fix comment and add a unit test

lightning/backend/local.go

@@ -1240,7 +1240,7 @@ func (local *local) isIngestRetryable(
 			}
 		}
 		return true, newRegion, errors.Errorf("not leader: %s", errPb.GetMessage())
-	case strings.Contains(errPb.Message, "Raft raft: proposal dropped"):
+	case strings.Contains(errPb.Message, "raft: proposal dropped"):
 		// TODO: we should change 'Raft raft: proposal dropped' to a error type like 'NotLeader'
 		newRegion, err = getRegion()
 		if err != nil {

lightning/backend/local_test.go

@@ -0,0 +1,33 @@
+package backend
+
+import (
+	"bytes"
+
+	. "github.com/pingcap/check"
+)
+
+type localSuite struct{}
+
+var _ = Suite(&localSuite{})
+
+func (s *localSuite) TestNextKey(c *C) {
+	c.Assert(nextKey([]byte{}), Equals, []byte{})
+
+	cases := [][]byte{
+		[]byte{0},
+		[]byte{255},
+		[]byte{1, 255},
+	}
+	for _, b := range cases {
+		next := nextKey(b)
+		c.Assert(next, Equals, append(b, 0))
+	}
+
+	// in the old logic, this should return []byte{} which is not the actually smallest eky
+	next := nextKey([]byte{1, 255})
+	c.Assert(bytes.Compare(next, []byte{2}), Equals, -1)
+
+	// another test case, nextkey()'s return should be smaller than key with a prefix of the origin key
+	next = nextKey([]byte{1, 255})
+	c.Assert(bytes.Compare(next, []byte{1, 255, 0, 1, 2}), Equals, -1)
+}