MM-54778 Fix mark as unread on GMs (#24880)

* Fix mark as unread on GMs

* Don't count own messages in gms when marking as unread

* Change argument name

* Rename userId

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/post.go

@@ -1795,7 +1795,7 @@ func (a *App) countThreadMentions(c request.CTX, user *model.User, post *model.P
 
 	count := 0
 
-	if channel.Type == model.ChannelTypeDirect {
+	if channel.Type == model.ChannelTypeDirect || channel.Type == model.ChannelTypeGroup {
 		// In a DM channel, every post made by the other user is a mention
 		otherId := channel.GetOtherUserIdForDM(user.Id)
 		for _, p := range posts {
@@ -1842,16 +1842,16 @@ func (a *App) countMentionsFromPost(c request.CTX, user *model.User, post *model
 		return 0, 0, 0, err
 	}
 
-	if channel.Type == model.ChannelTypeDirect {
+	if channel.Type == model.ChannelTypeDirect || channel.Type == model.ChannelTypeGroup {
 		// In a DM channel, every post made by the other user is a mention
-		count, countRoot, nErr := a.Srv().Store().Channel().CountPostsAfter(post.ChannelId, post.CreateAt-1, channel.GetOtherUserIdForDM(user.Id))
+		count, countRoot, nErr := a.Srv().Store().Channel().CountPostsAfter(post.ChannelId, post.CreateAt-1, user.Id)
 		if nErr != nil {
 			return 0, 0, 0, model.NewAppError("countMentionsFromPost", "app.channel.count_posts_since.app_error", nil, "", http.StatusInternalServerError).Wrap(nErr)
 		}
 
 		var urgentCount int
 		if a.IsPostPriorityEnabled() {
-			urgentCount, nErr = a.Srv().Store().Channel().CountUrgentPostsAfter(post.ChannelId, post.CreateAt-1, channel.GetOtherUserIdForDM(user.Id))
+			urgentCount, nErr = a.Srv().Store().Channel().CountUrgentPostsAfter(post.ChannelId, post.CreateAt-1, user.Id)
 			if nErr != nil {
 				return 0, 0, 0, model.NewAppError("countMentionsFromPost", "app.channel.count_urgent_posts_since.app_error", nil, "", http.StatusInternalServerError).Wrap(nErr)
 			}

server/channels/app/post_test.go

@@ -1946,6 +1946,49 @@ func TestCountMentionsFromPost(t *testing.T) {
 		assert.Equal(t, 0, count)
 	})
 
+	t.Run("should return the number of posts made by the other user for a group message", func(t *testing.T) {
+		th := Setup(t).InitBasic()
+		defer th.TearDown()
+
+		user1 := th.BasicUser
+		user2 := th.BasicUser2
+		user3 := th.SystemAdminUser
+
+		channel, err := th.App.createGroupChannel(th.Context, []string{user1.Id, user2.Id, user3.Id})
+		require.Nil(t, err)
+
+		post1, err := th.App.CreatePost(th.Context, &model.Post{
+			UserId:    user1.Id,
+			ChannelId: channel.Id,
+			Message:   "test",
+		}, channel, false, true)
+		require.Nil(t, err)
+
+		_, err = th.App.CreatePost(th.Context, &model.Post{
+			UserId:    user1.Id,
+			ChannelId: channel.Id,
+			Message:   "test2",
+		}, channel, false, true)
+		require.Nil(t, err)
+
+		_, err = th.App.CreatePost(th.Context, &model.Post{
+			UserId:    user3.Id,
+			ChannelId: channel.Id,
+			Message:   "test3",
+		}, channel, false, true)
+		require.Nil(t, err)
+
+		count, _, _, err := th.App.countMentionsFromPost(th.Context, user2, post1)
+
+		assert.Nil(t, err)
+		assert.Equal(t, 3, count)
+
+		count, _, _, err = th.App.countMentionsFromPost(th.Context, user1, post1)
+
+		assert.Nil(t, err)
+		assert.Equal(t, 1, count)
+	})
+
 	t.Run("should not count mentions from the before the given post", func(t *testing.T) {
 		th := Setup(t).InitBasic()
 		defer th.TearDown()