op-node: generalize and improve alt-sync

op-e2e/migration_test.go

@@ -277,6 +277,7 @@ func TestMigration(t *testing.T) {
 			L2EngineAddr:      gethNode.HTTPAuthEndpoint(),
 			L2EngineJWTSecret: testingJWTSecret,
 		},
+		L2Sync: &node.PreparedL2SyncEndpoint{Client: nil, TrustRPC: false},
 		Driver: driver.Config{
 			VerifierConfDepth:  0,
 			SequencerConfDepth: 0,

op-e2e/setup.go

@@ -193,6 +193,9 @@ type SystemConfig struct {
 
 	// If the proposer can make proposals for L2 blocks derived from L1 blocks which are not finalized on L1 yet.
 	NonFinalizedProposals bool
+
+	// Explicitly disable batcher, for tests that rely on unsafe L2 payloads
+	DisableBatcher bool
 }
 
 type System struct {
@@ -417,6 +420,10 @@ func (cfg SystemConfig) Start(_opts ...SystemConfigOption) (*System, error) {
 			L2EngineAddr:      l2EndpointConfig,
 			L2EngineJWTSecret: cfg.JWTSecret,
 		}
+		rollupCfg.L2Sync = &rollupNode.PreparedL2SyncEndpoint{
+			Client:   nil,
+			TrustRPC: false,
+		}
 	}
 
 	// Geth Clients
@@ -606,8 +613,11 @@ func (cfg SystemConfig) Start(_opts ...SystemConfigOption) (*System, error) {
 		return nil, fmt.Errorf("failed to setup batch submitter: %w", err)
 	}
 
-	if err := sys.BatchSubmitter.Start(); err != nil {
-		return nil, fmt.Errorf("unable to start batch submitter: %w", err)
+	// Batcher may be enabled later
+	if !sys.cfg.DisableBatcher {
+		if err := sys.BatchSubmitter.Start(); err != nil {
+			return nil, fmt.Errorf("unable to start batch submitter: %w", err)
+		}
 	}
 
 	return sys, nil

op-e2e/system_test.go

@@ -649,7 +649,7 @@ func TestSystemMockP2P(t *testing.T) {
 	require.Contains(t, received, receiptVerif.BlockHash)
 }
 
-// TestSystemMockP2P sets up a L1 Geth node, a rollup node, and a L2 geth node and then confirms that
+// TestSystemRPCAltSync sets up a L1 Geth node, a rollup node, and a L2 geth node and then confirms that
 // the nodes can sync L2 blocks before they are confirmed on L1.
 //
 // Test steps:
@@ -660,24 +660,28 @@ func TestSystemMockP2P(t *testing.T) {
 // 6. Wait for the RPC sync method to grab the block from the sequencer over RPC and insert it into the verifier's unsafe chain.
 // 7. Wait for the verifier to sync the unsafe chain into the safe chain.
 // 8. Verify that the TX is included in the verifier's safe chain.
-func TestSystemMockAltSync(t *testing.T) {
+func TestSystemRPCAltSync(t *testing.T) {
 	parallel(t)
 	if !verboseGethNodes {
 		log.Root().SetHandler(log.DiscardHandler())
 	}
 
 	cfg := DefaultSystemConfig(t)
-	// slow down L1 blocks so we can see the L2 blocks arrive well before the L1 blocks do.
-	// Keep the seq window small so the L2 chain is started quick
-	cfg.DeployConfig.L1BlockTime = 10
+	// the default is nil, but this may change in the future.
+	// This test must ensure the blocks are not synced via Gossip, but instead via the alt RPC based sync.
+	cfg.P2PTopology = nil
+	// Disable batcher, so there will not be any L1 data to sync from
+	cfg.DisableBatcher = true
 
