MM-34871 CRT: Participants of thread include non-replying followers (and past followers) (#17447)

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

app/post_test.go

@@ -1924,6 +1924,58 @@ func TestThreadMembership(t *testing.T) {
 	})
 }
 
+func TestFollowThreadSkipsParticipants(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+	os.Setenv("MM_FEATUREFLAGS_COLLAPSEDTHREADS", "true")
+	defer os.Unsetenv("MM_FEATUREFLAGS_COLLAPSEDTHREADS")
+
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.ServiceSettings.ThreadAutoFollow = true
+		*cfg.ServiceSettings.CollapsedThreads = model.COLLAPSED_THREADS_DEFAULT_ON
+	})
+
+	channel := th.BasicChannel
+	user := th.BasicUser
+	user2 := th.BasicUser2
+	sysadmin := th.SystemAdminUser
+
+	appErr := th.App.JoinChannel(channel, user.Id)
+	require.Nil(t, appErr)
+	appErr = th.App.JoinChannel(channel, user2.Id)
+	require.Nil(t, appErr)
+	_, appErr = th.App.JoinUserToTeam(th.BasicTeam, sysadmin, sysadmin.Id)
+	require.Nil(t, appErr)
+	appErr = th.App.JoinChannel(channel, sysadmin.Id)
+	require.Nil(t, appErr)
+
+	p1, err := th.App.CreatePost(&model.Post{UserId: user.Id, ChannelId: channel.Id, Message: "Hi @" + sysadmin.Username}, channel, false, false)
+	require.Nil(t, err)
+	_, err = th.App.CreatePost(&model.Post{RootId: p1.Id, UserId: user.Id, ChannelId: channel.Id, Message: "Hola"}, channel, false, false)
+	require.Nil(t, err)
+
+	thread, err := th.App.GetThreadForUser(user.Id, th.BasicTeam.Id, p1.Id, false)
+	require.Nil(t, err)
+	require.Len(t, thread.Participants, 1) // length should be 1, the original poster, since sysadmin was just mentioned but didn't post
+
+	_, err = th.App.CreatePost(&model.Post{RootId: p1.Id, UserId: sysadmin.Id, ChannelId: channel.Id, Message: "sysadmin reply"}, channel, false, false)
+	require.Nil(t, err)
+
+	thread, err = th.App.GetThreadForUser(user.Id, th.BasicTeam.Id, p1.Id, false)
+	require.Nil(t, err)
+	require.Len(t, thread.Participants, 2) // length should be 2, the original poster and sysadmin, since sysadmin participated now
+
+	// another user follows the thread
+	th.App.UpdateThreadFollowForUser(user2.Id, th.BasicTeam.Id, p1.Id, true)
+
+	thread, err = th.App.GetThreadForUser(user2.Id, th.BasicTeam.Id, p1.Id, false)
+	require.Nil(t, err)
+	require.Len(t, thread.Participants, 2) // length should be 2, since follow shouldn't update participant list, only user1 and sysadmin are participants
+	for _, p := range thread.Participants {
+		require.True(t, p.Id == sysadmin.Id || p.Id == user.Id)
+	}
+}
+
 func TestAutofollowBasedOnRootPost(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
@@ -1986,13 +2038,13 @@ func TestCollapsedThreadFetch(t *testing.T) {
 		require.Nil(t, err)
 		thread, nErr := th.App.Srv().Store.Thread().Get(postRoot.Id)
 		require.NoError(t, nErr)
-		require.Len(t, thread.Participants, 2)
+		require.Len(t, thread.Participants, 1)
 		th.App.MarkChannelAsUnreadFromPost(postRoot.Id, user1.Id)
 		l, err := th.App.GetPostsForChannelAroundLastUnread(channel.Id, user1.Id, 10, 10, true, true, false)
 		require.Nil(t, err)
 		require.Len(t, l.Order, 1)
 		require.EqualValues(t, 1, l.Posts[postRoot.Id].ReplyCount)
-		require.EqualValues(t, []string{user1.Id, user2.Id}, []string{l.Posts[postRoot.Id].Participants[0].Id, l.Posts[postRoot.Id].Participants[1].Id})
+		require.EqualValues(t, []string{user1.Id}, []string{l.Posts[postRoot.Id].Participants[0].Id})
 		require.Empty(t, l.Posts[postRoot.Id].Participants[0].Email)
 		require.NotZero(t, l.Posts[postRoot.Id].LastReplyAt)
 		require.True(t, l.Posts[postRoot.Id].IsFollowing)