LockFile.java

/*
 * Copyright (C) 2007, Robin Rosenberg <robin.rosenberg@dewire.com>
 * Copyright (C) 2006-2008, Shawn O. Pearce <spearce@spearce.org> and others
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Distribution License v. 1.0 which is available at
 * https://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

package org.eclipse.jgit.internal.storage.file;

import static org.eclipse.jgit.lib.Constants.LOCK_SUFFIX;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FilenameFilter;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import java.nio.file.Files;
import java.nio.file.StandardCopyOption;
import java.nio.file.attribute.FileTime;
import java.text.MessageFormat;
import java.time.Instant;
import java.util.concurrent.TimeUnit;

import org.eclipse.jgit.internal.JGitText;
import org.eclipse.jgit.lib.Constants;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.util.FS;
import org.eclipse.jgit.util.FS.LockToken;
import org.eclipse.jgit.util.FileUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Git style file locking and replacement.
 * <p>
 * To modify a ref file Git tries to use an atomic update approach: we write the
 * new data into a brand new file, then rename it in place over the old name.
 * This way we can just delete the temporary file if anything goes wrong, and
 * nothing has been damaged. To coordinate access from multiple processes at
 * once Git tries to atomically create the new temporary file under a well-known
 * name.
 */
public class LockFile {
    private static final Logger LOG = LoggerFactory.getLogger(LockFile.class);

    /**
     * Unlock the given file.
     * <p>
     * This method can be used for recovering from a thrown
     * {@link org.eclipse.jgit.errors.LockFailedException} . This method does
     * not validate that the lock is or is not currently held before attempting
     * to unlock it.
     *
     * @param file
     *            a {@link java.io.File} object.
     * @return true if unlocked, false if unlocking failed
     */
    public static boolean unlock(File file) {
        final File lockFile = getLockFile(file);
        final int flags = FileUtils.RETRY | FileUtils.SKIP_MISSING;
        try {
            FileUtils.delete(lockFile, flags);
        } catch (IOException ignored) {
            // Ignore and return whether lock file still exists
        }
        return !lockFile.exists();
    }

    /**
     * Get the lock file corresponding to the given file.
     *
     * @param file
     * @return lock file
     */
    static File getLockFile(File file) {
        return new File(file.getParentFile(),
                file.getName() + LOCK_SUFFIX);
    }

    /** Filter to skip over active lock files when listing a directory. */
    static final FilenameFilter FILTER = (File dir,
            String name) -> !name.endsWith(LOCK_SUFFIX);

    private final File ref;

    private final File lck;

    private boolean haveLck;

    FileOutputStream os;

    private boolean needSnapshot;

    boolean fsync;

    private FileSnapshot commitSnapshot;

    private LockToken token;

    /**
     * Create a new lock for any file.
     *
     * @param f
     *            the file that will be locked.
     */
    public LockFile(File f) {
        ref = f;
        lck = getLockFile(ref);
    }

    /**
     * Try to establish the lock.
     *
     * @return true if the lock is now held by the caller; false if it is held
     *         by someone else.
     * @throws java.io.IOException
     *             the temporary output file could not be created. The caller
     *             does not hold the lock.
     */
    public boolean lock() throws IOException {
        FileUtils.mkdirs(lck.getParentFile(), true);
        try {
            token = FS.DETECTED.createNewFileAtomic(lck);
        } catch (IOException e) {
            LOG.error(JGitText.get().failedCreateLockFile, lck, e);
            throw e;
        }
        if (token.isCreated()) {
            haveLck = true;
            try {
                os = new FileOutputStream(lck);
            } catch (IOException ioe) {
                unlock();
                throw ioe;
            }
        } else {
            closeToken();
        }
        return haveLck;
    }

    /**
     * Try to establish the lock for appending.
     *
     * @return true if the lock is now held by the caller; false if it is held
     *         by someone else.
     * @throws java.io.IOException
     *             the temporary output file could not be created. The caller
     *             does not hold the lock.
     */
    public boolean lockForAppend() throws IOException {
        if (!lock())
            return false;
        copyCurrentContent();
        return true;
    }

