Implement deep copy method for model.WebSocketEvent (#18977)

app/post.go

@@ -735,7 +735,9 @@ func (a *App) publishWebsocketEventForPermalinkPost(post *model.Post, message *m
 			}
 			return false, err
 		}
-		messageCopy := message.Copy()
+		// Using DeepCopy here to avoid a race condition
+		// between publishing the event and setting the "post" data value below.
+		messageCopy := message.DeepCopy()
 		broadcastCopy := messageCopy.GetBroadcast()
 		broadcastCopy.UserId = cm.UserId
 		messageCopy.SetBroadcast(broadcastCopy)