MM-35298: Follow thread when added to channel (#19311)

* MM-35298: Follow thread when added to channel

* return better error if thread doesn't exist

* update test for possible race

* use correct comparision operator

api4/channel.go

@@ -1698,6 +1698,14 @@ func addChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if postRootId != "" {
+		err := c.App.UpdateThreadFollowForUserFromChannelAdd(cm.UserId, channel.TeamId, postRootId)
+		if err != nil {
+			c.Err = err
+			return
+		}
+	}
+
 	auditRec.Success()
 	auditRec.AddMeta("add_user_id", cm.UserId)
 	c.LogAudit("name=" + channel.Name + " user_id=" + cm.UserId)

api4/channel_test.go

@@ -3012,6 +3012,63 @@ func TestAddChannelMember(t *testing.T) {
 	})
 }
 
+func TestAddChannelMemberFromThread(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+	team := th.BasicTeam
+	user := th.BasicUser
+	user2 := th.BasicUser2
+	user3 := th.CreateUserWithClient(th.SystemAdminClient)
+	_, _, err := th.SystemAdminClient.AddTeamMember(team.Id, user3.Id)
+	require.NoError(t, err)
+
+	publicChannel := th.CreatePublicChannel()
+
+	_, resp, err := th.Client.AddChannelMember(publicChannel.Id, user.Id)
+	require.NoError(t, err)
+	CheckCreatedStatus(t, resp)
+	_, resp, err = th.Client.AddChannelMember(publicChannel.Id, user2.Id)
+	require.NoError(t, err)
+	CheckCreatedStatus(t, resp)
+
+	post := &model.Post{
+		ChannelId: publicChannel.Id,
+		Message:   "A root post",
+		UserId:    user.Id,
+	}
+	rpost, _, err := th.SystemAdminClient.CreatePost(post)
+	require.NoError(t, err)
+
+	_, _, err = th.SystemAdminClient.CreatePost(
+		&model.Post{
+			ChannelId: publicChannel.Id,
+			Message:   "A reply post with mention @" + user3.Username,
+			UserId:    user2.Id,
+			RootId:    rpost.Id,
+		})
+	require.NoError(t, err)
+
+	_, _, err = th.SystemAdminClient.CreatePost(
+		&model.Post{
+			ChannelId: publicChannel.Id,
+			Message:   "Another reply post with mention @" + user3.Username,
+			UserId:    user2.Id,
+			RootId:    rpost.Id,
+		})
+	require.NoError(t, err)
+
+	// Simulate adding a user to a channel from a thread
+	_, _, err = th.SystemAdminClient.AddChannelMemberWithRootId(publicChannel.Id, user3.Id, rpost.Id)
+	require.NoError(t, err)
+
+	// Threadmembership should exist for added user
+	ut, _, err := th.SystemAdminClient.GetUserThread(user3.Id, team.Id, rpost.Id, false)
+	require.NoError(t, err)
+	// Should have two mentions. There might be a race condition
+	// here between the "added user to the channel" message and the GetUserThread call
+	require.LessOrEqual(t, int64(2), ut.UnreadMentions)
+}
+
 func TestAddChannelMemberAddMyself(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()