Add panics if an unclosed iterator is found. Or, if multiple iterator…

…s are run in one transaction.

backup.go

@@ -53,6 +53,8 @@ func (db *DB) Backup(w io.Writer, since uint64) (uint64, error) {
 		opts := DefaultIteratorOptions
 		opts.AllVersions = true
 		it := txn.NewIterator(opts)
+		defer it.Close()
+
 		for it.Rewind(); it.Valid(); it.Next() {
 			item := it.Item()
 			if item.Version() < since {

backup_test.go

@@ -91,6 +91,7 @@ func TestDumpLoad(t *testing.T) {
 		opts := DefaultIteratorOptions
 		opts.AllVersions = true
 		it := txn.NewIterator(opts)
+		defer it.Close()
 		var count int
 		for it.Rewind(); it.Valid(); it.Next() {
 			item := it.Item()

db.go

@@ -1121,6 +1121,8 @@ func (op *MergeOperator) iterateAndMerge(txn *Txn) (val []byte, err error) {
 	opt := DefaultIteratorOptions
 	opt.AllVersions = true
 	it := txn.NewIterator(opt)
+	defer it.Close()
+
 	var numVersions int
 	for it.Rewind(); it.ValidForPrefix(op.key); it.Next() {
 		item := it.Item()

db_test.go

@@ -862,6 +862,7 @@ func TestDiscardVersionsBelow(t *testing.T) {
 		// Verify that there are 4 versions, and record 3rd version (2nd from top in iteration)
 		db.View(func(txn *Txn) error {
 			it := txn.NewIterator(opts)
+			defer it.Close()
 			var count int
 			for it.Rewind(); it.Valid(); it.Next() {
 				count++
@@ -885,6 +886,7 @@ func TestDiscardVersionsBelow(t *testing.T) {
 		// below ts have been deleted.
 		db.View(func(txn *Txn) error {
 			it := txn.NewIterator(opts)
+			defer it.Close()
 			var count int
 			for it.Rewind(); it.Valid(); it.Next() {
 				count++
@@ -931,6 +933,7 @@ func TestExpiry(t *testing.T) {
 		opts.PrefetchValues = false
 		err = db.View(func(txn *Txn) error {
 			it := txn.NewIterator(opts)
+			defer it.Close()
 			var count int
 			for it.Rewind(); it.Valid(); it.Next() {
 				count++
@@ -1099,6 +1102,7 @@ func TestWriteDeadlock(t *testing.T) {
 		opt := DefaultIteratorOptions
 		opt.PrefetchValues = false
 		it := txn.NewIterator(opt)
+		defer it.Close()
 		for it.Rewind(); it.Valid(); it.Next() {
 			item := it.Item()
 
@@ -1558,6 +1562,7 @@ func ExampleTxn_NewIterator() {
 	var count int
 	err = db.View(func(txn *Txn) error {
 		it := txn.NewIterator(opt)
+		defer it.Close()
 		for it.Rewind(); it.Valid(); it.Next() {
 			count++
 		}

iterator.go

@@ -20,6 +20,7 @@ import (
 	"bytes"
 	"fmt"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/dgraph-io/badger/options"
@@ -316,6 +317,10 @@ type Iterator struct {
 // Using prefetch is highly recommended if you're doing a long running iteration.
 // Avoid long running iterations in update transactions.
 func (txn *Txn) NewIterator(opt IteratorOptions) *Iterator {
+	if atomic.AddInt32(&txn.numIterators, 1) > 1 {
+		panic("Only one iterator can be active at one time.")
+	}
+
 	tables, decr := txn.db.getMemTables()
 	defer decr()
 	txn.db.vlog.incrIteratorCount()
@@ -382,6 +387,7 @@ func (it *Iterator) Close() {
 
 	// TODO: We could handle this error.
 	_ = it.txn.db.vlog.decrIteratorCount()
+	atomic.AddInt32(&it.txn.numIterators, -1)
 }
 
