grandpa: voting rules shouldn't restrict past round base (#4155)

* grandpa: voting rules shouldn't restrict past round base

* grandpa: fix lower bound on vote restriction. add test

substrate/client/finality-grandpa/src/environment.rs

@@ -497,8 +497,18 @@ where
 				// note that we pass the original `best_header`, i.e. before the
 				// authority set limit filter, which can be considered a
 				// mandatory/implicit voting rule.
+				//
+				// we also make sure that the restricted vote is higher than the
+				// round base (i.e. last finalized), otherwise the value
+				// returned by the given voting rule is ignored and the original
+				// target is used instead.
 				self.voting_rule
 					.restrict_vote(&*self.client, &base_header, &best_header, target_header)
+					.filter(|(_, restricted_number)| {
+						// we can only restrict votes within the interval [base, target]
+						restricted_number >= base_header.number() &&
+							restricted_number < target_header.number()
+					})
 					.or(Some((target_header.hash(), *target_header.number())))
 			},
 			Ok(None) => {

substrate/client/finality-grandpa/src/tests.rs

@@ -1660,6 +1660,19 @@ fn grandpa_environment_respects_voting_rules() {
 		).unwrap().1,
 		19,
 	);
+
+	// we finalize block 20 with block 20 being the best block
+	peer.client().finalize_block(BlockId::Number(20), None, false).unwrap();
+
+	// even though the default environment will always try to not vote on the
+	// best block, there's a hard rule that we can't cast any votes lower than
+	// the given base (#20).
+	assert_eq!(
+		default_env.best_chain_containing(
+			peer.client().info().chain.finalized_hash
+		).unwrap().1,
+		20,
+	);
 }
 
 #[test]