MM-20649 Fix incorrect mention count when marking a DM channel as unread (#13245)

* MM-20649 Fix incorrect mention count when marking a DM channel as unread

* Satisfy govet

* Update missed test

store/sqlstore/channel_store.go

@@ -1019,7 +1019,7 @@ func (s SqlChannelStore) GetPublicChannelsForTeam(teamId string, offset int, lim
 			PublicChannels pc ON (pc.Id = Channels.Id)
 		WHERE
 			pc.TeamId = :TeamId
-		AND pc.DeleteAt = 0 
+		AND pc.DeleteAt = 0
 		ORDER BY pc.DisplayName
 		LIMIT :Limit
 		OFFSET :Offset
@@ -1229,15 +1229,15 @@ func (s SqlChannelStore) GetDeleted(teamId string, offset int, limit int, userId
 	channels := &model.ChannelList{}
 
 	query := `
-		SELECT * FROM Channels 
-		WHERE (TeamId = :TeamId OR TeamId = '') 
-		AND DeleteAt != 0 
+		SELECT * FROM Channels
+		WHERE (TeamId = :TeamId OR TeamId = '')
+		AND DeleteAt != 0
 		AND Type != 'P'
 		UNION
-			SELECT * FROM Channels 
-			WHERE (TeamId = :TeamId OR TeamId = '') 
-			AND DeleteAt != 0 
-			AND Type = 'P' 
+			SELECT * FROM Channels
+			WHERE (TeamId = :TeamId OR TeamId = '')
+			AND DeleteAt != 0
+			AND Type = 'P'
 			AND Id IN (SELECT ChannelId FROM ChannelMembers WHERE UserId = :UserId)
 		ORDER BY DisplayName LIMIT :Limit OFFSET :Offset
 	`
@@ -1758,25 +1758,30 @@ func (s SqlChannelStore) UpdateLastViewedAt(channelIds []string, userId string)
 	return times, nil
 }
 
-// countPostsAfter returns the number of posts in the given channel created after but not including the given timestamp.
-func (s SqlChannelStore) countPostsAfter(channelID string, since int64) (int64, *model.AppError) {
+// CountPostsAfter returns the number of posts in the given channel created after but not including the given timestamp. If given a non-empty user ID, only counts posts made by that user.
+func (s SqlChannelStore) CountPostsAfter(channelId string, timestamp int64, userId string) (int64, *model.AppError) {
 	countUnreadQuery := `
 	SELECT count(*)
 	FROM Posts
 	WHERE
-		ChannelId = :channelId
-		AND CreateAt > :createAt
-		AND Type = ''
+		ChannelId = :ChannelId
+		AND CreateAt > :CreateAt
+		AND Type = '' -- This line causes MM-20681
 		AND DeleteAt = 0
 	`
 	countParams := map[string]interface{}{
-		"channelId": channelID,
-		"createAt":  since,
+		"ChannelId": channelId,
+		"CreateAt":  timestamp,
+	}
+
+	if userId != "" {
+		countUnreadQuery += " AND UserId = :UserId"
+		countParams["UserId"] = userId
 	}
 
 	unread, err := s.GetReplica().SelectInt(countUnreadQuery, countParams)
 	if err != nil {
-		return 0, model.NewAppError("SqlChannelStore.countPostsAfter", "store.sql_channel.count_posts_since.app_error", countParams, fmt.Sprintf("channel_id=%s, since=%d, err=%s", channelID, since, err), http.StatusInternalServerError)
+		return 0, model.NewAppError("SqlChannelStore.CountPostsAfter", "store.sql_channel.count_posts_since.app_error", countParams, fmt.Sprintf("channel_id=%s, timestamp=%d, err=%s", channelId, timestamp, err), http.StatusInternalServerError)
 	}
 	return unread, nil
 }
@@ -1787,16 +1792,11 @@ func (s SqlChannelStore) countPostsAfter(channelID string, since int64) (int64,
 func (s SqlChannelStore) UpdateLastViewedAtPost(unreadPost *model.Post, userID string, mentionCount int) (*model.ChannelUnreadAt, *model.AppError) {
 	unreadDate := unreadPost.CreateAt - 1
 
-	unread, appErr := s.countPostsAfter(unreadPost.ChannelId, unreadDate)
+	unread, appErr := s.CountPostsAfter(unreadPost.ChannelId, unreadDate, "")
 	if appErr != nil {
 		return nil, appErr
 	}
 
-	if mentionCount == store.MentionAllPosts {
-		// Treat every unread post as a mention (like in a DM channel)
-		mentionCount = int(unread)
-	}
-
 	params := map[string]interface{}{
 		"mentions":     mentionCount,
 		"unreadCount":  unread,