[MM-55485] Ensure user added to channel receives websocket event regardless of channel membership info retrieved in HA (#27013)

* [MM-55485] Ensure user added to channel receives websocket event regardless of channel membership info retrieved in HA

* Testing to see if group messages are also affected

* Use context from master approach

* Hardcode to master to test

* Revert "Hardcode to master to test"

This reverts commit 50ef2baed1559861367d9c3b4cea74c21b4b9fd1.

* Revert "Use context from master approach"

This reverts commit 65d3584f83559554213cd9c726258d80775bc8bd.

* Remove group message changes, add comment

---------

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

server/channels/app/channel.go

@@ -1597,11 +1597,19 @@ func (a *App) AddUserToChannel(c request.CTX, user *model.User, channel *model.C
 		return nil, err
 	}
 
-	message := model.NewWebSocketEvent(model.WebsocketEventUserAdded, "", channel.Id, "", nil, "")
+	// We are sending separate websocket events to the user added and to the channel
+	// This is to get around potential cluster syncing issues where other nodes may not receive the most up to date channel members
+	// There is likely some issue syncing these that needs to be looked at, but this is the current fix.
+	message := model.NewWebSocketEvent(model.WebsocketEventUserAdded, "", channel.Id, "", map[string]bool{user.Id: true}, "")
 	message.Add("user_id", user.Id)
 	message.Add("team_id", channel.TeamId)
 	a.Publish(message)
 
+	userMessage := model.NewWebSocketEvent(model.WebsocketEventUserAdded, "", channel.Id, user.Id, nil, "")
+	userMessage.Add("user_id", user.Id)
+	userMessage.Add("team_id", channel.TeamId)
+	a.Publish(userMessage)
+
 	return newMember, nil
 }