op-node: Log sync start (#3716)

op-node/rollup/derive/engine_queue.go

@@ -405,7 +405,7 @@ func (eq *EngineQueue) forceNextSafeAttributes(ctx context.Context) error {
 // ResetStep Walks the L2 chain backwards until it finds an L2 block whose L1 origin is canonical.
 // The unsafe head is set to the head of the L2 chain, unless the existing safe head is not canonical.
 func (eq *EngineQueue) Reset(ctx context.Context, _ eth.L1BlockRef) error {
-	result, err := sync.FindL2Heads(ctx, eq.cfg, eq.l1Fetcher, eq.engine)
+	result, err := sync.FindL2Heads(ctx, eq.cfg, eq.l1Fetcher, eq.engine, eq.log)
 	if err != nil {
 		return NewTemporaryError(fmt.Errorf("failed to find the L2 Heads to start from: %w", err))
 	}

op-node/rollup/sync/start.go

@@ -32,6 +32,7 @@ import (
 	"github.com/ethereum-optimism/optimism/op-node/rollup"
 	"github.com/ethereum/go-ethereum"
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/log"
 )
 
 type L1Chain interface {
@@ -101,7 +102,7 @@ func currentHeads(ctx context.Context, cfg *rollup.Config, l2 L2Chain) (*FindHea
 // Plausible: meaning that the blockhash of the L2 block's L1 origin
 // (as reported in the L1 Attributes deposit within the L2 block) is not canonical at another height in the L1 chain,
 // and the same holds for all its ancestors.
-func FindL2Heads(ctx context.Context, cfg *rollup.Config, l1 L1Chain, l2 L2Chain) (result *FindHeadsResult, err error) {
+func FindL2Heads(ctx context.Context, cfg *rollup.Config, l1 L1Chain, l2 L2Chain, lgr log.Logger) (result *FindHeadsResult, err error) {
 	// Fetch current L2 forkchoice state
 	result, err = currentHeads(ctx, cfg, l2)
 	if err != nil {
@@ -137,6 +138,8 @@ func FindL2Heads(ctx context.Context, cfg *rollup.Config, l1 L1Chain, l2 L2Chain
 			ahead = notFound
 		}
 
+		lgr.Trace("walking sync start", "number", n.Number)
+
 		// Don't walk past genesis. If we were at the L2 genesis, but could not find its L1 origin,
 		// the L2 chain is building on the wrong L1 branch.
 		if n.Number == cfg.Genesis.L2.Number {

op-node/rollup/sync/start_test.go

@@ -73,7 +73,9 @@ func (c *syncStartTestCase) Run(t *testing.T) {
 		Genesis:       genesis,
 		SeqWindowSize: c.SeqWindowSize,
 	}
-	result, err := FindL2Heads(context.Background(), cfg, chain, chain)
+	lgr := log.New()
+	lgr.SetHandler(log.DiscardHandler())
+	result, err := FindL2Heads(context.Background(), cfg, chain, chain, lgr)
 	if c.ExpectedErr != nil {
 		require.ErrorIs(t, err, c.ExpectedErr, "expected error")
 		return