op-supervisor: Load the starting block from log db (#11090)

* finish TODO(optimism#11023)

* revert GetLogBlockNum() && use ClosestBlockInfo

* catch error

* rewind

* gofmt -w -s .

* op-supervisor: Remove unused import.

---------
Co-authored-by: Adrian Sutton <adrian@oplabs.co>

op-supervisor/supervisor/backend/backend.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"sync/atomic"
 	"time"
 
@@ -50,7 +51,21 @@ func NewSupervisorBackend(ctx context.Context, logger log.Logger, m Metrics, cfg
 			return nil, fmt.Errorf("failed to create logdb for chain %v at %v: %w", chainID, path, err)
 		}
 		logDBs[i] = logDB
-		monitor, err := source.NewChainMonitor(ctx, logger, cm, chainID, rpc, rpcClient)
+
+		// Get the last checkpoint that was written then Rewind the db
+		// to the block prior to that block and start from there.
+		// Guarantees we will always roll back at least one block
+		// so we know we're always starting from a fully written block.
+		checkPointBlock, _, err := logDB.ClosestBlockInfo(math.MaxUint64)
+		if err != nil {
+			return nil, fmt.Errorf("failed to get block from checkpoint: %w", err)
+		}
+		block := checkPointBlock - 1
+		err = logDB.Rewind(block)
+		if err != nil {
+			return nil, fmt.Errorf("failed to 'Rewind' the database: %w", err)
+		}
+		monitor, err := source.NewChainMonitor(ctx, logger, cm, chainID, rpc, rpcClient, block)
 		if err != nil {
 			return nil, fmt.Errorf("failed to create monitor for rpc %v: %w", rpc, err)
 		}

op-supervisor/supervisor/backend/source/chain.go

@@ -31,15 +31,16 @@ type ChainMonitor struct {
 	headMonitor *HeadMonitor
 }
 
-func NewChainMonitor(ctx context.Context, logger log.Logger, m Metrics, chainID types.ChainID, rpc string, client client.RPC) (*ChainMonitor, error) {
+func NewChainMonitor(ctx context.Context, logger log.Logger, m Metrics, chainID types.ChainID, rpc string, client client.RPC, block uint64) (*ChainMonitor, error) {
 	logger = logger.New("chainID", chainID)
 	cl, err := newClient(ctx, logger, m, rpc, client, pollInterval, trustRpc, rpcKind)
 	if err != nil {
 		return nil, err
 	}
 
-	// TODO(optimism#11023): Load the starting block from log db
-	startingHead := eth.L1BlockRef{}
+	startingHead := eth.L1BlockRef{
+		Number: block,
+	}
 
 	fetchReceipts := newLogFetcher(cl, &loggingReceiptProcessor{logger})
 	unsafeBlockProcessor := NewChainProcessor(logger, cl, startingHead, fetchReceipts)