Merge pull request #2189 from Fox2Code/1.4.7-concurrent-fix

Fix ConcurrentModificationException caused by timers

src/main/java/dan200/computercraft/core/apis/OSAPI.java

@@ -203,9 +203,11 @@ public class OSAPI implements ILuaAPI {
      */
     @LuaFunction
     public final int startTimer(double timer) throws LuaException {
+        synchronized (timers) {
             timers.put(nextTimerToken, new Timer((int) Math.round(checkFinite(0, timer) / 0.05)));
             return nextTimerToken++;
         }
+    }
 
     /**
      * Cancels a timer previously started with startTimer. This will stop the
@@ -217,8 +219,10 @@ public class OSAPI implements ILuaAPI {
      */
     @LuaFunction
     public final void cancelTimer(int token) {
+        synchronized (timers) {
             timers.remove(token);
         }
+    }
 
     /**
      * Sets an alarm that will fire at the specified in-game time. When it