org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java - jgit - Git at Google

 /*
  * Copyright (C) 2010, Google Inc.
  * and other copyright owners as documented in the project's IP log.
  *
  * This program and the accompanying materials are made available
  * under the terms of the Eclipse Distribution License v1.0 which
  * accompanies this distribution, is reproduced below, and is
  * available at http://www.eclipse.org/org/documents/edl-v10.php
  *
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or
  * without modification, are permitted provided that the following
  * conditions are met:
  *
  * - Redistributions of source code must retain the above copyright
  *   notice, this list of conditions and the following disclaimer.
  *
  * - Redistributions in binary form must reproduce the above
  *   copyright notice, this list of conditions and the following
  *   disclaimer in the documentation and/or other materials provided
  *   with the distribution.
  *
  * - Neither the name of the Eclipse Foundation, Inc. nor the
  *   names of its contributors may be used to endorse or promote
  *   products derived from this software without specific prior
  *   written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
  * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
  * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

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

 import static org.eclipse.jgit.util.FS.FileStoreAttributes.FALLBACK_FILESTORE_ATTRIBUTES;
 import static org.eclipse.jgit.util.FS.FileStoreAttributes.FALLBACK_TIMESTAMP_RESOLUTION;
 import java.io.File;
 import java.io.IOException;
 import java.nio.file.attribute.BasicFileAttributes;
 import java.time.Duration;
 import java.time.Instant;
 import java.time.ZoneId;
 import java.time.format.DateTimeFormatter;
 import java.util.Locale;
 import java.util.Objects;
 import java.util.concurrent.TimeUnit;

 import org.eclipse.jgit.annotations.NonNull;
 import org.eclipse.jgit.util.FS;
 import org.eclipse.jgit.util.FS.FileStoreAttributes;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;

 /**
  * Caches when a file was last read, making it possible to detect future edits.
  * <p>
  * This object tracks the last modified time of a file. Later during an
  * invocation of {@link #isModified(File)} the object will return true if the
  * file may have been modified and should be re-read from disk.
  * <p>
  * A snapshot does not "live update" when the underlying filesystem changes.
  * Callers must poll for updates by periodically invoking
  * {@link #isModified(File)}.
  * <p>
  * To work around the "racy git" problem (where a file may be modified multiple
  * times within the granularity of the filesystem modification clock) this class
  * may return true from isModified(File) if the last modification time of the
  * file is less than 3 seconds ago.
  */
 public class FileSnapshot {
 	private static final Logger LOG = LoggerFactory
 			.getLogger(FileSnapshot.class);
 	/**
 	 * An unknown file size.
 	 *
 	 * This value is used when a comparison needs to happen purely on the lastUpdate.
 	 */
 	public static final long UNKNOWN_SIZE = -1;

 	private static final Instant UNKNOWN_TIME = Instant.ofEpochMilli(-1);

 	private static final Object MISSING_FILEKEY = new Object();

 	private static final DateTimeFormatter dateFmt = DateTimeFormatter
 			.ofPattern("yyyy-MM-dd HH:mm:ss.nnnnnnnnn") //$NON-NLS-1$
 			.withLocale(Locale.getDefault()).withZone(ZoneId.systemDefault());

 	/**
 	 * A FileSnapshot that is considered to always be modified.
 	 * <p>
 	 * This instance is useful for application code that wants to lazily read a
 	 * file, but only after {@link #isModified(File)} gets invoked. The returned
 	 * snapshot contains only invalid status information.
 	 */
 	public static final FileSnapshot DIRTY = new FileSnapshot(UNKNOWN_TIME,
 			UNKNOWN_TIME, UNKNOWN_SIZE, Duration.ZERO, MISSING_FILEKEY);

 	/**
 	 * A FileSnapshot that is clean if the file does not exist.
 	 * <p>
 	 * This instance is useful if the application wants to consider a missing
 	 * file to be clean. {@link #isModified(File)} will return false if the file
 	 * path does not exist.
 	 */
 	public static final FileSnapshot MISSING_FILE = new FileSnapshot(
 			Instant.EPOCH, Instant.EPOCH, 0, Duration.ZERO, MISSING_FILEKEY) {
 		@Override
 		public boolean isModified(File path) {
 			return FS.DETECTED.exists(path);
 		}
 	};

 	/**
 	 * Record a snapshot for a specific file path.
 	 * <p>
 	 * This method should be invoked before the file is accessed.
 	 *
 	 * @param path
 	 *            the path to later remember. The path's current status
 	 *            information is saved.
 	 * @return the snapshot.
 	 */
 	public static FileSnapshot save(File path) {
 		return new FileSnapshot(path);
 	}

 	/**
 	 * Record a snapshot for a specific file path without using config file to
 	 * get filesystem timestamp resolution.
 	 * <p>
 	 * This method should be invoked before the file is accessed. It is used by
 	 * FileBasedConfig to avoid endless recursion.
 	 *
 	 * @param path
 	 *            the path to later remember. The path's current status
 	 *            information is saved.
 	 * @return the snapshot.
 	 */
 	public static FileSnapshot saveNoConfig(File path) {
 		return new FileSnapshot(path, false);
 	}

 	private static Object getFileKey(BasicFileAttributes fileAttributes) {
 		Object fileKey = fileAttributes.fileKey();
 		return fileKey == null ? MISSING_FILEKEY : fileKey;
 	}