    /**
     * Copy the current file content into the temporary file.
     * <p>
     * This method saves the current file content by inserting it into the
     * temporary file, so that the caller can safely append rather than replace
     * the primary file.
     * <p>
     * This method does nothing if the current file does not exist, or exists
     * but is empty.
     *
     * @throws java.io.IOException
     *             the temporary file could not be written, or a read error
     *             occurred while reading from the current file. The lock is
     *             released before throwing the underlying IO exception to the
     *             caller.
     * @throws java.lang.RuntimeException
     *             the temporary file could not be written. The lock is released
     *             before throwing the underlying exception to the caller.
     */
    public void copyCurrentContent() throws IOException {
        requireLock();
        try {
            try (FileInputStream fis = new FileInputStream(ref)) {
                if (fsync) {
                    FileChannel in = fis.getChannel();
                    long pos = 0;
                    long cnt = in.size();
                    while (0 < cnt) {
                        long r = os.getChannel().transferFrom(in, pos, cnt);
                        pos += r;
                        cnt -= r;
                    }
                } else {
                    final byte[] buf = new byte[2048];
                    int r;
                    while ((r = fis.read(buf)) >= 0)
                        os.write(buf, 0, r);
                }
            }
        } catch (FileNotFoundException fnfe) {
            if (ref.exists()) {
                unlock();
                throw fnfe;
            }
            // Don't worry about a file that doesn't exist yet, it
            // conceptually has no current content to copy.
            //
        } catch (IOException | RuntimeException | Error ioe) {
            unlock();
            throw ioe;
        }
    }

    /**
     * Write an ObjectId and LF to the temporary file.
     *
     * @param id
     *            the id to store in the file. The id will be written in hex,
     *            followed by a sole LF.
     * @throws java.io.IOException
     *             the temporary file could not be written. The lock is released
     *             before throwing the underlying IO exception to the caller.
     * @throws java.lang.RuntimeException
     *             the temporary file could not be written. The lock is released
     *             before throwing the underlying exception to the caller.
     */
    public void write(ObjectId id) throws IOException {
        byte[] buf = new byte[Constants.OBJECT_ID_STRING_LENGTH + 1];
        id.copyTo(buf, 0);
        buf[Constants.OBJECT_ID_STRING_LENGTH] = '\n';
        write(buf);
    }

    /**
     * Write arbitrary data to the temporary file.
     *
     * @param content
     *            the bytes to store in the temporary file. No additional bytes
     *            are added, so if the file must end with an LF it must appear
     *            at the end of the byte array.
     * @throws java.io.IOException
     *             the temporary file could not be written. The lock is released
     *             before throwing the underlying IO exception to the caller.
     * @throws java.lang.RuntimeException
     *             the temporary file could not be written. The lock is released
     *             before throwing the underlying exception to the caller.
     */
    public void write(byte[] content) throws IOException {
        requireLock();
        try {
            if (fsync) {
                FileChannel fc = os.getChannel();
                ByteBuffer buf = ByteBuffer.wrap(content);
                while (0 < buf.remaining())
                    fc.write(buf);
                fc.force(true);
            } else {
                os.write(content);
            }
            os.close();
            os = null;
        } catch (IOException | RuntimeException | Error ioe) {
            unlock();
            throw ioe;
        }
    }

    /**
     * Obtain the direct output stream for this lock.
     * <p>
     * The stream may only be accessed once, and only after {@link #lock()} has
     * been successfully invoked and returned true. Callers must close the
     * stream prior to calling {@link #commit()} to commit the change.
     *
     * @return a stream to write to the new file. The stream is unbuffered.
     */
    public OutputStream getOutputStream() {
        requireLock();

        final OutputStream out;
        if (fsync)
            out = Channels.newOutputStream(os.getChannel());
        else
            out = os;

        return new OutputStream() {
            @Override
            public void write(byte[] b, int o, int n)
                    throws IOException {
                out.write(b, o, n);
            }

            @Override
            public void write(byte[] b) throws IOException {
                out.write(b);
            }

            @Override
            public void write(int b) throws IOException {
                out.write(b);
            }

            @Override
            public void close() throws IOException {
                try {
                    if (fsync)
                        os.getChannel().force(true);
                    out.close();
                    os = null;
                } catch (IOException | RuntimeException | Error ioe) {
                    unlock();
                    throw ioe;
                }
            }
        };
    }

    void requireLock() {
        if (os == null) {
            unlock();
            throw new IllegalStateException(MessageFormat.format(JGitText.get().lockOnNotHeld, ref));
        }
    }

    /**
     * Request that {@link #commit()} remember modification time.
     * <p>
     * This is an alias for {@code setNeedSnapshot(true)}.
     *
     * @param on
     *            true if the commit method must remember the modification time.
     */
    public void setNeedStatInformation(boolean on) {
        setNeedSnapshot(on);
    }

