Merge branch 'develop' into disallow-lower-epoch-update

beacon-chain/core/epoch/precompute/justification_finalization.go

@@ -25,7 +25,7 @@ func UnrealizedCheckpoints(st state.BeaconState) (*ethpb.Checkpoint, *ethpb.Chec
 	}
 
 	justification := processJustificationBits(st, activeBalance, prevTarget, currentTarget)
-	return computeCheckpoints(st, justification)
+	return ComputeCheckpoints(st, justification)
 }
 
 // ProcessJustificationAndFinalizationPreCompute processes justification and finalization during
@@ -77,7 +77,7 @@ func processJustificationBits(state state.BeaconState, totalActiveBalance, prevE
 // updateJustificationAndFinalization processes justification and finalization during
 // epoch processing. This is where a beacon node can justify and finalize a new epoch.
 func weighJustificationAndFinalization(state state.BeaconState, newBits bitfield.Bitvector4) (state.BeaconState, error) {
-	jc, fc, err := computeCheckpoints(state, newBits)
+	jc, fc, err := ComputeCheckpoints(state, newBits)
 	if err != nil {
 		return nil, err
 	}
@@ -100,7 +100,7 @@ func weighJustificationAndFinalization(state state.BeaconState, newBits bitfield
 	return state, nil
 }
 
-// computeCheckpoints computes the new Justification and Finalization
+// ComputeCheckpoints computes the new Justification and Finalization
 // checkpoints at epoch transition
 // Spec pseudocode definition:
 // def weigh_justification_and_finalization(state: BeaconState,
@@ -139,7 +139,7 @@ func weighJustificationAndFinalization(state state.BeaconState, newBits bitfield
 //    # The 1st/2nd most recent epochs are justified, the 1st using the 2nd as source
 //    if all(bits[0:2]) and old_current_justified_checkpoint.epoch + 1 == current_epoch:
 //        state.finalized_checkpoint = old_current_justified_checkpoint
-func computeCheckpoints(state state.BeaconState, newBits bitfield.Bitvector4) (*ethpb.Checkpoint, *ethpb.Checkpoint, error) {
+func ComputeCheckpoints(state state.BeaconState, newBits bitfield.Bitvector4) (*ethpb.Checkpoint, *ethpb.Checkpoint, error) {
 	prevEpoch := time.PrevEpoch(state)
 	currentEpoch := time.CurrentEpoch(state)
 	oldPrevJustifiedCheckpoint := state.PreviousJustifiedCheckpoint()
@@ -149,14 +149,14 @@ func computeCheckpoints(state state.BeaconState, newBits bitfield.Bitvector4) (*
 	finalizedCheckpoint := state.FinalizedCheckpoint()
 
 	// If 2/3 or more of the total balance attested in the current epoch.
-	if newBits.BitAt(0) {
+	if newBits.BitAt(0) && currentEpoch >= justifiedCheckpoint.Epoch {
 		blockRoot, err := helpers.BlockRoot(state, currentEpoch)
 		if err != nil {
 			return nil, nil, errors.Wrapf(err, "could not get block root for current epoch %d", currentEpoch)
 		}
 		justifiedCheckpoint.Epoch = currentEpoch
 		justifiedCheckpoint.Root = blockRoot
-	} else if newBits.BitAt(1) {
+	} else if newBits.BitAt(1) && prevEpoch >= justifiedCheckpoint.Epoch {
 		// If 2/3 or more of total balance attested in the previous epoch.
 		blockRoot, err := helpers.BlockRoot(state, prevEpoch)
 		if err != nil {

beacon-chain/core/epoch/precompute/justification_finalization_test.go

@@ -250,3 +250,18 @@ func TestUnrealizedCheckpoints(t *testing.T) {
 		})
 	}
 }
+
+func Test_ComputeCheckpoints_CantUpdateToLower(t *testing.T) {
+	st, err := v2.InitializeFromProto(&ethpb.BeaconStateAltair{
+		Slot: params.BeaconConfig().SlotsPerEpoch * 2,
+		CurrentJustifiedCheckpoint: &ethpb.Checkpoint{
+			Epoch: 2,
+		},
+	})
+	require.NoError(t, err)
+	jb := make(bitfield.Bitvector4, 1)
+	jb.SetBitAt(1, true)
+	cp, _, err := precompute.ComputeCheckpoints(st, jb)
+	require.NoError(t, err)
+	require.Equal(t, types.Epoch(2), cp.Epoch)
+}