Merge pull request #6353 from ethereum-optimism/refcell/enforce-step-on-non-agree

fix(op-challenger): Enforce Step Response

op-challenger/fault/agent.go

@@ -95,7 +95,8 @@ func (a *Agent) step(claim Claim, game Game) error {
 		return nil
 	}
 
-	if game.AgreeWithClaimLevel(claim) {
+	agreeWithClaimLevel := game.AgreeWithClaimLevel(claim)
+	if agreeWithClaimLevel {
 		a.log.Warn("Agree with leaf claim, skipping step", "claim_depth", claim.Depth(), "maxDepth", a.maxDepth)
 		return nil
 	}
@@ -106,7 +107,7 @@ func (a *Agent) step(claim Claim, game Game) error {
 	}
 
 	a.log.Info("Attempting step", "claim_depth", claim.Depth(), "maxDepth", a.maxDepth)
-	step, err := a.solver.AttemptStep(claim)
+	step, err := a.solver.AttemptStep(claim, agreeWithClaimLevel)
 	if err != nil {
 		a.log.Warn("Failed to get a step", "err", err)
 		return err

op-challenger/fault/solver.go

@@ -71,10 +71,13 @@ type StepData struct {
 
 // AttemptStep determines what step should occur for a given leaf claim.
 // An error will be returned if the claim is not at the max depth.
-func (s *Solver) AttemptStep(claim Claim) (StepData, error) {
+func (s *Solver) AttemptStep(claim Claim, agreeWithClaimLevel bool) (StepData, error) {
 	if claim.Depth() != s.gameDepth {
 		return StepData{}, errors.New("cannot step on non-leaf claims")
 	}
+	if agreeWithClaimLevel {
+		return StepData{}, errors.New("cannot step on claims we agree with")
+	}
 	claimCorrect, err := s.agreeWithClaim(claim.ClaimData)
 	if err != nil {
 		return StepData{}, err

op-challenger/fault/solver_test.go

@@ -111,18 +111,29 @@ func TestAttemptStep(t *testing.T) {
 		},
 	}
 
-	step, err := solver.AttemptStep(bottom)
+	step, err := solver.AttemptStep(bottom, false)
 	require.NoError(t, err)
 	require.Equal(t, bottom, step.LeafClaim)
 	require.True(t, step.IsAttack)
 	require.Equal(t, step.PreState, BuildAlphabetPreimage(3, "d"))
 
-	_, err = solver.AttemptStep(middle)
+	_, err = solver.AttemptStep(middle, false)
 	require.Error(t, err)
 
-	step, err = solver.AttemptStep(zero)
+	step, err = solver.AttemptStep(zero, false)
 	require.NoError(t, err)
 	require.Equal(t, zero, step.LeafClaim)
 	require.True(t, step.IsAttack)
 	require.Equal(t, canonicalProvider.AbsolutePreState(), step.PreState)
 }
+
+func TestAttempStep_AgreeWithClaimLevel_Fails(t *testing.T) {
+	maxDepth := 3
+	canonicalProvider := NewAlphabetProvider("abcdefgh", uint64(maxDepth))
+	solver := NewSolver(maxDepth, canonicalProvider)
+	_, _, middle, _ := createTestClaims()
+
+	step, err := solver.AttemptStep(middle, true)
+	require.Error(t, err)
+	require.Equal(t, StepData{}, step)
+}