batch: update to allow for read-only calls

In this commit, we update the batch schedular so that it has the ability
to do read-only calls. It will do a best effort attempt at keeping a
transaction in read-only mode and then if any requests get added to a
batch that require a read-write tx, then the entire batch's tx will be
upgraded to use a read-write tx.

batch/batch.go

@@ -14,6 +14,19 @@ var errSolo = errors.New(
 	"batch function returned an error and should be re-run solo",
 )
 
+// txOpts implements the sqldb.TxOptions interface. It is used to indicate that
+// the transaction can be read-only or not transaction.
+type txOpts struct {
+	readOnly bool
+}
+
+// ReadOnly returns true if the transaction should be read only.
+//
+// NOTE: This is part of the sqldb.TxOptions interface.
+func (t *txOpts) ReadOnly() bool {
+	return t.readOnly
+}
+
 type request[Q any] struct {
 	*Request[Q]
 	errChan chan error
@@ -25,6 +38,7 @@ type batch[Q any] struct {
 	reqs   []*request[Q]
 	clear  func(b *batch[Q])
 	locker sync.Locker
+	txOpts txOpts
 }
 
 // trigger is the entry point for the batch and ensures that run is started at
@@ -52,12 +66,11 @@ func (b *batch[Q]) run(ctx context.Context) {
 
 	// Apply the batch until a subset succeeds or all of them fail. Requests
 	// that fail will be retried individually.
-	var writeTx writeOpts
 	for len(b.reqs) > 0 {
 		var failIdx = -1
-		err := b.db.ExecTx(ctx, &writeTx, func(tx Q) error {
+		err := b.db.ExecTx(ctx, &b.txOpts, func(tx Q) error {
 			for i, req := range b.reqs {
-				err := req.Update(tx)
+				err := req.Do(tx)
 				if err != nil {
 					// If we get a serialization error, we
 					// want the underlying SQL retry