batch-submitter: allow deposit only batches

Fixes a long standing bug that allows the batch submitter
to send batches that only contain deposit transactions.
Includes test coverage of the batch creation.

The on chain code requires that there must be at least one
context. A context contains information about the queue
transactions that should be included. It makes no sense
to error when there is a context but no sequencer txs
because the context must be submitted to the chain.

.changeset/fresh-pots-move.md

+---
+'@eth-optimism/batch-submitter-service': patch
+---
+
+Allow deposit only batches

batch-submitter/drivers/sequencer/batch_test.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/ethereum-optimism/optimism/batch-submitter/drivers/sequencer"
 	l2common "github.com/ethereum-optimism/optimism/l2geth/common"
+	"github.com/ethereum-optimism/optimism/l2geth/core/types"
 	l2types "github.com/ethereum-optimism/optimism/l2geth/core/types"
 	"github.com/stretchr/testify/require"
 )
@@ -47,3 +48,76 @@ func TestBatchElementFromBlock(t *testing.T) {
 	require.False(t, element.IsSequencerTx())
 	require.Nil(t, element.Tx)
 }
+
+func TestGenSequencerParams(t *testing.T) {
+	tx := types.NewTransaction(0, l2common.Address{}, big.NewInt(0), 0, big.NewInt(0), []byte{})
+
+	shouldStartAtElement := uint64(1)
+	blockOffset := uint64(1)
+	batches := []sequencer.BatchElement{
+		{Timestamp: 1, BlockNumber: 1},
+		{Timestamp: 1, BlockNumber: 1, Tx: sequencer.NewCachedTx(tx)},
+	}
+
+	params, err := sequencer.GenSequencerBatchParams(shouldStartAtElement, blockOffset, batches)
+	require.NoError(t, err)
+
+	require.Equal(t, uint64(0), params.ShouldStartAtElement)
+	require.Equal(t, uint64(len(batches)), params.TotalElementsToAppend)
+	require.Equal(t, len(batches), len(params.Contexts))
+	// There is only 1 sequencer tx
+	require.Equal(t, 1, len(params.Txs))
+
+	// There are 2 contexts
+	// The first context contains the deposit
+	context1 := params.Contexts[0]
+	require.Equal(t, uint64(0), context1.NumSequencedTxs)
+	require.Equal(t, uint64(1), context1.NumSubsequentQueueTxs)
+	require.Equal(t, uint64(1), context1.Timestamp)
+	require.Equal(t, uint64(1), context1.BlockNumber)
+
+	// The second context contains the sequencer tx
+	context2 := params.Contexts[1]
+	require.Equal(t, uint64(1), context2.NumSequencedTxs)
+	require.Equal(t, uint64(0), context2.NumSubsequentQueueTxs)
+	require.Equal(t, uint64(1), context2.Timestamp)
+	require.Equal(t, uint64(1), context2.BlockNumber)
+}
+
+func TestGenSequencerParamsOnlyDeposits(t *testing.T) {
+	shouldStartAtElement := uint64(1)
+	blockOffset := uint64(1)
+	batches := []sequencer.BatchElement{
+		{Timestamp: 1, BlockNumber: 1},
+		{Timestamp: 1, BlockNumber: 1},
+		{Timestamp: 2, BlockNumber: 2},
+	}
+
+	params, err := sequencer.GenSequencerBatchParams(shouldStartAtElement, blockOffset, batches)
+	require.NoError(t, err)
+
+	// The batches will pack deposits into the same context when their
+	// timestamps and blocknumbers are the same
+	require.Equal(t, uint64(0), params.ShouldStartAtElement)
+	require.Equal(t, uint64(len(batches)), params.TotalElementsToAppend)
+	// 2 deposits have the same timestamp + blocknumber, they go in the
+	// same context. 1 deposit has a different timestamp + blocknumber,
+	// it goes into a different context. Therefore there are 2 contexts
+	require.Equal(t, 2, len(params.Contexts))
+	// No sequencer txs
+	require.Equal(t, 0, len(params.Txs))
+
+	// There are 2 contexts
+	// The first context contains the deposit
+	context1 := params.Contexts[0]
+	require.Equal(t, uint64(0), context1.NumSequencedTxs)
+	require.Equal(t, uint64(2), context1.NumSubsequentQueueTxs)
+	require.Equal(t, uint64(1), context1.Timestamp)
+	require.Equal(t, uint64(1), context1.BlockNumber)
+
+	context2 := params.Contexts[1]
+	require.Equal(t, uint64(0), context2.NumSequencedTxs)
+	require.Equal(t, uint64(1), context2.NumSubsequentQueueTxs)
+	require.Equal(t, uint64(2), context2.Timestamp)
+	require.Equal(t, uint64(2), context2.BlockNumber)
+}

batch-submitter/drivers/sequencer/encoding.go

@@ -222,11 +222,6 @@ func (p *AppendSequencerBatchParams) Write(
 		return ErrMalformedBatch
 	}
 
-	// There must be transactions if there are contexts
-	if len(p.Txs) == 0 && len(p.Contexts) != 0 {
-		return ErrMalformedBatch
-	}
-
 	// copy the contexts as to not malleate the struct
 	// when it is a typed batch
 	contexts := make([]BatchContext, 0, len(p.Contexts)+1)
@@ -361,9 +356,6 @@ func (p *AppendSequencerBatchParams) Read(r io.Reader) error {
 			if len(p.Contexts) == 0 && len(p.Txs) != 0 {
 				return ErrMalformedBatch
 			}
-			if len(p.Txs) == 0 && len(p.Contexts) != 0 {
-				return ErrMalformedBatch
-			}
 			return closeReader()
 		} else if err != nil {
 			return err

batch-submitter/drivers/sequencer/testdata/valid_append_sequencer_batch_params.json

@@ -46,7 +46,7 @@
         }
       ],
       "txs": [],
-      "error": true
+      "error": false
     },
     {
       "name": "multiple-contexts-no-txs",
@@ -80,7 +80,7 @@
         }
       ],
       "txs": [],
-      "error": true
+      "error": false
     },
     {
       "name": "complex",