ThreadDeath problem

Jump to navigation Jump to search

Here is my current best guess.

Note that Beaming's stacktrace includes:

at net.sf.robocode.battle.peer.RobotPeer.checkSkippedTurn(

Therefore, RobotPeer.checkSkippedTurn() called BasicRobotProxy.forceStopThread()

The source code of RobotPeer.checkSkippedTurn() shows one and only one possible call to BasicRobotProxy.forceStopThread():

public void checkSkippedTurn() {
    // ...
    if (/* snip */) {
        println("SYSTEM: " + getShortName() + " has not performed any actions in a reasonable amount of time.");
        println("SYSTEM: No score will be generated.");
        /* snip */

Therefore, control flow must have passed through this path.

Note that this code sequence includes a println("SYSTEM: <Bot> has not performed any actions...") call, which must have been executed, yet no such SYSTEM message is present in Beaming's log.

The source code of the println() method:

public void println(String s) {
    synchronized (proxyText) {

Note the following:

  1. Robocode is multithreaded. Heavy multithreading causes race conditions.
  2. The variable battleText is a StringBuilder, which is not thread-safe (unlike a StringBuffer), which is.
  3. The code modifies battleText, but only synchronizes against proxyText.

Therefore, EvBotNG is skipping turns, but no log messages (from either Robocode or the bot) are printed, because they are silently dropped due to race conditions in the println() method. The only thing you ultimately see is the ThreadDeath exception caused by Thread.stop() forcibly terminating the thread.

Presumably, this occurs because of unlucky interaction between RobotPeer.println() and RobotPeer.readOutText().

readOutText()                                 | println()
============================================= | ===============================
final String robotText =                      |
battleText.toString() + proxyText.toString(); |
                                              | battleText.append(s);
                                              | battleText.append("\n");
battleText.setLength(0);                      |
proxyText.setLength(0);                       |
return robotText;                             |

Therefore, the solution is to add synchronized(battleText) { guards around readOutText() and println().

Or maybe I'm way off track.

MultiplyByZer0 (talk)21:46, 5 September 2017

I think you're on the right track, it definitely feels like race conditions to me. However in this case you're wrong - both code blocks (println and readOutText) are synchronized with the same object, so they can't run at the same time. It isn't important which one they are synchronized with, it could even be a different object. See the sharedLock example here.

I think a good place to start looking is in the 'fixes' to the skipped turn behavior, which were documented here, as well as a very busy discussion on the wiki.

Skilgannon (talk)22:37, 5 September 2017

Well, println() is definitely broken somehow. There is no way that checkSkippedTurn() could have called forceStopThread() (as is shown in the stacktrace), without going through println() first, yet the print message never made its way to the console (as is shown in Beaming's log).

battleText is also cleared in initializeRound() and cleanup(), both of which have no synchronization guards at all.

I think skipped turns are working as they should, they're just not being reported.

MultiplyByZer0 (talk)22:59, 5 September 2017

Good catch. I've added synchronization guards on the initializeRound() and cleanup() and now I get the SYSTEM: Neuromancer 5.4 has not performed any actions in a reasonable amount of time. printed inside the correct round. The ThreadDeath is still printed in the round after, but I can live with that I guess.

Now for the next question, why is this even happening?

Skilgannon (talk)23:18, 5 September 2017

Would you please share the patch? My multithreading skills are not that good.

Beaming (talk)01:52, 6 September 2017

You do not have permission to edit this page, for the following reasons:

  • The action you have requested is limited to users in the group: Users.
  • You must confirm your email address before editing pages. Please set and validate your email address through your user preferences.

You can view and copy the source of this page.

Return to Thread:Talk:RoboRumble/ThreadDeath problem/reply (27).

See it now. Thanks.

Beaming (talk)03:28, 6 September 2017

@ MultiplyByZer0, excellent analysis. I would never spotted it.

Beaming (talk)01:53, 6 September 2017

To add to mysteries. Why I do not see dramatic drop in scores if I run Voidious' RoboRunner? I see missing SYSTEM messages in logs but it does not affect scores the same drastic way as with the literumble client. I also see no ThreadDeath messages. I am guessing RoboRunner catches those somehow and process the score differently.

Beaming (talk)02:38, 6 September 2017