-	var published, received []common.Hash
+	var published, received []string
 	seqTracer, verifTracer := new(FnTracer), new(FnTracer)
+	// The sequencer still publishes the blocks to the tracer, even if they do not reach the network due to disabled P2P
 	seqTracer.OnPublishL2PayloadFn = func(ctx context.Context, payload *eth.ExecutionPayload) {
-		published = append(published, payload.BlockHash)
+		published = append(published, payload.ID().String())
 	}
+	// Blocks are now received via the RPC based alt-sync method
 	verifTracer.OnUnsafeL2PayloadFn = func(ctx context.Context, from peer.ID, payload *eth.ExecutionPayload) {
-		received = append(received, payload.BlockHash)
+		received = append(received, payload.ID().String())
 	}
 	cfg.Nodes["sequencer"].Tracer = seqTracer
 	cfg.Nodes["verifier"].Tracer = verifTracer
@@ -687,8 +691,8 @@ func TestSystemMockAltSync(t *testing.T) {
 		role: "sequencer",
 		action: func(sCfg *SystemConfig, system *System) {
 			rpc, _ := system.Nodes["sequencer"].Attach() // never errors
-			cfg.Nodes["verifier"].L2Sync = &rollupNode.L2SyncRPCConfig{
-				Rpc: client.NewBaseRPCClient(rpc),
+			cfg.Nodes["verifier"].L2Sync = &rollupNode.PreparedL2SyncEndpoint{
+				Client: client.NewBaseRPCClient(rpc),
 			}
 		},
 	})
@@ -726,7 +730,7 @@ func TestSystemMockAltSync(t *testing.T) {
 	require.Equal(t, receiptSeq, receiptVerif)
 
 	// Verify that the tx was received via RPC sync (P2P is disabled)
-	require.Contains(t, received, receiptVerif.BlockHash)
+	require.Contains(t, received, eth.BlockID{Hash: receiptVerif.BlockHash, Number: receiptVerif.BlockNumber.Uint64()}.String())
 
 	// Verify that everything that was received was published
 	require.GreaterOrEqual(t, len(published), len(received))

op-node/flags/flags.go

@@ -175,6 +175,13 @@ var (
 		EnvVar:   prefixEnvVar("L2_BACKUP_UNSAFE_SYNC_RPC"),
 		Required: false,
 	}
+	BackupL2UnsafeSyncRPCTrustRPC = cli.StringFlag{
+		Name: "l2.backup-unsafe-sync-rpc.trustrpc",
+		Usage: "Like l1.trustrpc, configure if response data from the RPC needs to be verified, e.g. blockhash computation." +
+			"This does not include checks if the blockhash is part of the canonical chain.",
+		EnvVar:   prefixEnvVar("L2_BACKUP_UNSAFE_SYNC_RPC_TRUST_RPC"),
+		Required: false,
+	}
 )
 
 var requiredFlags = []cli.Flag{
@@ -207,6 +214,7 @@ var optionalFlags = []cli.Flag{
 	HeartbeatMonikerFlag,
 	HeartbeatURLFlag,
 	BackupL2UnsafeSyncRPC,
+	BackupL2UnsafeSyncRPCTrustRPC,
 }
 
 // Flags contains the list of configuration options available to the binary.

op-node/node/client.go

@@ -20,7 +20,9 @@ type L2EndpointSetup interface {
 }
 
 type L2SyncEndpointSetup interface {
-	Setup(ctx context.Context, log log.Logger) (cl client.RPC, err error)
+	// Setup a RPC client to another L2 node to sync L2 blocks from.
+	// It may return a nil client with nil error if RPC based sync is not enabled.
+	Setup(ctx context.Context, log log.Logger) (cl client.RPC, trust bool, err error)
 	Check() error
 }
 
@@ -82,45 +84,45 @@ func (p *PreparedL2Endpoints) Setup(ctx context.Context, log log.Logger) (client
 
 // L2SyncEndpointConfig contains configuration for the fallback sync endpoint
 type L2SyncEndpointConfig struct {
-	// Address of the L2 RPC to use for backup sync
+	// Address of the L2 RPC to use for backup sync, may be empty if RPC alt-sync is disabled.
 	L2NodeAddr string
+	TrustRPC   bool
 }
 
 var _ L2SyncEndpointSetup = (*L2SyncEndpointConfig)(nil)
 
-func (cfg *L2SyncEndpointConfig) Setup(ctx context.Context, log log.Logger) (client.RPC, error) {
+// Setup creates an RPC client to sync from.
+// It will return nil without error if no sync method is configured.
+func (cfg *L2SyncEndpointConfig) Setup(ctx context.Context, log log.Logger) (cl client.RPC, trust bool, err error) {
+	if cfg.L2NodeAddr == "" {
+		return nil, false, nil
+	}
 	l2Node, err := client.NewRPC(ctx, log, cfg.L2NodeAddr)
 	if err != nil {
-		return nil, err
+		return nil, false, err
 	}
 
