ref impl: return both refL1 and nextRefL1 for driver

Author: protolambda

opnode/l2/sync_start.go

@@ -15,8 +15,7 @@ var WrongChainErr = errors.New("wrong chain")
 
 // FindSyncStart finds nextRefL1: the L1 block needed next for sync, to derive into a L2 block on top of refL2.
 // If the L1 reorgs then this will find the common history to build on top of and then follow the first step of the reorg.
-func FindSyncStart(ctx context.Context, reference SyncReference, genesis *Genesis) (nextRefL1 eth.BlockID, refL2 eth.BlockID, err error) {
-	var refL1 eth.BlockID    // the L1 block that was derived into refL2
+func FindSyncStart(ctx context.Context, reference SyncReference, genesis *Genesis) (refL1, nextRefL1, refL2 eth.BlockID, err error) {
 	var parentL2 common.Hash // the parent of refL2
 	// Start at L2 head
 	refL1, refL2, parentL2, err = reference.RefByL2Num(ctx, nil, genesis)

opnode/l2/sync_start_test.go

@@ -139,7 +139,7 @@ func (c *syncStartTestCase) Run(t *testing.T) {
 	}
 	expectedRefL2 := mockID(c.ExpectedRefL2, expectedRefL2Num)
 
-	nextRefL1, refL2, err := FindSyncStart(context.Background(), msr, genesis)
+	_, nextRefL1, refL2, err := FindSyncStart(context.Background(), msr, genesis)
 	if c.ExpectedErr != nil {
 		assert.Error(t, err, "got next L1 %s (%d), onto L2: %s (%d)", nextRefL1.Hash[:1], nextRefL1.Number, refL2.Hash[:1], refL2.Number)
 		assert.ErrorIs(t, err, c.ExpectedErr)