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/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */

package org.elasticsearch.index.translog;

import org.apache.lucene.store.AlreadyClosedException;
import org.elasticsearch.common.io.Channels;
import org.elasticsearch.core.IOUtils;
import org.elasticsearch.index.seqno.SequenceNumbers;

import java.io.Closeable;
import java.io.EOFException;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
import java.util.concurrent.atomic.AtomicBoolean;

import static org.elasticsearch.index.translog.Translog.getCommitCheckpointFileName;

/**
 * an immutable translog filereader
 */
public class TranslogReader extends BaseTranslogReader implements Closeable {
    protected final long length;
    private final int totalOperations;
    private final Checkpoint checkpoint;
    protected final AtomicBoolean closed = new AtomicBoolean(false);
    private long lastModifiedTime = -1;

    /**
     * Create a translog writer against the specified translog file channel.
     *
     * @param checkpoint the translog checkpoint
     * @param channel    the translog file channel to open a translog reader against
     * @param path       the path to the translog
     * @param header     the header of the translog file
     */
    TranslogReader(final Checkpoint checkpoint, final FileChannel channel, final Path path, final TranslogHeader header) {
        super(checkpoint.generation, channel, path, header);
        this.length = checkpoint.offset;
        this.totalOperations = checkpoint.numOps;
        this.checkpoint = checkpoint;
    }

    /**
     * Given a file channel, opens a {@link TranslogReader}, taking care of checking and validating the file header.
     *
     * @param channel the translog file channel
     * @param path the path to the translog
     * @param checkpoint the translog checkpoint
     * @param translogUUID the tranlog UUID
     * @return a new TranslogReader
     * @throws IOException if any of the file operations resulted in an I/O exception
     */
    public static TranslogReader open(final FileChannel channel, final Path path, final Checkpoint checkpoint, final String translogUUID)
        throws IOException {
        final TranslogHeader header = TranslogHeader.read(translogUUID, path, channel);
        return new TranslogReader(checkpoint, channel, path, header);
    }

    /**
     * Closes current reader and creates new one with new checkoint and same file channel
     */
    TranslogReader closeIntoTrimmedReader(long aboveSeqNo, ChannelFactory channelFactory) throws IOException {
        if (closed.compareAndSet(false, true)) {
            Closeable toCloseOnFailure = channel;
            final TranslogReader newReader;
            try {
                if (aboveSeqNo < checkpoint.trimmedAboveSeqNo
                    || aboveSeqNo < checkpoint.maxSeqNo && checkpoint.trimmedAboveSeqNo == SequenceNumbers.UNASSIGNED_SEQ_NO) {
                    final Path checkpointFile = path.getParent().resolve(getCommitCheckpointFileName(checkpoint.generation));
                    final Checkpoint newCheckpoint = new Checkpoint(
                        checkpoint.offset,
                        checkpoint.numOps,
                        checkpoint.generation,
                        checkpoint.minSeqNo,
                        checkpoint.maxSeqNo,
                        checkpoint.globalCheckpoint,
                        checkpoint.minTranslogGeneration,
                        aboveSeqNo
                    );
                    Checkpoint.write(channelFactory, checkpointFile, newCheckpoint, StandardOpenOption.WRITE);
                    IOUtils.fsync(checkpointFile.getParent(), true);

                    newReader = new TranslogReader(newCheckpoint, channel, path, header);
                } else {
                    newReader = new TranslogReader(checkpoint, channel, path, header);
                }
                toCloseOnFailure = null;
                return newReader;
            } finally {
                IOUtils.close(toCloseOnFailure);
            }
        } else {
            throw new AlreadyClosedException(toString() + " is already closed");
        }
    }

    public long sizeInBytes() {
        return length;
    }

    public int totalOperations() {
        return totalOperations;
    }

    @Override
    final Checkpoint getCheckpoint() {
        return checkpoint;
    }

    /**
     * reads an operation at the given position into the given buffer.
     */
    protected void readBytes(ByteBuffer buffer, long position) throws IOException {
        if (position >= length) {
            throw new EOFException("read requested past EOF. pos [" + position + "] end: [" + length + "]");
        }
        if (position < getFirstOperationOffset()) {
            throw new IOException(
                "read requested before position of first ops. pos [" + position + "] first op on: [" + getFirstOperationOffset() + "]"
            );
        }
        Channels.readFromFileChannelWithEofException(channel, position, buffer);
    }

    @Override
    public final void close() throws IOException {
        if (closed.compareAndSet(false, true)) {
            channel.close();
        }
    }

    protected final boolean isClosed() {
        return closed.get();
    }

    protected void ensureOpen() {
        if (isClosed()) {
            throw new AlreadyClosedException(toString() + " is already closed");
        }
    }

    @Override
    public long getLastModifiedTime() throws IOException {
        long modified = this.lastModifiedTime;
        if (modified == -1) {
            // cache the lastModifiedTime and return it forever, translogs are immutable
            modified = super.getLastModifiedTime();
            this.lastModifiedTime = modified;
        }
        return modified;
    }
}