add ErrNoInput, returned when calling batch/tx APIs without input

see #174

batch_test.go

@@ -137,10 +137,23 @@ func TestBatchGetEmptySets(t *testing.T) {
 
 	results = []widget{}
 	err = table.Batch("UserID", "Time").Get(keysToCheck[:len(keysToCheck)-1]...).Consistent(true).All(&results)
-	if err != nil {
+	if err != ErrNotFound {
 		t.Error(err)
 	}
 	if len(results) != 0 {
 		t.Error("batch get empty set, unexpected length:", len(results), "want:", 0)
 	}
 }
+
+func TestBatchEmptyInput(t *testing.T) {
+	table := testDB.Table(testTable)
+	err := table.Batch("UserID", "Time").Get().All(nil)
+	if err != ErrNoInput {
+		t.Error("unexpected error", err)
+	}
+
+	_, err = table.Batch("UserID", "Time").Write().Run()
+	if err != ErrNoInput {
+		t.Error("unexpected error", err)
+	}
+}

batchget.go

@@ -179,6 +179,10 @@ type bgIter struct {
 }
 
 func newBGIter(bg *BatchGet, fn unmarshalFunc, err error) *bgIter {
+	if err == nil && len(bg.reqs) == 0 {
+		err = ErrNoInput
+	}
+
 	iter := &bgIter{
 		bg:        bg,
 		err:       err,

batchwrite.go

@@ -75,6 +75,9 @@ func (bw *BatchWrite) RunWithContext(ctx aws.Context) (wrote int, err error) {
 	if bw.err != nil {
 		return 0, bw.err
 	}
+	if len(bw.ops) == 0 {
+		return 0, ErrNoInput
+	}
 
 	// TODO: this could be made to be more efficient,
 	// by combining unprocessed items with the next request.

tx.go

@@ -1,11 +1,17 @@
 package dynamo
 
 import (
+	"errors"
+
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/service/dynamodb"
 	"github.com/gofrs/uuid"
 )
 
+// ErrNoInput is returned when APIs that can take multiple inputs are run with zero inputs.
+// For example, in a transaction with no operations.
+var ErrNoInput = errors.New("dynamo: no input items")
+
 type getTxOp interface {
 	getTxItem() (*dynamodb.TransactGetItem, error)
 }
@@ -143,6 +149,9 @@ func (tx *GetTx) AllWithContext(ctx aws.Context, out interface{}) error {
 }
 
 func (tx *GetTx) input() (*dynamodb.TransactGetItemsInput, error) {
+	if len(tx.items) == 0 {
+		return nil, ErrNoInput
+	}
 	input := &dynamodb.TransactGetItemsInput{}
 	for _, item := range tx.items {
 		tgi, err := item.getTxItem()
@@ -269,6 +278,9 @@ func (tx *WriteTx) RunWithContext(ctx aws.Context) error {
 }
 
 func (tx *WriteTx) input() (*dynamodb.TransactWriteItemsInput, error) {
+	if len(tx.items) == 0 {
+		return nil, ErrNoInput
+	}
 	input := &dynamodb.TransactWriteItemsInput{}
 	for _, item := range tx.items {
 		wti, err := item.writeTxItem()

tx_test.go

@@ -164,4 +164,15 @@ func TestTx(t *testing.T) {
 
 	t.Logf("1: %+v 2: %+v 3: %+v", record1, record2, record3)
 	t.Logf("All: %+v (len: %d)", records, len(records))
+
+	// no input
+	err = testDB.GetTx().All(nil)
+	if err != ErrNoInput {
+		t.Error("unexpected error", err)
+	}
+
+	err = testDB.WriteTx().Run()
+	if err != ErrNoInput {
+		t.Error("unexpected error", err)
+	}
 }