-	return l2Node, nil
+	return l2Node, cfg.TrustRPC, nil
 }
 
 func (cfg *L2SyncEndpointConfig) Check() error {
-	if cfg.L2NodeAddr == "" {
-		return errors.New("empty L2 Node Address")
-	}
-
+	// empty addr is valid, as it is optional.
 	return nil
 }
 
-type L2SyncRPCConfig struct {
-	// RPC endpoint to use for syncing
-	Rpc client.RPC
+type PreparedL2SyncEndpoint struct {
+	// RPC endpoint to use for syncing, may be nil if RPC alt-sync is disabled.
+	Client   client.RPC
+	TrustRPC bool
 }
 
-var _ L2SyncEndpointSetup = (*L2SyncRPCConfig)(nil)
+var _ L2SyncEndpointSetup = (*PreparedL2SyncEndpoint)(nil)
 
-func (cfg *L2SyncRPCConfig) Setup(ctx context.Context, log log.Logger) (client.RPC, error) {
-	return cfg.Rpc, nil
+func (cfg *PreparedL2SyncEndpoint) Setup(ctx context.Context, log log.Logger) (cl client.RPC, trust bool, err error) {
+	return cfg.Client, cfg.TrustRPC, nil
 }
 
-func (cfg *L2SyncRPCConfig) Check() error {
-	if cfg.Rpc == nil {
-		return errors.New("rpc cannot be nil")
-	}
-
+func (cfg *PreparedL2SyncEndpoint) Check() error {
 	return nil
 }
 

op-node/node/config.go

@@ -80,6 +80,9 @@ func (cfg *Config) Check() error {
 	if err := cfg.L2.Check(); err != nil {
 		return fmt.Errorf("l2 endpoint config error: %w", err)
 	}
+	if err := cfg.L2Sync.Check(); err != nil {
+		return fmt.Errorf("sync config error: %w", err)
+	}
 	if err := cfg.Rollup.Check(); err != nil {
 		return fmt.Errorf("rollup config error: %w", err)
 	}

op-node/node/node.go

@@ -33,6 +33,7 @@ type OpNode struct {
 	l1Source  *sources.L1Client     // L1 Client to fetch data from
 	l2Driver  *driver.Driver        // L2 Engine to Sync
 	l2Source  *sources.EngineClient // L2 Execution Engine RPC bindings
+	rpcSync   *sources.SyncClient   // Alt-sync RPC client, optional (may be nil)
 	server    *rpcServer            // RPC server hosting the rollup-node API
 	p2pNode   *p2p.NodeP2P          // P2P node functionality
 	p2pSigner p2p.Signer            // p2p gogssip application messages will be signed with this signer
@@ -86,6 +87,9 @@ func (n *OpNode) init(ctx context.Context, cfg *Config, snapshotLog log.Logger)
 	if err := n.initL2(ctx, cfg, snapshotLog); err != nil {
 		return err
 	}
+	if err := n.initRPCSync(ctx, cfg); err != nil {
+		return err
+	}
 	if err := n.initP2PSigner(ctx, cfg); err != nil {
 		return err
 	}
@@ -197,29 +201,28 @@ func (n *OpNode) initL2(ctx context.Context, cfg *Config, snapshotLog log.Logger
 		return err
 	}
 
-	var syncClient *sources.SyncClient
-	// If the L2 sync config is present, use it to create a sync client
-	if cfg.L2Sync != nil {
-		if err := cfg.L2Sync.Check(); err != nil {
-			log.Info("L2 sync config is not present, skipping L2 sync client setup", "err", err)
-		} else {
-			rpcSyncClient, err := cfg.L2Sync.Setup(ctx, n.log)
-			if err != nil {
-				return fmt.Errorf("failed to setup L2 execution-engine RPC client for backup sync: %w", err)
-			}
+	n.l2Driver = driver.NewDriver(&cfg.Driver, &cfg.Rollup, n.l2Source, n.l1Source, n, n, n.log, snapshotLog, n.metrics)
 
