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Neo4j kernel is a lightweight, embedded Java database designed to store data structured as graphs rather than tables. For more information, see http://neo4j.org.

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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [http://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.kernel.internal.locker;

import java.io.IOException;
import java.nio.channels.FileLock;
import java.nio.file.Path;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

import org.neo4j.io.fs.FileSystemAbstraction;

/**
 * File locker that guarantee that only single channel ever will be opened and then closed
 * file to prevent cases where lock will be released when any of the opened channels for file will be released as
 * described in {@link FileLock} javadoc:
 * 
 * 
 * On some systems, closing a channel releases all locks held by the Java virtual machine on the underlying
 * file regardless of whether the locks were acquired via that channel or via another channel open on the same file.
 * It is strongly recommended that, within a program, a unique channel be used to acquire all locks on any given file.
 * 
 * 
 *
 * The guarantee is achieved by tracking all locked files over all instances of {@link GlobalFileLocker}.
 *
 * Class guarantee visibility of locked files over multiple thread but do not guarantee atomicity of operations.
 */
class GlobalFileLocker extends Locker
{
    private static final Set lockedFiles = ConcurrentHashMap.newKeySet();

    GlobalFileLocker( FileSystemAbstraction fileSystemAbstraction, Path lockFile )
    {
        super( fileSystemAbstraction, lockFile );
    }

    @Override
    public void checkLock()
    {
        super.checkLock();
        lockedFiles.add( lockFile() );
    }

    @Override
    protected boolean haveLockAlready()
    {
        if ( lockedFiles.contains( lockFile() ) )
        {
            if ( lockFileLock != null )
            {
                return true;
            }
            throw unableToObtainLockException();
        }
        return false;
    }

    @Override
    protected void releaseLock() throws IOException
    {
        lockedFiles.remove( lockFile() );
        super.releaseLock();
    }
}