// Copyright (C) 2015 The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package com.google.gerrit.server.util;

import com.google.gerrit.extensions.events.LifecycleListener;
import com.google.gerrit.extensions.systemstatus.ServerInformation;
import org.apache.log4j.AsyncAppender;
import org.apache.log4j.Layout;
import org.apache.log4j.LogManager;
import org.apache.log4j.Logger;

public abstract class PluginLogFile implements LifecycleListener {

  private final SystemLog systemLog;
  private final ServerInformation serverInfo;
  private final String logName;
  private final Layout layout;

  public PluginLogFile(
      SystemLog systemLog, ServerInformation serverInfo, String logName, Layout layout) {
    this.systemLog = systemLog;
    this.serverInfo = serverInfo;
    this.logName = logName;
    this.layout = layout;
  }

  @Override
  public void start() {
    AsyncAppender asyncAppender = systemLog.createAsyncAppender(logName, layout, true, true);
    Logger logger = LogManager.getLogger(logName);
    logger.removeAppender(logName);
    logger.addAppender(asyncAppender);
    logger.setAdditivity(false);
  }

  @Override
  public void stop() {
    // stop is called when plugin is unloaded or when the server shutdown.
    // Only clean up when the server is shutting down to prevent issue when a
    // plugin is reloaded. The issue is that gerrit load the new plugin and then
    // unload the old one so because loggers are static, the unload of the old
    // plugin would remove the appenders just created by the new plugin.
    if (serverInfo.getState() == ServerInformation.State.SHUTDOWN) {
      LogManager.getLogger(logName).removeAllAppenders();
    }
  }
}

@@ -1555,40 +1555,136 @@
     
+if (
 logger.
-removeAppender(logName);%0A
+getAppender(logName) == null) %7B%0A      synchronized (this) %7B%0A        if (logger.getAppender(logName) == null) %7B%0A      
     
@@ -1718,16 +1718,40 @@
 ender);%0A
+        %7D%0A      %7D%0A    %7D%0A
     logg

Make start method in PluginLogFile thread safe