-			// The sync client's RPC is always trusted
-			config := sources.SyncClientDefaultConfig(&cfg.Rollup, true)
+	return nil
+}
 
-			syncClient, err = sources.NewSyncClient(n.OnUnsafeL2Payload, rpcSyncClient, n.log, n.metrics.L2SourceCache, config)
-			if err != nil {
-				return fmt.Errorf("failed to create sync client: %w", err)
-			}
-		}
+func (n *OpNode) initRPCSync(ctx context.Context, cfg *Config) error {
+	rpcSyncClient, trustRPC, err := cfg.L2Sync.Setup(ctx, n.log)
+	if err != nil {
+		return fmt.Errorf("failed to setup L2 execution-engine RPC client for backup sync: %w", err)
+	}
+	if rpcSyncClient == nil { // if no RPC client is configured to sync from, then don't add the RPC sync client
+		return nil
 	}
 
-	n.l2Driver = driver.NewDriver(&cfg.Driver, &cfg.Rollup, n.l2Source, n.l1Source, syncClient, n, n.log, snapshotLog, n.metrics)
+	// The sync client's RPC is always trusted
+	config := sources.SyncClientDefaultConfig(&cfg.Rollup, trustRPC)
 
+	syncClient, err := sources.NewSyncClient(n.OnUnsafeL2Payload, rpcSyncClient, n.log, n.metrics.L2SourceCache, config)
+	if err != nil {
+		return fmt.Errorf("failed to create sync client: %w", err)
+	}
+	n.rpcSync = syncClient
 	return nil
 }
 
@@ -292,11 +295,12 @@ func (n *OpNode) Start(ctx context.Context) error {
 	}
 
 	// If the backup unsafe sync client is enabled, start its event loop
-	if n.l2Driver.L2SyncCl != nil {
-		if err := n.l2Driver.L2SyncCl.Start(); err != nil {
+	if n.rpcSync != nil {
+		if err := n.rpcSync.Start(); err != nil {
 			n.log.Error("Could not start the backup sync client", "err", err)
 			return err
 		}
+		n.log.Info("Started L2-RPC sync service")
 	}
 
 	return nil
@@ -375,6 +379,14 @@ func (n *OpNode) OnUnsafeL2Payload(ctx context.Context, from peer.ID, payload *e
 	return nil
 }
 
+func (n *OpNode) RequestL2Range(ctx context.Context, start, end uint64) error {
+	if n.rpcSync != nil {
+		return n.rpcSync.RequestL2Range(ctx, start, end)
+	}
+	n.log.Debug("ignoring request to sync L2 range, no sync method available")
+	return nil
+}
+
 func (n *OpNode) P2P() p2p.Node {
 	return n.p2pNode
 }
@@ -413,8 +425,8 @@ func (n *OpNode) Close() error {
 		}
 
 		// If the L2 sync client is present & running, close it.
-		if n.l2Driver.L2SyncCl != nil {
-			if err := n.l2Driver.L2SyncCl.Close(); err != nil {
+		if n.rpcSync != nil {
+			if err := n.rpcSync.Close(); err != nil {
 				result = multierror.Append(result, fmt.Errorf("failed to close L2 engine backup sync client cleanly: %w", err))
 			}
 		}

op-node/rollup/derive/engine_queue.go

@@ -682,7 +682,8 @@ func (eq *EngineQueue) Reset(ctx context.Context, _ eth.L1BlockRef, _ eth.System
 	return io.EOF
 }
 
-// GetUnsafeQueueGap retrieves the current [start, end] range of the gap between the tip of the unsafe priority queue and the unsafe head.
+// GetUnsafeQueueGap retrieves the current [start, end) range (incl. start, excl. end)
+// of the gap between the tip of the unsafe priority queue and the unsafe head.
 // If there is no gap, the difference between end and start will be 0.
 func (eq *EngineQueue) GetUnsafeQueueGap(expectedNumber uint64) (start uint64, end uint64) {
 	// The start of the gap is always the unsafe head + 1
@@ -691,9 +692,11 @@ func (eq *EngineQueue) GetUnsafeQueueGap(expectedNumber uint64) (start uint64, e
 	// If the priority queue is empty, the end is the first block number at the top of the priority queue
 	// Otherwise, the end is the expected block number
 	if first := eq.unsafePayloads.Peek(); first != nil {
+		// Don't include the payload we already have in the sync range
 		end = first.ID().Number
 	} else {
-		end = expectedNumber
+		// Include the expected payload in the sync range
+		end = expectedNumber + 1
 	}
 
