Fixed issue with help thread creation actions being send too fast (#724)

* delayed by 5 seconds

Author: Zabuzard

application/src/main/java/org/togetherjava/tjbot/commands/help/HelpThreadCreatedListener.java

@@ -9,13 +9,17 @@
 import net.dv8tion.jda.api.events.channel.ChannelCreateEvent;
 import net.dv8tion.jda.api.hooks.ListenerAdapter;
 import net.dv8tion.jda.api.requests.RestAction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import org.togetherjava.tjbot.commands.EventReceiver;
 
 import javax.annotation.Nonnull;
 
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -25,6 +29,9 @@
  * user.
  */
 public final class HelpThreadCreatedListener extends ListenerAdapter implements EventReceiver {
+    private static final Logger logger = LoggerFactory.getLogger(HelpThreadCreatedListener.class);
+    private static final ScheduledExecutorService SERVICE = Executors.newScheduledThreadPool(2);
+
     private final HelpSystemHelper helper;
     private final Cache<Long, Instant> threadIdToCreatedAtCache = Caffeine.newBuilder()
         .maximumSize(1_000)
@@ -69,7 +76,20 @@ private boolean wasThreadAlreadyHandled(long threadChannelId) {
     private void handleHelpThreadCreated(ThreadChannel threadChannel) {
         helper.writeHelpThreadToDatabase(threadChannel.getOwnerIdLong(), threadChannel);
 
-        createMessages(threadChannel).queue();
+        Runnable createMessages = () -> {
+            try {
+                createMessages(threadChannel).queue();
+            } catch (Exception e) {
+                logger.error(
+                        "Unknown error while creating messages after help-thread ({}) creation",
+                        threadChannel.getId(), e);
+            }
+        };
+
+        // The creation is delayed, because otherwise it could be too fast and be executed
+        // after Discord created the thread, but before Discord send OPs initial message.
+        // Sending messages at that moment is not allowed.
+        SERVICE.schedule(createMessages, 5, TimeUnit.SECONDS);
     }
 
     private RestAction<Message> createMessages(ThreadChannel threadChannel) {