op-node/rollup/sequencing: Fix temporary engine error handling (#12258)

Do schedule a next sequencer action even if the building state is empty
in `onEngineTemporaryError`.
This can happen if the previous successful payload cleared the building
state and then when `startBuildingBlock` is entered, it hits a temp
error but never modified the cleared building state, so the temp error
handler would never schedule the next action.

Towards #12240 #12041

op-node/rollup/sequencing/sequencer.go

@@ -55,8 +55,7 @@ type AsyncGossiper interface {
 // This event is used to prioritize sequencer work over derivation work,
 // by emitting it before e.g. a derivation-pipeline step.
 // A future sequencer in an async world may manage its own execution.
-type SequencerActionEvent struct {
-}
+type SequencerActionEvent struct{}
 
 func (ev SequencerActionEvent) String() string {
 	return "sequencer-action"
@@ -129,7 +128,8 @@ func NewSequencer(driverCtx context.Context, log log.Logger, rollupCfg *rollup.C
 	listener SequencerStateListener,
 	conductor conductor.SequencerConductor,
 	asyncGossip AsyncGossiper,
-	metrics Metrics) *Sequencer {
+	metrics Metrics,
+) *Sequencer {
 	return &Sequencer{
 		ctx:              driverCtx,
 		log:              log,
@@ -270,7 +270,8 @@ func (d *Sequencer) onBuildSealed(x engine.BuildSealedEvent) {
 	defer cancel()
 	if err := d.conductor.CommitUnsafePayload(ctx, x.Envelope); err != nil {
 		d.emitter.Emit(rollup.EngineTemporaryErrorEvent{
-			Err: fmt.Errorf("failed to commit unsafe payload to conductor: %w", err)})
+			Err: fmt.Errorf("failed to commit unsafe payload to conductor: %w", err),
+		})
 		return
 	}
 
@@ -382,8 +383,7 @@ func (d *Sequencer) onSequencerAction(x SequencerActionEvent) {
 
 func (d *Sequencer) onEngineTemporaryError(x rollup.EngineTemporaryErrorEvent) {
 	if d.latest == (BuildingState{}) {
-		d.log.Debug("Engine reported temporary error, but sequencer is not using engine", "err", x.Err)
-		return
+		d.log.Debug("Engine reported temporary error while building state is empty", "err", x.Err)
 	}
 	d.log.Error("Engine failed temporarily, backing off sequencer", "err", x.Err)
 	if errors.Is(x.Err, engine.ErrEngineSyncing) { // if it is syncing we can back off by more