discard candidate when ahead (#11874)

* discard candidate when ahead

* also discard result.Unsafe for consistency

* remove unnecessary type convertion for FakeGenesis

* Revert "also discard result.Unsafe for consistency"

This reverts commit d2288d4a01bb4176fd1a3c6e1901646c7bb404c7.

* remove type cast

op-node/rollup/sync/start.go

@@ -214,6 +214,8 @@ func FindL2Heads(ctx context.Context, cfg *rollup.Config, l1 L1Chain, l2 L2Chain
 		}
 
 		if ahead {
+			// discard previous candidate
+			highestL2WithCanonicalL1Origin = eth.L2BlockRef{}
 			// keep the unsafe head if we can't tell if its L1 origin is canonical or not yet.
 		} else if l1Block.Hash == n.L1Origin.Hash {
 			// if L2 matches canonical chain, even if unsafe,

op-node/rollup/sync/start_test.go

@@ -26,7 +26,7 @@ func (c *syncStartTestCase) generateFakeL2(t *testing.T) (*testutils.FakeChainSo
 	log := testlog.Logger(t, log.LevelError)
 	chain := testutils.NewFakeChainSource([]string{c.L1, c.NewL1}, []string{c.L2}, int(c.GenesisL1Num), log)
 	chain.SetL2Head(len(c.L2) - 1)
-	genesis := testutils.FakeGenesis(c.GenesisL1, c.GenesisL2, int(c.GenesisL1Num))
+	genesis := testutils.FakeGenesis(c.GenesisL1, c.GenesisL2, c.GenesisL1Num)
 	chain.ReorgL1()
 	for i := 0; i < len(c.NewL1)-1; i++ {
 		chain.AdvanceL1()

op-service/testutils/fake_chain.go

@@ -14,9 +14,9 @@ import (
 	"github.com/ethereum-optimism/optimism/op-service/eth"
 )
 
-func FakeGenesis(l1 rune, l2 rune, l1GenesisNumber int) rollup.Genesis {
+func FakeGenesis(l1 rune, l2 rune, l1GenesisNumber uint64) rollup.Genesis {
 	return rollup.Genesis{
-		L1: fakeID(l1, uint64(l1GenesisNumber)),
+		L1: fakeID(l1, l1GenesisNumber),
 		L2: fakeID(l2, 0),
 	}
 }