op-node: Fix invalid advance

op-node/rollup/derive/batch_queue.go

@@ -220,10 +220,11 @@ batchLoop:
 	// i.e. if the sequence window expired, we create empty batches for the current epoch
 	expiryEpoch := epoch.Number + bq.config.SeqWindowSize
 	forceEmptyBatches := (expiryEpoch == bq.origin.Number && outOfData) || expiryEpoch < bq.origin.Number
+	firstOfEpoch := epoch.Number == l2SafeHead.L1Origin.Number+1
 
 	bq.log.Trace("Potentially generating an empty batch",
 		"expiryEpoch", expiryEpoch, "forceEmptyBatches", forceEmptyBatches, "nextTimestamp", nextTimestamp,
-		"epoch_time", epoch.Time, "len_l1_blocks", len(bq.l1Blocks))
+		"epoch_time", epoch.Time, "len_l1_blocks", len(bq.l1Blocks), "firstOfEpoch", firstOfEpoch)
 
 	if !forceEmptyBatches {
 		// sequence window did not expire yet, still room to receive batches for the current epoch,
@@ -237,8 +238,9 @@ batchLoop:
 
 	nextEpoch := bq.l1Blocks[1]
 	// Fill with empty L2 blocks of the same epoch until we meet the time of the next L1 origin,
-	// to preserve that L2 time >= L1 time
-	if nextTimestamp < nextEpoch.Time {
+	// to preserve that L2 time >= L1 time. If this is the first block of the epoch, always generate a
+	// batch to ensure that we at least have one batch per epoch.
+	if nextTimestamp < nextEpoch.Time || firstOfEpoch {
 		bq.log.Trace("Generating next batch", "epoch", epoch, "timestamp", nextTimestamp)
 		return &BatchData{
 			BatchV1{

op-node/rollup/derive/batch_queue_test.go

@@ -183,7 +183,8 @@ func TestBatchQueueEager(t *testing.T) {
 	}
 }
 
-// Tests CLI-3378
+// TestBatchQueueInvalidInternalAdvance asserts that we do not miss an epoch when generating batches.
+// This is a regression test for CLI-3378.
 func TestBatchQueueInvalidInternalAdvance(t *testing.T) {
 	log := testlog.Logger(t, log.LvlTrace)
 	l1 := L1Chain([]uint64{10, 15, 20, 25, 30})
@@ -245,28 +246,30 @@ func TestBatchQueueInvalidInternalAdvance(t *testing.T) {
 
 	// Advance to origin 3. Should generate one empty batch.
 	input.origin = l1[3]
-	// b, e = bq.NextBatch(context.Background(), safeHead)
-	// require.Nil(t, e)
-	// require.NotNil(t, b)
-	// require.Equal(t, safeHead.Time+2, b.Timestamp)
-	// safeHead.Number += 1
-	// safeHead.Time += 2
-	// safeHead.Hash = mockHash(b.Timestamp, 2)
-	// safeHead.L1Origin = b.Epoch()
+	b, e = bq.NextBatch(context.Background(), safeHead)
+	require.Nil(t, e)
+	require.NotNil(t, b)
+	require.Equal(t, safeHead.Time+2, b.Timestamp)
+	require.Equal(t, rollup.Epoch(1), b.EpochNum)
+	safeHead.Number += 1
+	safeHead.Time += 2
+	safeHead.Hash = mockHash(b.Timestamp, 2)
+	safeHead.L1Origin = b.Epoch()
 	b, e = bq.NextBatch(context.Background(), safeHead)
 	require.ErrorIs(t, e, io.EOF)
 	require.Nil(t, b)
 
 	// Advance to origin 4. Should generate one empty batch.
 	input.origin = l1[4]
-	// b, e = bq.NextBatch(context.Background(), safeHead)
-	// require.Nil(t, e)
-	// require.NotNil(t, b)
-	// require.Equal(t, safeHead.Time+2, b.Timestamp)
-	// safeHead.Number += 1
-	// safeHead.Time += 2
-	// safeHead.Hash = mockHash(b.Timestamp, 2)
-	// safeHead.L1Origin = b.Epoch()
+	b, e = bq.NextBatch(context.Background(), safeHead)
+	require.Nil(t, e)
+	require.NotNil(t, b)
+	require.Equal(t, rollup.Epoch(2), b.EpochNum)
+	require.Equal(t, safeHead.Time+2, b.Timestamp)
+	safeHead.Number += 1
+	safeHead.Time += 2
+	safeHead.Hash = mockHash(b.Timestamp, 2)
+	safeHead.L1Origin = b.Epoch()
 	b, e = bq.NextBatch(context.Background(), safeHead)
 	require.ErrorIs(t, e, io.EOF)
 	require.Nil(t, b)

specs/derivation.md

@@ -617,6 +617,10 @@ then an empty batch can be derived with the following properties:
 - If `next_timestamp < next_epoch.time`: the current L1 origin is repeated, to preserve the L2 time invariant.
   - `epoch_num = epoch.number`
   - `epoch_hash = epoch.hash`
+- If the batch is the first batch of the epoch, that epoch is used instead of advancing the epoch to ensure that
+there is at least one L2 block per epoch.
+  - `epoch_num = epoch.number`
+  - `epoch_hash = epoch.hash`
 - Otherwise,
   - `epoch_num = next_epoch.number`
   - `epoch_hash = next_epoch.hash`