Use only fresh DisputeStatement-s for dispute spam detection (#4921) (#4957)

handle_import_statements() from  Dispute-Coordinator does spam detection
by counting unconfirmed dispute statements from each validator.

This patch adjusts the logic so that only fresh dispute statements are
counted.

Fresh dispute statement in this case means a statement which was not
loaded from overlay db.

polkadot/node/core/dispute-coordinator/src/real/initialized.rs

@@ -831,6 +831,9 @@ impl Initialized {
 			.find(|(_, index)| controlled_indices.contains(index))
 			.is_some();
 
+		// Indexes of the validators issued 'invalid' statements. Will be used to populate spam slots.
+		let mut fresh_invalid_statement_issuers = Vec::new();
+
 		// Update candidate votes.
 		for (statement, val_index) in &statements {
 			if validators
@@ -876,6 +879,7 @@ impl Initialized {
 						continue
 					}
 
+					fresh_invalid_statement_issuers.push(*val_index);
 					votes_changed = true;
 					self.metrics.on_invalid_vote();
 				},
@@ -892,17 +896,17 @@ impl Initialized {
 			byzantine_threshold(n_validators);
 
 		// Potential spam:
-		if !is_confirmed {
+		if !is_confirmed && !fresh_invalid_statement_issuers.is_empty() {
 			let mut free_spam_slots_available = true;
 			// Only allow import if all validators voting invalid, have not exceeded
 			// their spam slots:
-			for (statement, index) in statements.iter() {
+			for index in fresh_invalid_statement_issuers {
 				// Disputes can only be triggered via an invalidity stating vote, thus we only
 				// need to increase spam slots on invalid votes. (If we did not, we would also
 				// increase spam slots for backing validators for example - as validators have to
 				// provide some opposing vote for dispute-distribution).
-				free_spam_slots_available &= statement.statement().indicates_validity() ||
-					self.spam_slots.add_unconfirmed(session, candidate_hash, *index);
+				free_spam_slots_available &=
+					self.spam_slots.add_unconfirmed(session, candidate_hash, index);
 			}
 			// Only validity stating votes or validator had free spam slot?
 			if !free_spam_slots_available {