 	return start, end

op-node/rollup/driver/driver.go

@@ -10,7 +10,6 @@ import (
 	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum-optimism/optimism/op-node/rollup"
 	"github.com/ethereum-optimism/optimism/op-node/rollup/derive"
-	"github.com/ethereum-optimism/optimism/op-node/sources"
 )
 
 type Metrics interface {
@@ -82,8 +81,18 @@ type Network interface {
 	PublishL2Payload(ctx context.Context, payload *eth.ExecutionPayload) error
 }
 
+type AltSync interface {
+	// RequestL2Range informs the sync source that the given range of L2 blocks is missing,
+	// and should be retrieved from any available alternative syncing source.
+	// The sync results should be returned back to the driver via the OnUnsafeL2Payload(ctx, payload) method.
+	// The latest requested range should always take priority over previous requests.
+	// There may be overlaps in requested ranges.
+	// An error may be returned if the scheduling fails immediately, e.g. a context timeout.
+	RequestL2Range(ctx context.Context, start, end uint64) error
+}
+
 // NewDriver composes an events handler that tracks L1 state, triggers L2 derivation, and optionally sequences new L2 blocks.
-func NewDriver(driverCfg *Config, cfg *rollup.Config, l2 L2Chain, l1 L1Chain, syncClient *sources.SyncClient, network Network, log log.Logger, snapshotLog log.Logger, metrics Metrics) *Driver {
+func NewDriver(driverCfg *Config, cfg *rollup.Config, l2 L2Chain, l1 L1Chain, altSync AltSync, network Network, log log.Logger, snapshotLog log.Logger, metrics Metrics) *Driver {
 	l1State := NewL1State(log, metrics)
 	sequencerConfDepth := NewConfDepth(driverCfg.SequencerConfDepth, l1State.L1Head, l1)
 	findL1Origin := NewL1OriginSelector(log, cfg, sequencerConfDepth)
@@ -115,6 +124,6 @@ func NewDriver(driverCfg *Config, cfg *rollup.Config, l2 L2Chain, l1 L1Chain, sy
 		l1SafeSig:        make(chan eth.L1BlockRef, 10),
 		l1FinalizedSig:   make(chan eth.L1BlockRef, 10),
 		unsafeL2Payloads: make(chan *eth.ExecutionPayload, 10),
-		L2SyncCl:         syncClient,
+		altSync:          altSync,
 	}
 }

op-node/rollup/driver/state.go

@@ -16,7 +16,6 @@ import (
 	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum-optimism/optimism/op-node/rollup"
 	"github.com/ethereum-optimism/optimism/op-node/rollup/derive"
-	"github.com/ethereum-optimism/optimism/op-node/sources"
 	"github.com/ethereum-optimism/optimism/op-service/backoff"
 )
 
@@ -64,8 +63,8 @@ type Driver struct {
 	l1SafeSig      chan eth.L1BlockRef
 	l1FinalizedSig chan eth.L1BlockRef
 
-	// Backup unsafe sync client
-	L2SyncCl *sources.SyncClient
+	// Interface to signal the L2 block range to sync.
+	altSync AltSync
 
 	// L2 Signals:
 
@@ -200,11 +199,12 @@ func (s *Driver) eventLoop() {
 		sequencerTimer.Reset(delay)
 	}
 
-	// Create a ticker to check if there is a gap in the engine queue every 15 seconds
-	// If there is, we send requests to the backup RPC to retrieve the missing payloads
-	// and add them to the unsafe queue.
-	altSyncTicker := time.NewTicker(15 * time.Second)
+	// Create a ticker to check if there is a gap in the engine queue, whenever
+	// If there is, we send requests to sync source to retrieve the missing payloads.
+	syncCheckInterval := time.Duration(s.config.BlockTime) * time.Second * 2
+	altSyncTicker := time.NewTicker(syncCheckInterval)
 	defer altSyncTicker.Stop()
+	lastUnsafeL2 := s.derivation.UnsafeL2Head()
 
 	for {
 		// If we are sequencing, and the L1 state is ready, update the trigger for the next sequencer action.
@@ -220,6 +220,13 @@ func (s *Driver) eventLoop() {
 			sequencerCh = nil
 		}
 
