[FLINK-2392][yarn tests] Address concurrent modification exception

flink-yarn-tests/src/main/java/org/apache/flink/yarn/UtilsTest.java

@@ -76,22 +76,27 @@ public static LoggingEvent getEventContainingString(String expected) {
  throw new NullPointerException("Initialize test appender first");
  }
  LoggingEvent found = null;
- for(LoggingEvent event: testAppender.events) {
- if(event.getMessage().toString().contains(expected)) {
- found = event;
- break;
+ // make sure that different threads are not logging while the logs are checked
+ synchronized (testAppender.events) {
+ for (LoggingEvent event : testAppender.events) {
+ if (event.getMessage().toString().contains(expected)) {
+ found = event;
+ break;
+ }
  }
  }
  return found;
  }
 
  public static class TestAppender extends AppenderSkeleton {
- public List<LoggingEvent> events = new ArrayList<LoggingEvent>();
+ public final List<LoggingEvent> events = new ArrayList<>();
  public void close() {}
  public boolean requiresLayout() {return false;}
  @Override
  protected void append(LoggingEvent event) {
- events.add(event);
+ synchronized (events){
+ events.add(event);
+ }
  }
  }
 }