    /**
     * Request that {@link #commit()} remember the
     * {@link org.eclipse.jgit.internal.storage.file.FileSnapshot}.
     *
     * @param on
     *            true if the commit method must remember the FileSnapshot.
     */
    public void setNeedSnapshot(boolean on) {
        needSnapshot = on;
    }

    /**
     * Request that {@link #commit()} force dirty data to the drive.
     *
     * @param on
     *            true if dirty data should be forced to the drive.
     */
    public void setFSync(boolean on) {
        fsync = on;
    }

    /**
     * Wait until the lock file information differs from the old file.
     * <p>
     * This method tests the last modification date. If both are the same, this
     * method sleeps until it can force the new lock file's modification date to
     * be later than the target file.
     *
     * @throws java.lang.InterruptedException
     *             the thread was interrupted before the last modified date of
     *             the lock file was different from the last modified date of
     *             the target file.
     */
    public void waitForStatChange() throws InterruptedException {
        FileSnapshot o = FileSnapshot.save(ref);
        FileSnapshot n = FileSnapshot.save(lck);
        long fsTimeResolution = FS.getFileStoreAttributes(lck.toPath())
                .getFsTimestampResolution().toNanos();
        while (o.equals(n)) {
            TimeUnit.NANOSECONDS.sleep(fsTimeResolution);
            try {
                Files.setLastModifiedTime(lck.toPath(),
                        FileTime.from(Instant.now()));
            } catch (IOException e) {
                n.waitUntilNotRacy();
            }
            n = FileSnapshot.save(lck);
        }
    }

    /**
     * Commit this change and release the lock.
     * <p>
     * If this method fails (returns false) the lock is still released.
     *
     * @return true if the commit was successful and the file contains the new
     *         data; false if the commit failed and the file remains with the
     *         old data.
     * @throws java.lang.IllegalStateException
     *             the lock is not held.
     */
    public boolean commit() {
        if (os != null) {
            unlock();
            throw new IllegalStateException(MessageFormat.format(JGitText.get().lockOnNotClosed, ref));
        }

        saveStatInformation();
        try {
            FileUtils.rename(lck, ref, StandardCopyOption.ATOMIC_MOVE);
            haveLck = false;
            closeToken();
            return true;
        } catch (IOException e) {
            unlock();
            return false;
        }
    }

    private void closeToken() {
        if (token != null) {
            token.close();
            token = null;
        }
    }

    private void saveStatInformation() {
        if (needSnapshot)
            commitSnapshot = FileSnapshot.save(lck);
    }

    /**
     * Get the modification time of the output file when it was committed.
     *
     * @return modification time of the lock file right before we committed it.
     * @deprecated use {@link #getCommitLastModifiedInstant()} instead
     */
    @Deprecated
    public long getCommitLastModified() {
        return commitSnapshot.lastModified();
    }

    /**
     * Get the modification time of the output file when it was committed.
     *
     * @return modification time of the lock file right before we committed it.
     */
    public Instant getCommitLastModifiedInstant() {
        return commitSnapshot.lastModifiedInstant();
    }

    /**
     * Get the {@link FileSnapshot} just before commit.
     *
     * @return get the {@link FileSnapshot} just before commit.
     */
    public FileSnapshot getCommitSnapshot() {
        return commitSnapshot;
    }

    /**
     * Update the commit snapshot {@link #getCommitSnapshot()} before commit.
     * <p>
     * This may be necessary if you need time stamp before commit occurs, e.g
     * while writing the index.
     */
    public void createCommitSnapshot() {
        saveStatInformation();
    }

    /**
     * Unlock this file and abort this change.
     * <p>
     * The temporary file (if created) is deleted before returning.
     */
    public void unlock() {
        if (os != null) {
            try {
                os.close();
            } catch (IOException e) {
                LOG.error(MessageFormat
                        .format(JGitText.get().unlockLockFileFailed, lck), e);
            }
            os = null;
        }

        if (haveLck) {
            haveLck = false;
            try {
                FileUtils.delete(lck, FileUtils.RETRY);
            } catch (IOException e) {
                LOG.error(MessageFormat
                        .format(JGitText.get().unlockLockFileFailed, lck), e);
            } finally {
                closeToken();
            }
        }
    }

    /** {@inheritDoc} */
    @SuppressWarnings("nls")
    @Override
    public String toString() {
        return "LockFile[" + lck + ", haveLck=" + haveLck + "]";
    }
}