/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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 org.apache.jackrabbit.core.util;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Properties;
import java.util.UUID;
import javax.jcr.RepositoryException;
/**
* The file lock is used to ensure a resource is only open once at any time.
* It uses a cooperative locking protocol.
*/
public class CooperativeFileLock implements RepositoryLockMechanism {
/**
* Logger instance.
*/
private static final Logger LOG =
LoggerFactory.getLogger(CooperativeFileLock.class);
private static final String MAGIC = "CooperativeFileLock";
private static final String FILE_NAME = "lock.properties";
private static final int MAX_FILE_RETRY = 16;
private static final int SLEEP_GAP = 25;
private static final int TIME_GRANULARITY = 2000;
private static final int LOCK_SLEEP = 1000;
private String fileName;
private long lastWrite;
private Properties properties;
private boolean locked;
private volatile boolean stop;
private Thread watchdog;
/**
* Create a new file locking object using the given file name.
*
* @param path basic path to append {@link #FILE_NAME} to.
*/
public void init(String path) {
this.fileName = path + File.separatorChar + FILE_NAME;
}
/**
* Lock the directory if possible.
* This method will also start a background watchdog thread.
* A file may only be locked once.
*
* @throws RepositoryException if locking was not successful
*/
public synchronized void acquire() throws RepositoryException {
if (locked) {
throw new RepositoryException("Already locked " + fileName);
}
stop = false;
lockFile();
locked = true;
}
/**
* Unlock the directory.
* The watchdog thread is stopped.
* This method does nothing if the file is already unlocked.
*/
public synchronized void release() {
if (!locked) {
return;
}
try {
stop = true;
if (fileName != null) {
if (load().equals(properties)) {
delete(fileName);
}
}
} catch (Exception e) {
LOG.warn("Error unlocking " + fileName, e);
} finally {
locked = false;
}
try {
if (watchdog != null) {
watchdog.interrupt();
}
} catch (Exception e) {
LOG.debug("Error stopping watchdog " + fileName, e);
}
}
/**
* Save the properties file.
*/
private void save() throws RepositoryException {
try {
OutputStream out = new FileOutputStream(fileName);
try {
properties.store(out, MAGIC);
} finally {
out.close();
}
lastWrite = new File(fileName).lastModified();
if (LOG.isDebugEnabled()) {
LOG.debug("Save " + properties);
}
} catch (IOException e) {
throw getException(e);
}
}
/**
* Load the properties file.
*
* @return the properties object
*/
private Properties load() throws RepositoryException {
try {
Properties p2 = new Properties();
InputStream in = new FileInputStream(fileName);
try {
p2.load(in);
} finally {
in.close();
}
if (LOG.isDebugEnabled()) {
LOG.debug("Load " + p2);
}
return p2;
} catch (IOException e) {
throw getException(e);
}
}
/**
* Wait until the file is old (not modified for a certain time).
*/
private void waitUntilOld() throws RepositoryException {
for (int i = 0; i < TIME_GRANULARITY / SLEEP_GAP; i++) {
File f = new File(fileName);
long last = f.lastModified();
long dist = System.currentTimeMillis() - last;
if (dist < -TIME_GRANULARITY) {
// lock file modified in the future -
// wait for a bit longer than usual
try {
Thread.sleep(2 * LOCK_SLEEP);
} catch (Exception e) {
LOG.debug("Sleep", e);
}
return;
} else if (dist > TIME_GRANULARITY) {
return;
}
try {
Thread.sleep(SLEEP_GAP);
} catch (Exception e) {
LOG.debug("Sleep", e);
}
}
throw error("Lock file recently modified");
}
/**
* Lock the file.
*/
private void lockFile() throws RepositoryException {
properties = new Properties();
UUID uuid = UUID.randomUUID();
properties.setProperty("id", uuid.toString());
if (!createNewFile(fileName)) {
waitUntilOld();
save();
// wait twice the watchdog sleep time
for (int i = 0; i < 8; i++) {
sleep(LOCK_SLEEP / 4);
if (!load().equals(properties)) {
throw error("Locked by another process");
}
}
delete(fileName);
if (!createNewFile(fileName)) {
throw error("Another process was faster");
}
}
save();
sleep(SLEEP_GAP);
if (!load().equals(properties)) {
stop = true;
throw error("Concurrent update");
}
watchdog = new Thread(new Runnable() {
public void run() {
try {
while (!stop) {
// debug("Watchdog check");
try {
File f = new File(fileName);
if (!f.exists() || f.lastModified() != lastWrite) {
save();
}
Thread.sleep(LOCK_SLEEP);
} catch (OutOfMemoryError e) {
// ignore
} catch (InterruptedException e) {
// ignore
} catch (NullPointerException e) {
// ignore
} catch (Exception e) {
LOG.debug("Watchdog", e);
}
}
} catch (Exception e) {
LOG.debug("Watchdog", e);
}
LOG.debug("Watchdog end");
}
});
watchdog.setName(MAGIC + " Watchdog " + fileName);
watchdog.setDaemon(true);
watchdog.setPriority(Thread.MAX_PRIORITY - 1);
watchdog.start();
}
private RepositoryException getException(Throwable t) {
return new RepositoryException("Internal error in file lock " + fileName, t);
}
private RepositoryException error(String reason) {
return new RepositoryException("Error locking " + fileName + ", reason: " + reason);
}
private void sleep(int time) throws RepositoryException {
try {
Thread.sleep(time);
} catch (InterruptedException e) {
throw getException(e);
}
}
/**
* Create a new file, and retry if this doesn't work.
* If it still doesn't work after some time, this method returns false.
*
* @param fileName the name of the file to create
* @return if the file was created
*/
private static boolean createNewFile(String fileName) {
File file = new File(fileName);
for (int i = 0; i < MAX_FILE_RETRY; i++) {
try {
return file.createNewFile();
} catch (IOException e) {
// 'access denied' is really a concurrent access problem
wait(i);
}
}
return false;
}
/**
* Delete a file, and retry if this doesn't work.
* If it still doesn't work after some time, an exception is thrown.
*
* @param fileName the name of the file to delete
* @throws RepositoryException if the file could not be deleted
*/
private static void delete(String fileName) throws RepositoryException {
File file = new File(fileName);
if (file.exists()) {
for (int i = 0; i < MAX_FILE_RETRY; i++) {
if (LOG.isDebugEnabled()) {
LOG.debug("Deleting " + fileName);
}
boolean ok = file.delete();
if (ok) {
return;
}
wait(i);
}
throw new RepositoryException("Could not delete file " + fileName);
}
}
private static void wait(int i) {
if (i > 8) {
System.gc();
}
try {
// sleep at most 256 ms
long sleep = Math.min(256, i * i);
Thread.sleep(sleep);
} catch (InterruptedException e) {
// ignore
}
}
}
|