MM-43939: fixes LastReplyAt when deleting the last reply (#20615)

* MM-43939: fixes lastreplyat when deleting the last reply

Currently we are not updating the Threads.LastReplyAt when the last
reply gets deleted. This can lead to threads appearing unread when
actually there is no unread thread.

This commit updates the value of Threads.LastReplyAt when a reply gets
deleted, to the most recent post's timestamp in the thread.

* Updates ReplyCount to current value on post delete

* Addresses review comments

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/post_store.go

@@ -2840,8 +2840,23 @@ func (s *SqlPostStore) updateThreadAfterReplyDeletion(transaction *sqlxTxWrapper
 			}
 		}
 
+		lastReplyAtSubquery := sq.Select("COALESCE(MAX(CreateAt), 0)").
+			From("Posts").
+			Where(sq.Eq{
+				"RootId":   rootId,
+				"DeleteAt": 0,
+			})
+
+		lastReplyCountSubquery := sq.Select("Count(*)").
+			From("Posts").
+			Where(sq.Eq{
+				"RootId":   rootId,
+				"DeleteAt": 0,
+			})
+
 		updateQueryString, updateArgs, err := updateQuery.
-			Set("ReplyCount", sq.Expr("ReplyCount - 1")).
+			Set("LastReplyAt", lastReplyAtSubquery).
+			Set("ReplyCount", lastReplyCountSubquery).
 			Where(sq.And{
 				sq.Eq{"PostId": rootId},
 				sq.Gt{"ReplyCount": 0},

store/storetest/post_store.go

@@ -979,6 +979,74 @@ func testPostStoreDelete(t *testing.T, ss store.Store) {
 		_, err = ss.Post().Get(context.Background(), rootPost2.Id, model.GetPostsOptions{}, "", map[string]bool{})
 		require.NoError(t, err)
 	})
+
+	t.Run("thread with multiple replies, update thread last reply at", func(t *testing.T) {
+		// Create a root post
+		rootPost1, err := ss.Post().Save(&model.Post{
+			ChannelId: model.NewId(),
+			UserId:    model.NewId(),
+			Message:   NewTestId(),
+		})
+		require.NoError(t, err)
+
+		// Reply to that root post
+		replyPost1, err := ss.Post().Save(&model.Post{
+			ChannelId: rootPost1.ChannelId,
+			UserId:    model.NewId(),
+			Message:   NewTestId(),
+			RootId:    rootPost1.Id,
+		})
+		require.NoError(t, err)
+
+		// Reply to that root post a second time
+		replyPost2, err := ss.Post().Save(&model.Post{
+			ChannelId: rootPost1.ChannelId,
+			UserId:    model.NewId(),
+			Message:   NewTestId(),
+			RootId:    rootPost1.Id,
+		})
+		require.NoError(t, err)
+
+		// Reply to that root post a third time
+		replyPost3, err := ss.Post().Save(&model.Post{
+			ChannelId: rootPost1.ChannelId,
+			UserId:    model.NewId(),
+			Message:   NewTestId(),
+			RootId:    rootPost1.Id,
+		})
+		require.NoError(t, err)
+
+		thread, err := ss.Thread().Get(rootPost1.Id)
+		require.NoError(t, err)
+		require.Equal(t, replyPost3.CreateAt, thread.LastReplyAt)
+
+		// Delete the reply previous to last
+		err = ss.Post().Delete(replyPost2.Id, model.GetMillis(), "")
+		require.NoError(t, err)
+
+		thread, err = ss.Thread().Get(rootPost1.Id)
+		require.NoError(t, err)
+		// last reply at should be unchanged
+		require.Equal(t, replyPost3.CreateAt, thread.LastReplyAt)
+
+		// Delete the last reply
+		err = ss.Post().Delete(replyPost3.Id, model.GetMillis(), "")
+		require.NoError(t, err)
+
+		thread, err = ss.Thread().Get(rootPost1.Id)
+		require.NoError(t, err)
+		// last reply at should have changed
+		require.Equal(t, replyPost1.CreateAt, thread.LastReplyAt)
+
+		// Delete the last reply
+		err = ss.Post().Delete(replyPost1.Id, model.GetMillis(), "")
+		require.NoError(t, err)
+
+		thread, err = ss.Thread().Get(rootPost1.Id)
+		require.NoError(t, err)
+		// last reply at should be 0
+		require.Equal(t, int64(0), thread.LastReplyAt)
+	})
 }
 
 func testPostStorePermDelete1Level(t *testing.T, ss store.Store) {