op-challenger: Cache the cannon absolute prestate (#9756)

The prestate is large so can be slow to load. Verifying the prestate for hundreds of games at startup is repeatedly loading the configured prestate file, adding a large delay to being able to start playing games.

op-challenger/game/fault/register.go

@@ -163,6 +163,7 @@ func registerCannon(
 	l2Client cannon.L2HeaderSource,
 	l1HeaderSource L1HeaderSource,
 ) error {
+	cannonPrestateProvider := cannon.NewPrestateProvider(cfg.CannonAbsolutePreState)
 	playerCreator := func(game types.GameMetadata, dir string) (scheduler.GamePlayer, error) {
 		contract, err := contracts.NewFaultDisputeGameContract(game.Proxy, caller)
 		if err != nil {
@@ -188,7 +189,7 @@ func registerCannon(
 			}
 			return accessor, nil
 		}
-		prestateValidator := NewPrestateValidator("cannon", contract.GetAbsolutePrestateHash, cannon.NewPrestateProvider(cfg.CannonAbsolutePreState))
+		prestateValidator := NewPrestateValidator("cannon", contract.GetAbsolutePrestateHash, cannonPrestateProvider)
 		genesisValidator := NewPrestateValidator("output root", contract.GetGenesisOutputRoot, prestateProvider)
 		return NewGamePlayer(ctx, cl, logger, m, dir, game.Proxy, txSender, contract, syncValidator, []Validator{prestateValidator, genesisValidator}, creator, l1HeaderSource)
 	}

op-challenger/game/fault/trace/cannon/prestate.go

@@ -14,10 +14,12 @@ var _ types.PrestateProvider = (*CannonPrestateProvider)(nil)
 
 type CannonPrestateProvider struct {
 	prestate string
+
+	prestateCommitment common.Hash
 }
 
 func NewPrestateProvider(prestate string) *CannonPrestateProvider {
-	return &CannonPrestateProvider{prestate}
+	return &CannonPrestateProvider{prestate: prestate}
 }
 
 func (p *CannonPrestateProvider) absolutePreState() ([]byte, error) {
@@ -29,6 +31,9 @@ func (p *CannonPrestateProvider) absolutePreState() ([]byte, error) {
 }
 
 func (p *CannonPrestateProvider) AbsolutePreStateCommitment(_ context.Context) (common.Hash, error) {
+	if p.prestateCommitment != (common.Hash{}) {
+		return p.prestateCommitment, nil
+	}
 	state, err := p.absolutePreState()
 	if err != nil {
 		return common.Hash{}, fmt.Errorf("cannot load absolute pre-state: %w", err)
@@ -37,5 +42,6 @@ func (p *CannonPrestateProvider) AbsolutePreStateCommitment(_ context.Context) (
 	if err != nil {
 		return common.Hash{}, fmt.Errorf("cannot hash absolute pre-state: %w", err)
 	}
+	p.prestateCommitment = hash
 	return hash, nil
 }

op-challenger/game/fault/trace/cannon/prestate_test.go

@@ -58,6 +58,21 @@ func TestAbsolutePreStateCommitment(t *testing.T) {
 		require.NoError(t, err)
 		require.Equal(t, expected, actual)
 	})
+
+	t.Run("CacheAbsolutePreState", func(t *testing.T) {
+		setupPreState(t, dataDir, "state.json")
+		provider := newCannonPrestateProvider(dataDir, prestate)
+		first, err := provider.AbsolutePreStateCommitment(context.Background())
+		require.NoError(t, err)
+
+		// Remove the prestate from disk
+		require.NoError(t, os.Remove(provider.prestate))
+
+		// Value should still be available from cache
+		cached, err := provider.AbsolutePreStateCommitment(context.Background())
+		require.NoError(t, err)
+		require.Equal(t, first, cached)
+	})
 }
 
 func setupPreState(t *testing.T, dataDir string, filename string) {