ref impl: return both refL1 and nextRefL1 for driver

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)

specs/rollup-node.md

@@ -58,6 +58,7 @@ First inputs are derived from L1 source data, then outputs are derived with L2 s
 The L2 block has the same format as a L1 block: a block-header and a list of transactions.
 
 The list of transaction carries:
+
 - A *[L1 attributes transaction]* (always first item)
 - L2 transactions deposited by users in the L1 block (*[deposits]*, if any)
 
@@ -86,7 +87,6 @@ These are then encoded as a [L1 attributes deposit] to update the [L1 Attributes
 [L1 attributes deposit]: deposits.md#l1-attributes-deposit
 [L1 Attributes Predeploy]: deposits.md#l1-attributes-predeploy
 
-
 #### Transaction deposits derivation
 
 A [transaction deposit][transaction deposits] is an L2 transaction that has been submitted on L1, via a call to the
@@ -115,7 +115,6 @@ The object properties must be set as follows:
 [EIP-2718]: https://eips.ethereum.org/EIPS/eip-2718
 [EIP-2930]: https://eips.ethereum.org/EIPS/eip-2930
 
-
 ### Output derivation
 
 Building a full block requires the earlier [derived payload attributes](#payload-attributes-derivation)