 // Next would advance the iterator by one. Always check it.Valid() after a Next()

transaction.go

@@ -149,8 +149,9 @@ type Txn struct {
 	callbacks []func()
 	discarded bool
 
-	size  int64
-	count int64
+	size         int64
+	count        int64
+	numIterators int32
 }
 
 type pendingWritesIterator struct {
@@ -423,10 +424,12 @@ func (txn *Txn) Discard() {
 	if txn.discarded { // Avoid a re-run.
 		return
 	}
+	if atomic.LoadInt32(&txn.numIterators) > 0 {
+		panic("Unclosed iterator at time of Txn.Discard.")
+	}
 	txn.discarded = true
 	txn.db.orc.readMark.Done(txn.readTs)
 	txn.runCallbacks()
-
 	if txn.update {
 		txn.db.orc.decrRef()
 	}

transaction_test.go

@@ -156,6 +156,7 @@ func TestTxnVersions(t *testing.T) {
 		}
 
 		checkIterator := func(itr *Iterator, i int) {
+			defer itr.Close()
 			count := 0
 			for itr.Rewind(); itr.Valid(); itr.Next() {
 				item := itr.Item()
@@ -225,12 +226,14 @@ func TestTxnVersions(t *testing.T) {
 			opt.AllVersions = true
 			itr = txn.NewIterator(opt)
 			checkAllVersions(itr, i)
+			itr.Close()
 
 			opt = DefaultIteratorOptions
 			opt.AllVersions = true
 			opt.Reverse = true
 			itr = txn.NewIterator(opt)
 			checkAllVersions(itr, i)
+			itr.Close()
 
 			txn.Discard()
 		}
@@ -352,6 +355,7 @@ func TestTxnIterationEdgeCase(t *testing.T) {
 		require.Equal(t, uint64(4), db.orc.readTs())
 
 		checkIterator := func(itr *Iterator, expected []string) {
+			defer itr.Close()
 			var i int
 			for itr.Rewind(); itr.Valid(); itr.Next() {
 				item := itr.Item()
@@ -435,6 +439,7 @@ func TestTxnIterationEdgeCase2(t *testing.T) {
 		require.Equal(t, uint64(4), db.orc.readTs())
 
 		checkIterator := func(itr *Iterator, expected []string) {
+			defer itr.Close()
 			var i int
 			for itr.Rewind(); itr.Valid(); itr.Next() {
 				item := itr.Item()
@@ -463,6 +468,7 @@ func TestTxnIterationEdgeCase2(t *testing.T) {
 		itr.Seek(kc)
 		require.True(t, itr.Valid())
 		require.Equal(t, itr.item.Key(), kc)
+		itr.Close()
 
 		itr = txn.NewIterator(rev)
 		itr.Seek(ka)
@@ -471,6 +477,7 @@ func TestTxnIterationEdgeCase2(t *testing.T) {
 		itr.Seek(kc)
 		require.True(t, itr.Valid())
 		require.Equal(t, itr.item.Key(), kc)
+		itr.Close()
 
 		txn.readTs = 3
 		itr = txn.NewIterator(DefaultIteratorOptions)
@@ -537,6 +544,7 @@ func TestTxnIterationEdgeCase3(t *testing.T) {
 		itr.Seek([]byte("ac"))
 		require.True(t, itr.Valid())
 		require.Equal(t, itr.item.Key(), kc)
+		itr.Close()
 
 		//  Keys: "abc", "ade"
 		// Read pending writes.
@@ -558,6 +566,7 @@ func TestTxnIterationEdgeCase3(t *testing.T) {
 		itr.Seek([]byte("ad"))
 		require.True(t, itr.Valid())
 		require.Equal(t, itr.item.Key(), kd)
+		itr.Close()
 
 		itr = txn.NewIterator(rev)
 		itr.Seek([]byte("ac"))
@@ -576,6 +585,7 @@ func TestTxnIterationEdgeCase3(t *testing.T) {
 		itr.Seek([]byte("ad"))
 		require.True(t, itr.Valid())
 		require.Equal(t, itr.item.Key(), kc)
+		itr.Close()
 
 		//  Keys: "abc", "ade"
 		itr = txn2.NewIterator(rev)
@@ -595,6 +605,7 @@ func TestTxnIterationEdgeCase3(t *testing.T) {
 		itr.Seek([]byte("ac"))
 		require.True(t, itr.Valid())
 		require.Equal(t, itr.item.Key(), kb)
+		itr.Close()
 	})
 }
 
@@ -630,6 +641,7 @@ func TestIteratorAllVersionsWithDeleted(t *testing.T) {
 		// Verify that deleted shows up when AllVersions is set.
 		err = db.View(func(txn *Txn) error {
 			it := txn.NewIterator(opts)
+			defer it.Close()
 			var count int
 			for it.Rewind(); it.Valid(); it.Next() {
 				count++
@@ -670,6 +682,7 @@ func TestIteratorAllVersionsWithDeleted2(t *testing.T) {
 		// Verify that deleted shows up when AllVersions is set.
 		err := db.View(func(txn *Txn) error {
 			it := txn.NewIterator(opts)
+			defer it.Close()
 			var count int
 			for it.Rewind(); it.Valid(); it.Next() {
 				item := it.Item()