+		// If the engine is not ready, or if the L2 head is actively changing, then reset the alt-sync:
+		// there is no need to request L2 blocks when we are syncing already.
+		if head := s.derivation.UnsafeL2Head(); head != lastUnsafeL2 || !s.derivation.EngineReady() {
+			lastUnsafeL2 = head
+			altSyncTicker.Reset(syncCheckInterval)
+		}
+
 		select {
 		case <-sequencerCh:
 			payload, err := s.sequencer.RunNextSequencerAction(ctx)
@@ -237,10 +244,12 @@ func (s *Driver) eventLoop() {
 			}
 			planSequencerAction() // schedule the next sequencer action to keep the sequencing looping
 		case <-altSyncTicker.C:
-			// Check if there is a gap in the current unsafe payload queue. If there is, attempt to fetch
-			// missing payloads from the backup RPC (if it is configured).
-			if s.L2SyncCl != nil {
-				s.checkForGapInUnsafeQueue(ctx)
+			// Check if there is a gap in the current unsafe payload queue.
+			ctx, cancel := context.WithTimeout(ctx, time.Second*2)
+			err := s.checkForGapInUnsafeQueue(ctx)
+			cancel()
+			if err != nil {
+				s.log.Warn("failed to check for unsafe L2 blocks to sync", "err", err)
 			}
 		case payload := <-s.unsafeL2Payloads:
 			s.snapshot("New unsafe payload")
@@ -462,35 +471,29 @@ type hashAndErrorChannel struct {
 	err  chan error
 }
 
-// checkForGapInUnsafeQueue checks if there is a gap in the unsafe queue and attempts to retrieve the missing payloads from the backup RPC.
-// WARNING: The sync client's attempt to retrieve the missing payloads is not guaranteed to succeed, and it will fail silently (besides
-// emitting warning logs) if the requests fail.
-func (s *Driver) checkForGapInUnsafeQueue(ctx context.Context) {
+// checkForGapInUnsafeQueue checks if there is a gap in the unsafe queue and attempts to retrieve the missing payloads from an alt-sync method.
+// WARNING: This is only an outgoing signal, the blocks are not guaranteed to be retrieved.
+// Results are received through OnUnsafeL2Payload.
+func (s *Driver) checkForGapInUnsafeQueue(ctx context.Context) error {
 	// subtract genesis time from wall clock to get the time elapsed since genesis, and then divide that
 	// difference by the block time to get the expected L2 block number at the current time. If the
 	// unsafe head does not have this block number, then there is a gap in the queue.
 	wallClock := uint64(time.Now().Unix())
 	genesisTimestamp := s.config.Genesis.L2Time
+	if wallClock < genesisTimestamp {
+		s.log.Debug("nothing to sync, did not reach genesis L2 time yet", "genesis", genesisTimestamp)
+		return nil
+	}
 	wallClockGenesisDiff := wallClock - genesisTimestamp
-	expectedL2Block := wallClockGenesisDiff / s.config.BlockTime
+	// Note: round down, we should not request blocks into the future.
+	blocksSinceGenesis := wallClockGenesisDiff / s.config.BlockTime
+	expectedL2Block := s.config.Genesis.L2.Number + blocksSinceGenesis
 
 	start, end := s.derivation.GetUnsafeQueueGap(expectedL2Block)
-	size := end - start
-
 	// Check if there is a gap between the unsafe head and the expected L2 block number at the current time.
-	if size > 0 {
-		s.log.Warn("Gap in payload queue tip and expected unsafe chain detected", "start", start, "end", end, "size", size)
-		s.log.Info("Attempting to fetch missing payloads from backup RPC", "start", start, "end", end, "size", size)
-
-		// Attempt to fetch the missing payloads from the backup unsafe sync RPC concurrently.
-		// Concurrent requests are safe here due to the engine queue being a priority queue.
-		for blockNumber := start; blockNumber <= end; blockNumber++ {
-			select {
-			case s.L2SyncCl.FetchUnsafeBlock <- blockNumber:
-				// Do nothing- the block number was successfully sent into the channel
-			default:
-				return // If the channel is full, return and wait for the next iteration of the event loop
-			}
-		}
+	if end > start {
+		s.log.Debug("requesting missing unsafe L2 block range", "start", start, "end", end, "size", end-start)
+		return s.altSync.RequestL2Range(ctx, start, end)
 	}
+	return nil
 }

