Parachains runtime: properly filter backed candidate votes

polkadot/runtime/parachains/src/paras_inherent/mod.rs

@@ -1328,16 +1328,24 @@ fn filter_backed_statements_from_disabled_validators<
 		// The indices of statements from disabled validators in `BackedCandidate`. We have to drop
 		// these.
 		let indices_to_drop = disabled_indices.clone() & &validator_indices;
-		// Apply the bitmask to drop the disabled validator from `validator_indices`
-		validator_indices &= !disabled_indices;
-		// Update the backed candidate
-		bc.set_validator_indices_and_core_index(validator_indices, maybe_injected_core_index);
 
 		// Remove the corresponding votes from `validity_votes`
 		for idx in indices_to_drop.iter_ones().rev() {
-			bc.validity_votes_mut().remove(idx);
+			// Map the index in `indices_to_drop` (which is an index into the validator group)
+			// to the index in the validity votes vector, which might have less number of votes,
+			// than validators assigned to the group.
+			//
+			// For each index `idx` in `indices_to_drop`, the corresponding index in the
+			// validity votes vector is the number of `1` bits in `validator_indices` before `idx`.
+			let mapped_idx = validator_indices[..idx].count_ones();
+			bc.validity_votes_mut().remove(mapped_idx);
 		}
 
+		// Apply the bitmask to drop the disabled validator from `validator_indices`
+		validator_indices &= !disabled_indices;
+		// Update the backed candidate
+		bc.set_validator_indices_and_core_index(validator_indices, maybe_injected_core_index);
+
 		// By filtering votes we might render the candidate invalid and cause a failure in
 		// [`process_candidates`]. To avoid this we have to perform a sanity check here. If there
 		// are not enough backing votes after filtering we will remove the whole candidate.

polkadot/runtime/parachains/src/paras_inherent/tests.rs

@@ -4776,5 +4776,43 @@ mod sanitizers {
 				});
 			}
 		}
+
+		// Disable Bob which is second validator in group for core 0.
+		// Para 0 candidate has a single valid vote that is removed
+		// because Bob is disabled.
+		//
+		// This test ensures we remove the right validty votes from the backed candidate
+		#[test]
+		fn drop_right_indices() {
+			new_test_ext(default_config()).execute_with(|| {
+				let TestData { mut expected_backed_candidates_with_core, .. } =
+					get_test_data_one_core_per_para();
+
+				set_disabled_validators(vec![1]);
+
+				let (backed, _) = expected_backed_candidates_with_core
+					.get_mut(&ParaId::from(1))
+					.unwrap()
+					.first_mut()
+					.unwrap();
+				let (indices, core) = backed.validator_indices_and_core_index();
+				let mut indices = BitVec::<_>::from(indices);
+
+				indices.set(0, false);
+				backed.validity_votes_mut().remove(0);
+				backed.set_validator_indices_and_core_index(indices, core);
+
+				let mut untouched = expected_backed_candidates_with_core.clone();
+
+				filter_backed_statements_from_disabled_validators::<Test>(
+					&mut expected_backed_candidates_with_core,
+					&shared::AllowedRelayParents::<Test>::get(),
+				);
+
+				untouched.remove(&ParaId::from(1)).unwrap();
+
+				assert_eq!(expected_backed_candidates_with_core, untouched);
+			});
+		}
 	}
 }