Fix flakiness in LoggerTest (#885)

If any other threads logs a message while we iterate over the events it can throw a ConcurrentModificationException.

Create our own implementation of ListAppender that uses the synchronized keyword to protect reading/writing to the backing list with a mutex. Create a copy of the list while holding to the lock to support iterating over it.

src/test/java/com/uber/cadence/workflow/LoggerTest.java

@@ -22,7 +22,7 @@
 
 import ch.qos.logback.classic.LoggerContext;
 import ch.qos.logback.classic.spi.ILoggingEvent;
-import ch.qos.logback.core.read.ListAppender;
+import ch.qos.logback.core.AppenderBase;
 import com.uber.cadence.client.WorkflowClient;
 import com.uber.cadence.client.WorkflowClientOptions;
 import com.uber.cadence.client.WorkflowOptions;
@@ -31,14 +31,16 @@
 import com.uber.cadence.testing.TestWorkflowEnvironment;
 import com.uber.cadence.worker.Worker;
 import java.time.Duration;
+import java.util.ArrayList;
+import java.util.List;
 import java.util.UUID;
 import org.junit.Test;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 public class LoggerTest {
 
-  private static final ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
+  private static final ThreadSafeListAppender listAppender = new ThreadSafeListAppender();
 
   static {
     LoggerContext context = (LoggerContext) LoggerFactory.getILoggerFactory();
@@ -118,7 +120,7 @@ public void testWorkflowLogger() {
 
   private int matchingLines(String message) {
     int i = 0;
-    for (ILoggingEvent event : listAppender.list) {
+    for (ILoggingEvent event : listAppender.getEvents()) {
       if (event.getFormattedMessage().contains(message)) {
         assertTrue(event.getMDCPropertyMap().containsKey(LoggerTag.WORKFLOW_ID));
         assertTrue(event.getMDCPropertyMap().containsKey(LoggerTag.WORKFLOW_TYPE));
@@ -129,4 +131,18 @@ private int matchingLines(String message) {
     }
     return i;
   }
+
+  private static class ThreadSafeListAppender extends AppenderBase<ILoggingEvent> {
+
+    private final List<ILoggingEvent> events = new ArrayList<>();
+
+    @Override
+    protected synchronized void append(ILoggingEvent event) {
+      events.add(event);
+    }
+
+    public synchronized List<ILoggingEvent> getEvents() {
+      return new ArrayList<>(events);
+    }
+  }
 }