fix: fix reverse iterator broken by seek (#2109)

Fixes #2108

Author: Harshil Goel

db_test.go

@@ -164,6 +164,44 @@ func runBadgerTest(t *testing.T, opts *Options, test func(t *testing.T, db *DB))
 	test(t, db)
 }
 
+func TestReverseIterator(t *testing.T) {
+	runBadgerTest(t, nil, func(t *testing.T, db *DB) {
+		key := make([]byte, 6)
+		err := db.Update(func(txn *Txn) error {
+			binary.BigEndian.PutUint16(key, 5)
+			binary.BigEndian.PutUint32(key[2:], 1)
+			err1 := txn.Set(key, []byte("value1"))
+			require.NoError(t, err1)
+
+			binary.BigEndian.PutUint32(key[2:], 2)
+			err1 = txn.Set(key, []byte("value2"))
+			require.NoError(t, err1)
+			return nil
+		})
+
+		require.NoError(t, err)
+
+		err = db.View(func(txn *Txn) error {
+			searchBuffer := make([]byte, 3)
+			binary.BigEndian.PutUint16(searchBuffer, 5)
+			searchBuffer[2] = 0xFF
+
+			iteratorOptions := DefaultIteratorOptions
+			iteratorOptions.Reverse = true
+			iteratorOptions.PrefetchValues = false
+			iteratorOptions.Prefix = searchBuffer
+			it := txn.NewIterator(iteratorOptions)
+			defer it.Close()
+
+			it.Rewind()
+			require.Equal(t, it.Item().Key(), key)
+			return nil
+		})
+
+		require.NoError(t, err)
+	})
+}
+
 func TestWrite(t *testing.T) {
 	runBadgerTest(t, nil, func(t *testing.T, db *DB) {
 		for i := 0; i < 100; i++ {

iterator.go

@@ -589,7 +589,7 @@ func (it *Iterator) Next() {
 
 	// Set next item to current
 	it.item = it.data.pop()
-	for it.iitr.Valid() && hasPrefix(it.iitr, it.opt.Prefix) {
+	for it.iitr.Valid() && hasPrefix(it) {
 		if it.parseItem() {
 			// parseItem calls one extra next.
 			// This is used to deal with the complexity of reverse iteration.
@@ -725,9 +725,11 @@ func (it *Iterator) fill(item *Item) {
 	}
 }
 
-func hasPrefix(it y.Iterator, prefix []byte) bool {
-	if len(prefix) > 0 {
-		return bytes.HasPrefix(y.ParseKey(it.Key()), prefix)
+func hasPrefix(it *Iterator) bool {
+	// We shouldn't check prefix in case the iterator is going in reverse. Since in reverse we expect
+	// people to append items to the end of prefix.
+	if !it.opt.Reverse && len(it.opt.Prefix) > 0 {
+		return bytes.HasPrefix(y.ParseKey(it.iitr.Key()), it.opt.Prefix)
 	}
 	return true
 }
@@ -741,7 +743,7 @@ func (it *Iterator) prefetch() {
 	i := it.iitr
 	var count int
 	it.item = nil
-	for i.Valid() && hasPrefix(i, it.opt.Prefix) {
+	for i.Valid() && hasPrefix(it) {
 		if !it.parseItem() {
 			continue
 		}