op-node/service.go

@@ -136,6 +136,7 @@ func NewL2EndpointConfig(ctx *cli.Context, log log.Logger) (*node.L2EndpointConf
 func NewL2SyncEndpointConfig(ctx *cli.Context) *node.L2SyncEndpointConfig {
 	return &node.L2SyncEndpointConfig{
 		L2NodeAddr: ctx.GlobalString(flags.BackupL2UnsafeSyncRPC.Name),
+		TrustRPC:   ctx.GlobalBool(flags.BackupL2UnsafeSyncRPCTrustRPC.Name),
 	}
 }
 

op-node/sources/sync_client.go

@@ -3,7 +3,10 @@ package sources
 import (
 	"context"
 	"errors"
+	"fmt"
+	"io"
 	"sync"
+	"time"
 
 	"github.com/ethereum-optimism/optimism/op-node/client"
 	"github.com/ethereum-optimism/optimism/op-node/eth"
@@ -18,23 +21,34 @@ var ErrNoUnsafeL2PayloadChannel = errors.New("unsafeL2Payloads channel must not
 // RpcSyncPeer is a mock PeerID for the RPC sync client.
 var RpcSyncPeer peer.ID = "ALT_RPC_SYNC"
 
+// Limit the maximum range to request at a time.
+const maxRangePerWorker = 10
+
 type receivePayload = func(ctx context.Context, from peer.ID, payload *eth.ExecutionPayload) error
 
-type SyncClientInterface interface {
+type syncRequest struct {
+	start, end uint64
+}
+
+type RPCSync interface {
+	io.Closer
+	// Start starts an additional worker syncing job
 	Start() error
-	Close() error
-	fetchUnsafeBlockFromRpc(ctx context.Context, blockNumber uint64)
+	// RequestL2Range signals that the given range should be fetched, implementing the alt-sync interface.
+	RequestL2Range(ctx context.Context, start, end uint64) error
 }
 
 type SyncClient struct {
 	*L2Client
-	FetchUnsafeBlock chan uint64
-	done             chan struct{}
-	receivePayload   receivePayload
-	wg               sync.WaitGroup
-}
 
-var _ SyncClientInterface = (*SyncClient)(nil)
+	requests chan syncRequest
+
+	resCtx    context.Context
+	resCancel context.CancelFunc
+
+	receivePayload receivePayload
+	wg             sync.WaitGroup
+}
 
 type SyncClientConfig struct {
 	L2ClientConfig
@@ -51,30 +65,58 @@ func NewSyncClient(receiver receivePayload, client client.RPC, log log.Logger, m
 	if err != nil {
 		return nil, err
 	}
-
+	// This resource context is shared between all workers that may be started
+	resCtx, resCancel := context.WithCancel(context.Background())
 	return &SyncClient{
-		L2Client:         l2Client,
-		FetchUnsafeBlock: make(chan uint64, 128),
-		done:             make(chan struct{}),
-		receivePayload:   receiver,
+		L2Client:       l2Client,
+		resCtx:         resCtx,
+		resCancel:      resCancel,
+		requests:       make(chan syncRequest, 128),
+		receivePayload: receiver,
 	}, nil
 }
 
-// Start starts up the state loop.
-// The loop will have been started if err is not nil.
+// Start starts the syncing background work. This may not be called after Close().
 func (s *SyncClient) Start() error {
+	// TODO(CLI-3635): we can start multiple event loop runners as workers, to parallelize the work
 	s.wg.Add(1)
 	go s.eventLoop()
 	return nil
 }
 
-// Close sends a signal to the event loop to stop.
+// Close sends a signal to close all concurrent syncing work.
 func (s *SyncClient) Close() error {
-	s.done <- struct{}{}
+	s.resCancel()
 	s.wg.Wait()
 	return nil
 }
 
