op-node: Notify sequencer state listener when starting and stopping sequencer

op-node/node/node.go

@@ -199,7 +199,7 @@ func (n *OpNode) initL2(ctx context.Context, cfg *Config, snapshotLog log.Logger
 		return err
 	}
 
-	n.l2Driver = driver.NewDriver(&cfg.Driver, &cfg.Rollup, n.l2Source, n.l1Source, n, n, n.log, snapshotLog, n.metrics)
+	n.l2Driver = driver.NewDriver(&cfg.Driver, &cfg.Rollup, n.l2Source, n.l1Source, n, n, n.log, snapshotLog, n.metrics, DisabledConfigPersistence{})
 
 	return nil
 }

op-node/rollup/driver/driver.go

@@ -102,8 +102,13 @@ type AltSync interface {
 	RequestL2Range(ctx context.Context, start, end eth.L2BlockRef) error
 }
 
+type SequencerStateListener interface {
+	SequencerStarted() error
+	SequencerStopped() 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, altSync AltSync, 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, sequencerStateListener SequencerStateListener) *Driver {
 	l1 = NewMeteredL1Fetcher(l1, metrics)
 	l1State := NewL1State(log, metrics)
 	sequencerConfDepth := NewConfDepth(driverCfg.SequencerConfDepth, l1State.L1Head, l1)
@@ -122,6 +127,7 @@ func NewDriver(driverCfg *Config, cfg *rollup.Config, l2 L2Chain, l1 L1Chain, al
 		forceReset:       make(chan chan struct{}, 10),
 		startSequencer:   make(chan hashAndErrorChannel, 10),
 		stopSequencer:    make(chan chan hashAndError, 10),
+		sequencerNotifs:  sequencerStateListener,
 		config:           cfg,
 		driverConfig:     driverCfg,
 		done:             make(chan struct{}),

op-node/rollup/driver/state.go

@@ -47,6 +47,9 @@ type Driver struct {
 	// It tells the caller that the sequencer stopped by returning the latest sequenced L2 block hash.
 	stopSequencer chan chan hashAndError
 
+	// sequencerNotifs is notified when the sequencer is started or stopped
+	sequencerNotifs SequencerStateListener
+
 	// Rollup config: rollup chain configuration
 	config *rollup.Config
 
@@ -334,6 +337,10 @@ func (s *Driver) eventLoop() {
 			} else if !bytes.Equal(unsafeHead[:], resp.hash[:]) {
 				resp.err <- fmt.Errorf("block hash does not match: head %s, received %s", unsafeHead.String(), resp.hash.String())
 			} else {
+				if err := s.sequencerNotifs.SequencerStarted(); err != nil {
+					resp.err <- fmt.Errorf("sequencer start notification: %w", err)
+					continue
+				}
 				s.log.Info("Sequencer has been started")
 				s.driverConfig.SequencerStopped = false
 				close(resp.err)
@@ -343,6 +350,10 @@ func (s *Driver) eventLoop() {
 			if s.driverConfig.SequencerStopped {
 				respCh <- hashAndError{err: errors.New("sequencer not running")}
 			} else {
+				if err := s.sequencerNotifs.SequencerStopped(); err != nil {
+					respCh <- hashAndError{err: fmt.Errorf("sequencer start notification: %w", err)}
+					continue
+				}
 				s.log.Warn("Sequencer has been stopped")
 				s.driverConfig.SequencerStopped = true
 				respCh <- hashAndError{hash: s.derivation.UnsafeL2Head().Hash}