Add standalone set head operations (#1600)

* Refactor head setting operation to a separate function

* Fix commit issue and implement set_head standalone in db

* Add standalone set head operations

* Address grumbles

* Change number_and_hash_to_lookup key in light mod to take reference

* Fix bug in set head commit

* Add a convenience fn

* Fix a deadlock

* Fix missing argument

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

@@ -50,6 +50,7 @@ pub struct ImportOperation<Block: BlockT, S, F, H> {
 	leaf_state: NewBlockState,
 	aux_ops: Vec<(Vec<u8>, Option<Vec<u8>>)>,
 	finalized_blocks: Vec<BlockId<Block>>,
+	set_head: Option<BlockId<Block>>,
 	storage_update: Option<InMemoryState<H>>,
 	_phantom: ::std::marker::PhantomData<(S, F)>,
 }
@@ -120,6 +121,7 @@ impl<S, F, Block, H> ClientBackend<Block, H> for Backend<S, F, H> where
 			leaf_state: NewBlockState::Normal,
 			aux_ops: Vec::new(),
 			finalized_blocks: Vec::new(),
+			set_head: None,
 			storage_update: None,
 			_phantom: Default::default(),
 		})
@@ -165,6 +167,10 @@ impl<S, F, Block, H> ClientBackend<Block, H> for Backend<S, F, H> where
 			}
 		}
 
+		if let Some(set_head) = operation.set_head {
+			self.blockchain.storage().set_head(set_head)?;
+		}
+
 		Ok(())
 	}
 
@@ -294,6 +300,11 @@ where
 		self.finalized_blocks.push(block);
 		Ok(())
 	}
+
+	fn mark_head(&mut self, block: BlockId<Block>) -> ClientResult<()> {
+		self.set_head = Some(block);
+		Ok(())
+	}
 }
 
 impl<Block, S, F, H> StateBackend<H> for OnDemandState<Block, S, F>