MM-34895: LastViewed should be updated correctly for new thread membership (#17673)

Summary
Update LastViewed correctly for new thread membership

Ticket Link
https://mattermost.atlassian.net/browse/MM-34895

api4/user_test.go

@@ -5766,8 +5766,8 @@ func TestMaintainUnreadRepliesInThread(t *testing.T) {
 	// replying to the thread clears reply count, so it should be 0
 	checkThreadListReplies(t, th, th.Client, th.BasicUser.Id, 0, 1, nil)
 
-	// the other user should have 2 replies
+	// the other user should have 1 reply - the reply from the regular user
-	checkThreadListReplies(t, th, th.SystemAdminClient, th.SystemAdminUser.Id, 2, 1, nil)
+	checkThreadListReplies(t, th, th.SystemAdminClient, th.SystemAdminUser.Id, 1, 1, nil)
 
 	// mark all as read for user
 	resp := th.Client.UpdateThreadsReadForUser(th.BasicUser.Id, th.BasicTeam.Id)

store/sqlstore/thread_store.go

@@ -521,11 +521,15 @@ func (s *SqlThreadStore) MaintainMembership(userId, postId string, following, in
 	if incrementMentions {
 		mentions = 1
 	}
+	var lastViewed int64
+	if updateViewedTimestamp {
+		lastViewed = now
+	}
 	membership, err = s.SaveMembership(&model.ThreadMembership{
 		PostId:         postId,
 		UserId:         userId,
 		Following:      following,
-		LastViewed:     0,
+		LastViewed:     lastViewed,
 		LastUpdated:    now,
 		UnreadMentions: int64(mentions),
 	})

store/storetest/thread_store.go

@@ -297,11 +297,9 @@ func testThreadStorePopulation(t *testing.T, ss store.Store) {
 	t.Run("Thread membership 'viewed' timestamp is updated properly", func(t *testing.T) {
 		newPosts := makeSomePosts()
 
-		_, e := ss.Thread().MaintainMembership(newPosts[0].UserId, newPosts[0].Id, true, false, true, false, false)
+		tm, e := ss.Thread().MaintainMembership(newPosts[0].UserId, newPosts[0].Id, true, false, true, false, false)
 		require.NoError(t, e)
-		m, err1 := ss.Thread().GetMembershipForUser(newPosts[0].UserId, newPosts[0].Id)
+		require.Equal(t, int64(0), tm.LastViewed)
-		require.NoError(t, err1)
-		require.Equal(t, int64(0), m.LastViewed)
 
 		_, e = ss.Thread().MaintainMembership(newPosts[0].UserId, newPosts[0].Id, true, false, true, true, false)
 		require.NoError(t, e)
@@ -310,6 +308,13 @@ func testThreadStorePopulation(t *testing.T, ss store.Store) {
 		require.Greater(t, m2.LastViewed, int64(0))
 	})
 
+	t.Run("Thread membership 'viewed' timestamp is updated properly for new membership", func(t *testing.T) {
+		newPosts := makeSomePosts()
+		tm, e := ss.Thread().MaintainMembership(newPosts[0].UserId, newPosts[0].Id, true, false, false, true, false)
+		require.NoError(t, e)
+		require.NotEqual(t, int64(0), tm.LastViewed)
+	})
+
 	t.Run("Thread last updated is changed when channel is updated after UpdateLastViewedAtPost for mark unread", func(t *testing.T) {
 		newPosts := makeSomePosts()
 		_, e := ss.Thread().MaintainMembership(newPosts[0].UserId, newPosts[0].Id, true, false, true, false, false)