From 91df0112c7942f844178027877a0613636ea5003 Mon Sep 17 00:00:00 2001
From: Victor Farazdagi <simple.square@gmail.com>
Date: Tue, 6 Apr 2021 09:08:44 +0300
Subject: [PATCH] Add LatestWeakSubjectivityEpoch() method (#8707)

* Add IsWithinWeakSubjectivityPeriod helper method

* Add LatestWeakSubjectivityEpoch method

Co-authored-by: Raul Jordan <raul@prysmaticlabs.com>
---
 beacon-chain/core/helpers/weak_subjectivity.go | 15 +++++++++++++++
 beacon-chain/rpc/beacon/blocks.go              |  2 +-
 beacon-chain/rpc/beacon/blocks_test.go         | 17 ++++++++++++++++-
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/beacon-chain/core/helpers/weak_subjectivity.go b/beacon-chain/core/helpers/weak_subjectivity.go
index ce41bbaa8..a8981c04c 100644
--- a/beacon-chain/core/helpers/weak_subjectivity.go
+++ b/beacon-chain/core/helpers/weak_subjectivity.go
@@ -141,3 +141,18 @@ func IsWithinWeakSubjectivityPeriod(
 
 	return currentEpoch <= wsStateEpoch+wsPeriod, nil
 }
+
+// LatestWeakSubjectivityEpoch returns epoch of the most recent weak subjectivity checkpoint known to a node.
+//
+// Within the weak subjectivity period, if two conflicting blocks are finalized, 1/3 - D (D := safety decay)
+// of validators will get slashed. Therefore, it is safe to assume that any finalized checkpoint within that
+// period is protected by this safety margin.
+func LatestWeakSubjectivityEpoch(st iface.ReadOnlyBeaconState) (types.Epoch, error) {
+	wsPeriod, err := ComputeWeakSubjectivityPeriod(st)
+	if err != nil {
+		return 0, err
+	}
+
+	finalizedEpoch := st.FinalizedCheckpointEpoch()
+	return finalizedEpoch - (finalizedEpoch % wsPeriod), nil
+}
diff --git a/beacon-chain/rpc/beacon/blocks.go b/beacon-chain/rpc/beacon/blocks.go
index d46554b37..acf295171 100644
--- a/beacon-chain/rpc/beacon/blocks.go
+++ b/beacon-chain/rpc/beacon/blocks.go
@@ -368,7 +368,7 @@ func (bs *Server) GetWeakSubjectivityCheckpoint(ctx context.Context, _ *ptypes.E
 	if err != nil {
 		return nil, status.Error(codes.Internal, "Could not get head state")
 	}
-	wsEpoch, err := helpers.ComputeWeakSubjectivityPeriod(hs)
+	wsEpoch, err := helpers.LatestWeakSubjectivityEpoch(hs)
 	if err != nil {
 		return nil, status.Error(codes.Internal, "Could not get weak subjectivity epoch")
 	}
diff --git a/beacon-chain/rpc/beacon/blocks_test.go b/beacon-chain/rpc/beacon/blocks_test.go
index 8521d192e..aa784963f 100644
--- a/beacon-chain/rpc/beacon/blocks_test.go
+++ b/beacon-chain/rpc/beacon/blocks_test.go
@@ -711,6 +711,18 @@ func TestServer_GetWeakSubjectivityCheckpoint(t *testing.T) {
 	require.NoError(t, db.SaveState(ctx, beaconState, genesisBlockRoot))
 	require.NoError(t, db.SaveGenesisBlockRoot(ctx, genesisBlockRoot))
 
+	// Finalized checkpoint.
+	finalizedEpoch := types.Epoch(1020)
+	require.NoError(t, beaconState.SetSlot(types.Slot(finalizedEpoch.Mul(uint64(params.BeaconConfig().SlotsPerEpoch)))))
+	require.NoError(t, beaconState.SetCurrentJustifiedCheckpoint(&ethpb.Checkpoint{
+		Epoch: finalizedEpoch - 1,
+		Root:  bytesutil.PadTo([]byte{'A'}, 32),
+	}))
+	require.NoError(t, beaconState.SetFinalizedCheckpoint(&ethpb.Checkpoint{
+		Epoch: finalizedEpoch,
+		Root:  bytesutil.PadTo([]byte{'B'}, 32),
+	}))
+
 	chainService := &chainMock.ChainService{State: beaconState}
 	server := &Server{
 		Ctx:           ctx,
@@ -720,9 +732,12 @@ func TestServer_GetWeakSubjectivityCheckpoint(t *testing.T) {
 		StateGen:      stategen.New(db),
 	}
 
+	wsEpoch, err := helpers.ComputeWeakSubjectivityPeriod(beaconState)
+	require.NoError(t, err)
+
 	c, err := server.GetWeakSubjectivityCheckpoint(ctx, &ptypes.Empty{})
 	require.NoError(t, err)
-	e := types.Epoch(257)
+	e := finalizedEpoch - (finalizedEpoch % wsEpoch)
 	require.Equal(t, e, c.Epoch)
 	wsState, err := server.StateGen.StateBySlot(ctx, params.BeaconConfig().SlotsPerEpoch.Mul(uint64(e)))
 	require.NoError(t, err)