core: restrict grandpa voter on authority set change (#1564)

substrate/core/finality-grandpa/src/lib.rs

@@ -415,6 +415,14 @@ impl<Block: BlockT<Hash=H256>, B, E, N, RA> grandpa::Chain<Block::Hash, NumberFo
 	}
 
 	fn best_chain_containing(&self, block: Block::Hash) -> Option<(Block::Hash, NumberFor<Block>)> {
+		// NOTE: when we finalize an authority set change through the sync protocol the voter is
+		//       signalled asynchronously. therefore the voter could still vote in the next round
+		//       before activating the new set. the `authority_set` is updated immediately thus we
+		//       restrict the voter based on that.
+		if self.set_id != self.authority_set.inner().read().current().0 {
+			return None;
+		}
+
 		// we refuse to vote beyond the current limit number where transitions are scheduled to
 		// occur.
 		// once blocks are finalized that make that transition irrelevant or activate it,