Fix Sequence generates duplicate values (#1281)

Fix issue #1280

(cherry picked from commit fa94030)

db.go

@@ -1106,9 +1106,26 @@ func (seq *Sequence) Release() error {
 	seq.Lock()
 	defer seq.Unlock()
 	err := seq.db.Update(func(txn *Txn) error {
-		var buf [8]byte
-		binary.BigEndian.PutUint64(buf[:], seq.next)
-		return txn.SetEntry(NewEntry(seq.key, buf[:]))
+		item, err := txn.Get(seq.key)
+		if err != nil {
+			return err
+		}
+
+		var num uint64
+		if err := item.Value(func(v []byte) error {
+			num = binary.BigEndian.Uint64(v)
+			return nil
+		}); err != nil {
+			return err
+		}
+
+		if num == seq.leased {
+			var buf [8]byte
+			binary.BigEndian.PutUint64(buf[:], seq.next)
+			return txn.SetEntry(NewEntry(seq.key, buf[:]))
+		}
+
+		return nil
 	})
 	if err != nil {
 		return err

db_test.go

@@ -1373,6 +1373,33 @@ func TestSequence_Release(t *testing.T) {
 	})
 }
 
+func TestTestSequence2(t *testing.T) {
+	runBadgerTest(t, nil, func(t *testing.T, db *DB) {
+		key := []byte("key")
+		seq1, err := db.GetSequence(key, 2)
+		require.NoError(t, err)
+
+		seq2, err := db.GetSequence(key, 2)
+		require.NoError(t, err)
+		num, err := seq2.Next()
+		require.NoError(t, err)
+		require.Equal(t, uint64(2), num)
+
+		require.NoError(t, seq2.Release())
+		require.NoError(t, seq1.Release())
+
+		seq3, err := db.GetSequence(key, 2)
+		require.NoError(t, err)
+		for i := 0; i < 5; i++ {
+			num2, err := seq3.Next()
+			require.NoError(t, err)
+			require.Equal(t, uint64(i)+3, num2)
+		}
+
+		require.NoError(t, seq3.Release())
+	})
+}
+
 func TestReadOnly(t *testing.T) {
 	dir, err := ioutil.TempDir("", "badger-test")
 	require.NoError(t, err)