Paged keys rpc for child storage. (#9100)

* childstate_getKeysPaged rpc

* Rename `v` to `iter`.

* Update client/api/src/backend.rs

Co-authored-by: Alexander Popiak <alexander.popiak@gmail.com>

* Update client/api/src/backend.rs

Co-authored-by: Alexander Popiak <alexander.popiak@gmail.com>

Co-authored-by: Alexander Popiak <alexander.popiak@gmail.com>

substrate/client/rpc/src/state/mod.rs

@@ -402,6 +402,16 @@ pub trait ChildStateBackend<Block: BlockT, Client>: Send + Sync + 'static
 		prefix: StorageKey,
 	) -> FutureResult<Vec<StorageKey>>;
 
+	/// Returns the keys with prefix from a child storage with pagination support.
+	fn storage_keys_paged(
+		&self,
+		block: Option<Block::Hash>,
+		storage_key: PrefixedStorageKey,
+		prefix: Option<StorageKey>,
+		count: u32,
+		start_key: Option<StorageKey>,
+	) -> FutureResult<Vec<StorageKey>>;
+
 	/// Returns a child storage entry at a specific block's state.
 	fn storage(
 		&self,
@@ -469,6 +479,17 @@ impl<Block, Client> ChildStateApi<Block::Hash> for ChildState<Block, Client>
 		self.backend.storage_keys(block, storage_key, key_prefix)
 	}
 
+	fn storage_keys_paged(
+		&self,
+		storage_key: PrefixedStorageKey,
+		prefix: Option<StorageKey>,
+		count: u32,
+		start_key: Option<StorageKey>,
+		block: Option<Block::Hash>,
+	) -> FutureResult<Vec<StorageKey>> {
+		self.backend.storage_keys_paged(block, storage_key, prefix, count, start_key)
+	}
+
 	fn storage_hash(
 		&self,
 		storage_key: PrefixedStorageKey,

substrate/client/rpc/src/state/state_full.rs

@@ -296,7 +296,7 @@ impl<BE, Block, Client> StateBackend<Block, Client> for FullState<BE, Block, Cli
 						&BlockId::Hash(block), prefix.as_ref(), start_key.as_ref()
 					)
 				)
-				.map(|v| v.take(count as usize).collect())
+				.map(|iter| iter.take(count as usize).collect())
 				.map_err(client_err)))
 	}
 
@@ -618,6 +618,29 @@ impl<BE, Block, Client> ChildStateBackend<Block, Client> for FullState<BE, Block
 				.map_err(client_err)))
 	}
 
+	fn storage_keys_paged(
+		&self,
+		block: Option<Block::Hash>,
+		storage_key: PrefixedStorageKey,
+		prefix: Option<StorageKey>,
+		count: u32,
+		start_key: Option<StorageKey>,
+	) -> FutureResult<Vec<StorageKey>> {
+		Box::new(result(
+			self.block_or_best(block)
+				.and_then(|block| {
+					let child_info = match ChildType::from_prefixed_key(&storage_key) {
+						Some((ChildType::ParentKeyId, storage_key)) => ChildInfo::new_default(storage_key),
+						None => return Err(sp_blockchain::Error::InvalidChildStorageKey),
+					};
+					self.client.child_storage_keys_iter(
+						&BlockId::Hash(block), child_info, prefix.as_ref(), start_key.as_ref(),
+					)
+				})
+				.map(|iter| iter.take(count as usize).collect())
+				.map_err(client_err)))
+	}
+
 	fn storage(
 		&self,
 		block: Option<Block::Hash>,

substrate/client/rpc/src/state/state_light.rs

@@ -509,6 +509,17 @@ impl<Block, F, Client> ChildStateBackend<Block, Client> for LightState<Block, F,
 		Box::new(result(Err(client_err(ClientError::NotAvailableOnLightClient))))
 	}
 
+	fn storage_keys_paged(
+		&self,
+		_block: Option<Block::Hash>,
+		_storage_key: PrefixedStorageKey,
+		_prefix: Option<StorageKey>,
+		_count: u32,
+		_start_key: Option<StorageKey>,
+	) -> FutureResult<Vec<StorageKey>> {
+		Box::new(result(Err(client_err(ClientError::NotAvailableOnLightClient))))
+	}
+
 	fn storage(
 		&self,
 		block: Option<Block::Hash>,