[MM-59911] Force individual users to receive the user_added and `gr…

…oup_added` events instead of relying on the channel membership (mattermost#27846)

* [MM-59911] Force individual users to receive the `user_added` and `group_added` events instead of relying on the channel membership

* Fix redundant JSON serializing

server/channels/app/channel.go

@@ -497,13 +497,14 @@ func (a *App) CreateGroupChannel(c request.CTX, userIDs []string, creatorId stri
 		return nil, err
 	}
 
+	jsonIDs := model.ArrayToJSON(userIDs)
 	for _, userID := range userIDs {
 		a.InvalidateCacheForUser(userID)
-	}
 
-	message := model.NewWebSocketEvent(model.WebsocketEventGroupAdded, "", channel.Id, "", nil, "")
-	message.Add("teammate_ids", model.ArrayToJSON(userIDs))
-	a.Publish(message)
+		message := model.NewWebSocketEvent(model.WebsocketEventGroupAdded, "", channel.Id, userID, nil, "")
+		message.Add("teammate_ids", jsonIDs)
+		a.Publish(message)
+	}
 
 	return channel, nil
 }
@@ -1597,11 +1598,18 @@ 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
+	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
 }