+func (s *SyncClient) RequestL2Range(ctx context.Context, start, end uint64) error {
+	// Drain previous requests now that we have new information
+	for len(s.requests) > 0 {
+		<-s.requests
+	}
+
+	// TODO(CLI-3635): optimize the by-range fetching with the Engine API payloads-by-range method.
+
+	s.log.Info("Scheduling to fetch missing payloads from backup RPC", "start", start, "end", end, "size", end-start)
+
+	for i := start; i < end; i += maxRangePerWorker {
+		r := syncRequest{start: i, end: i + maxRangePerWorker}
+		if r.end > end {
+			r.end = end
+		}
+		s.log.Info("Scheduling range request", "start", r.start, "end", r.end, "size", r.end-r.start)
+		// schedule new range to be requested
+		select {
+		case s.requests <- r:
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+	return nil
+}
+
 // eventLoop is the main event loop for the sync client.
 func (s *SyncClient) eventLoop() {
 	defer s.wg.Done()
@@ -82,10 +124,28 @@ func (s *SyncClient) eventLoop() {
 
 	for {
 		select {
-		case <-s.done:
+		case <-s.resCtx.Done():
+			s.log.Debug("Shutting down RPC sync worker")
 			return
-		case blockNumber := <-s.FetchUnsafeBlock:
-			s.fetchUnsafeBlockFromRpc(context.Background(), blockNumber)
+		case r := <-s.requests:
+			// Limit the maximum time for fetching payloads
+			ctx, cancel := context.WithTimeout(s.resCtx, time.Second*10)
+			// We are only fetching one block at a time here.
+			err := s.fetchUnsafeBlockFromRpc(ctx, r.start)
+			cancel()
+			if err != nil {
+				s.log.Error("failed syncing L2 block via RPC", "err", err)
+			} else {
+				r.start += 1 // continue with next block
+			}
+			// Reschedule
+			if r.start < r.end {
+				select {
+				case s.requests <- r:
+				default:
+					// drop syncing job if we are too busy with sync jobs already.
+				}
+			}
 		}
 	}
 }
@@ -95,28 +155,22 @@ func (s *SyncClient) eventLoop() {
 //
 // Post Shanghai hardfork, the engine API's `PayloadBodiesByRange` method will be much more efficient, but for now,
 // the `eth_getBlockByNumber` method is more widely available.
-func (s *SyncClient) fetchUnsafeBlockFromRpc(ctx context.Context, blockNumber uint64) {
+func (s *SyncClient) fetchUnsafeBlockFromRpc(ctx context.Context, blockNumber uint64) error {
 	s.log.Info("Requesting unsafe payload from backup RPC", "block number", blockNumber)
 
 	payload, err := s.PayloadByNumber(ctx, blockNumber)
 	if err != nil {
-		s.log.Warn("Failed to convert block to execution payload", "block number", blockNumber, "err", err)
-		return
-	}
-
-	// Signature validation is not necessary here since the backup RPC is trusted.
-	if _, ok := payload.CheckBlockHash(); !ok {
-		s.log.Warn("Received invalid payload from backup RPC; invalid block hash", "payload", payload.ID())
-		return
+		return fmt.Errorf("failed to fetch payload by number (%d): %w", blockNumber, err)
 	}
+	// Note: the underlying RPC client used for syncing verifies the execution payload blockhash, if set to untrusted.
 
 	s.log.Info("Received unsafe payload from backup RPC", "payload", payload.ID())
 
 	// Send the retrieved payload to the `unsafeL2Payloads` channel.
 	if err = s.receivePayload(ctx, RpcSyncPeer, payload); err != nil {
-		s.log.Warn("Failed to send payload into the driver's unsafeL2Payloads channel", "payload", payload.ID(), "err", err)
-		return
+		return fmt.Errorf("failed to send payload %s into the driver's unsafeL2Payloads channel: %w", payload.ID(), err)
 	} else {
-		s.log.Info("Sent received payload into the driver's unsafeL2Payloads channel", "payload", payload.ID())
+		s.log.Debug("Sent received payload into the driver's unsafeL2Payloads channel", "payload", payload.ID())
+		return nil
 	}
 }