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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.impl.api.index;

import java.io.IOException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;

import org.neo4j.io.pagecache.context.CursorContext;
import org.neo4j.kernel.api.exceptions.index.IndexEntryConflictException;
import org.neo4j.kernel.api.index.IndexUpdater;
import org.neo4j.kernel.impl.api.index.updater.DelegatingIndexUpdater;
import org.neo4j.util.VisibleForTesting;

/**
 * {@link IndexProxy} layer that enforces the dynamic contract of {@link IndexProxy} (cf. Test)
 *
 * @see org.neo4j.kernel.impl.api.index.IndexProxy
 */
class ContractCheckingIndexProxy extends DelegatingIndexProxy
{
    /**
     * State machine for {@link IndexProxy proxies}
     *
     * The logic of {@link ContractCheckingIndexProxy} hinges on the fact that all states
     * are always entered and checked in this order (States may be skipped though):
     *
     * INIT > STARTING > STARTED > CLOSED
     *
     * Valid state transitions are:
     *
     * INIT -[:start]-> STARTING -[:implicit]-> STARTED -[:close|drop]-> CLOSED
     * INIT -[:close] -> CLOSED
     *
     * Additionally, {@link ContractCheckingIndexProxy} keeps track of the number of open
     * calls that started in STARTED state and are still running.  This allows us
     * to prevent calls to close() or drop() to go through while there are pending
     * commits.
     **/
    private enum State
    {
        INIT, STARTING, STARTED, CLOSED
    }

    private final AtomicReference state;
    private final AtomicInteger openCalls;

    ContractCheckingIndexProxy( IndexProxy delegate )
    {
        super( delegate );
        this.state = new AtomicReference<>( State.INIT );
        this.openCalls = new AtomicInteger( 0 );
    }

    @Override
    public void start()
    {
        if ( state.compareAndSet( State.INIT, State.STARTING ) )
        {
            try
            {
                super.start();
            }
            finally
            {
                this.state.set( State.STARTED );
            }
        }
        else
        {
            throw new IllegalStateException( "An IndexProxy can only be started once" );
        }
    }

    @Override
    public IndexUpdater newUpdater( IndexUpdateMode mode, CursorContext cursorContext )
    {
        if ( IndexUpdateMode.ONLINE == mode )
        {
            if ( tryOpenCall() )
            {
                try
                {
                    return new DelegatingIndexUpdater( super.newUpdater( mode, cursorContext ) )
                    {
                        @Override
                        public void close() throws IndexEntryConflictException
                        {
                            try
                            {
                                delegate.close();
                            }
                            finally
                            {
                                closeCall();
                            }
                        }
                    };
                }
                catch ( Throwable e )
                {
                    closeCall();
                    throw e;
                }
            }
            throw new IllegalStateException( "Cannot create new updater when index state is " + state.get() );
        }
        else
        {
            return super.newUpdater( mode, cursorContext );
        }
    }

    @Override
    public void force( CursorContext cursorContext ) throws IOException
    {
        if ( tryOpenCall() )
        {
            try
            {
                super.force( cursorContext );
            }
            finally
            {
                closeCall();
            }
        }
    }

    @Override
    public void drop()
    {
        if ( state.compareAndSet( State.INIT, State.CLOSED ) )
        {
            super.drop();
            return;
        }

        if ( State.STARTING == state.get() )
        {
            throw new IllegalStateException( "Concurrent drop while creating index" );
        }

        if ( state.compareAndSet( State.STARTED, State.CLOSED ) )
        {
            waitOpenCallsToClose();
            super.drop();
            return;
        }

        throw new IllegalStateException( "IndexProxy already closed" );
    }

    @Override
    public void close( CursorContext cursorContext ) throws IOException
    {
        if ( state.compareAndSet( State.INIT, State.CLOSED ) )
        {
            super.close( cursorContext );
            return;
        }

        if ( state.compareAndSet( State.STARTING, State.CLOSED ) )
        {
            throw new IllegalStateException( "Concurrent close while creating index" );
        }

        if ( state.compareAndSet( State.STARTED, State.CLOSED ) )
        {
            waitOpenCallsToClose();
            super.close( cursorContext );
            return;
        }

        throw new IllegalStateException( "IndexProxy already closed" );
    }

    private void waitOpenCallsToClose()
    {
        while ( openCalls.intValue() > 0 )
        {
            LockSupport.parkNanos( TimeUnit.MILLISECONDS.toNanos( 10 ) );
        }
    }

    @VisibleForTesting
    int getOpenCalls()
    {
        return openCalls.intValue();
    }

    private boolean tryOpenCall()
    {
        // do not open call unless we are in STARTED
        if ( State.STARTED == state.get() )
        {
            // increment openCalls for closers to see
            openCalls.incrementAndGet();
            if ( State.STARTED == state.get() )
            {
                return true;
            }
            openCalls.decrementAndGet();
        }
        return false;
    }

    private void closeCall()
    {
        // rollback once the call finished or failed
        openCalls.decrementAndGet();
    }
}