_proof(
 		&self,
@@ -338,6 +368,19 @@ impl<E, Block, H, S, F> FetchChecker<Block> for LightDataChecker<E, H, Block, S,
 			.map_err(Into::into)
 	}
 
+	fn check_read_child_proof(
+		&self,
+		request: &RemoteReadChildRequest<Block::Header>,
+		remote_proof: Vec<Vec<u8>>
+	) -> ClientResult<Option<Vec<u8>>> {
+		read_child_proof_check::<H>(
+			convert_hash(request.header.state_root()),
+			remote_proof,
+			&request.storage_key,
+			&request.key)
+			.map_err(Into::into)
+	}
+
 	fn check_execution_proof(
 		&self,
 		request: &RemoteCallRequest<Block::Header>,
@@ -425,6 +468,10 @@ pub mod tests {
 			err("Not implemented on test node".into())
 		}
 
+		fn remote_read_child(&self, _request: RemoteReadChildRequest<Header>) -> Self::RemoteReadResult {
+			err("Not implemented on test node".into())
+		}
+
 		fn remote_call(&self, _request: RemoteCallRequest<Header>) -> Self::RemoteCallResult {
 			ok((*self.lock()).clone())
 		}