fix(msg): correct gap message document limit

pkg/common/storage/model/msg.go

@@ -133,7 +133,7 @@ func (*MsgDocModel) GetMsgIndex(seq int64) int64 {
 }
 
 func (*MsgDocModel) GetLimitForSingleDoc(seq int64) int64 {
-	return seq % singleGocMsgNum
+	return (seq - 1) % singleGocMsgNum
 }
 
 func (*MsgDocModel) indexGen(conversationID string, seqSuffix int64) string {

pkg/common/storage/model/msg_test.go

@@ -0,0 +1,12 @@
+package model
+
+import "testing"
+
+func TestMsgDocModelGetLimitForSingleDoc(t *testing.T) {
+	model := new(MsgDocModel)
+	for _, test := range []struct{ seq, want int64 }{{1, 0}, {99, 98}, {100, 99}, {101, 0}} {
+		if got := model.GetLimitForSingleDoc(test.seq); got != test.want {
+			t.Errorf("GetLimitForSingleDoc(%d) = %d, want %d", test.seq, got, test.want)
+		}
+	}
+}