 	/**
 	 * Record a snapshot for a file for which the last modification time is
 	 * already known.
 	 * <p>
 	 * This method should be invoked before the file is accessed.
 	 * <p>
 	 * Note that this method cannot rely on measuring file timestamp resolution
 	 * to avoid racy git issues caused by finite file timestamp resolution since
 	 * it's unknown in which filesystem the file is located. Hence the worst
 	 * case fallback for timestamp resolution is used.
 	 *
 	 * @param modified
 	 *            the last modification time of the file
 	 * @return the snapshot.
 	 * @deprecated use {@link #save(Instant)} instead.
 	 */
 	@Deprecated
 	public static FileSnapshot save(long modified) {
 		final Instant read = Instant.now();
 		return new FileSnapshot(read, Instant.ofEpochMilli(modified),
 				UNKNOWN_SIZE, FALLBACK_TIMESTAMP_RESOLUTION, MISSING_FILEKEY);
 	}

 	/**
 	 * Record a snapshot for a file for which the last modification time is
 	 * already known.
 	 * <p>
 	 * This method should be invoked before the file is accessed.
 	 * <p>
 	 * Note that this method cannot rely on measuring file timestamp resolution
 	 * to avoid racy git issues caused by finite file timestamp resolution since
 	 * it's unknown in which filesystem the file is located. Hence the worst
 	 * case fallback for timestamp resolution is used.
 	 *
 	 * @param modified
 	 *            the last modification time of the file
 	 * @return the snapshot.
 	 */
 	public static FileSnapshot save(Instant modified) {
 		final Instant read = Instant.now();
 		return new FileSnapshot(read, modified, UNKNOWN_SIZE,
 				FALLBACK_TIMESTAMP_RESOLUTION, MISSING_FILEKEY);
 	}

 	/** Last observed modification time of the path. */
 	private final Instant lastModified;

 	/** Last wall-clock time the path was read. */
 	private volatile Instant lastRead;

 	/** True once {@link #lastRead} is far later than {@link #lastModified}. */
 	private boolean cannotBeRacilyClean;

 	/** Underlying file-system size in bytes.
 	 *
 	 * When set to {@link #UNKNOWN_SIZE} the size is not considered for modification checks. */
 	private final long size;

 	/** measured FileStore attributes */
 	private FileStoreAttributes fileStoreAttributeCache;

 	/**
 	 * Object that uniquely identifies the given file, or {@code
 	 * null} if a file key is not available
 	 */
 	private final Object fileKey;

 	private final File file;

 	/**
 	 * Record a snapshot for a specific file path.
 	 * <p>
 	 * This method should be invoked before the file is accessed.
 	 *
 	 * @param file
 	 *            the path to remember meta data for. The path's current status
 	 *            information is saved.
 	 */
 	protected FileSnapshot(File file) {
 		this(file, true);
 	}

 	/**
 	 * Record a snapshot for a specific file path.
 	 * <p>
 	 * This method should be invoked before the file is accessed.
 	 *
 	 * @param file
 	 *            the path to remember meta data for. The path's current status
 	 *            information is saved.
 	 * @param useConfig
 	 *            if {@code true} read filesystem time resolution from
 	 *            configuration file otherwise use fallback resolution
 	 */
 	protected FileSnapshot(File file, boolean useConfig) {
 		this.file = file;
 		this.lastRead = Instant.now();
 		this.fileStoreAttributeCache = useConfig
 				? FS.getFileStoreAttributes(file.toPath().getParent())
 				: FALLBACK_FILESTORE_ATTRIBUTES;
 		BasicFileAttributes fileAttributes = null;
 		try {
 			fileAttributes = FS.DETECTED.fileAttributes(file);
 		} catch (IOException e) {
 			this.lastModified = Instant.ofEpochMilli(file.lastModified());
 			this.size = file.length();
 			this.fileKey = MISSING_FILEKEY;
 			return;
 		}
 		this.lastModified = fileAttributes.lastModifiedTime().toInstant();
 		this.size = fileAttributes.size();
 		this.fileKey = getFileKey(fileAttributes);
 		if (LOG.isDebugEnabled()) {
 			LOG.debug("file={}, create new FileSnapshot: lastRead={}, lastModified={}, size={}, fileKey={}", //$NON-NLS-1$
 					file, dateFmt.format(lastRead),
 					dateFmt.format(lastModified), Long.valueOf(size),
 					fileKey.toString());
 		}
 	}

 	private boolean sizeChanged;

 	private boolean fileKeyChanged;

 	private boolean lastModifiedChanged;

 	private boolean wasRacyClean;

 	private long delta;

 	private long racyThreshold;

 	private FileSnapshot(Instant read, Instant modified, long size,
 			@NonNull Duration fsTimestampResolution, @NonNull Object fileKey) {
 		this.file = null;
 		this.lastRead = read;
 		this.lastModified = modified;
 		this.fileStoreAttributeCache = new FileStoreAttributes(
 				fsTimestampResolution);
 		this.size = size;
 		this.fileKey = fileKey;
 	}

 	/**
 	 * Get time of last snapshot update
 	 *
 	 * @return time of last snapshot update
 	 * @deprecated use {@link #lastModifiedInstant()} instead
 	 */
 	@Deprecated
 	public long lastModified() {
 		return lastModified.toEpochMilli();
 	}

 	/**
 	 * Get time of last snapshot update
 	 *
 	 * @return time of last snapshot update
 	 */
 	public Instant lastModifiedInstant() {
 		return lastModified;
 	}

 	/**
 	 * @return file size in bytes of last snapshot update
 	 */
 	public long size() {
 		return size;
 	}

 	/**
 	 * Check if the path may have been modified since the snapshot was saved.
 	 *
 	 * @param path
 	 *            the path the snapshot describes.
 	 * @return true if the path needs to be read again.
 	 */
 	public boolean isModified(File path) {
 		Instant currLastModified;
 		long currSize;
 		Object currFileKey;
 		try {
 			BasicFileAttributes fileAttributes = FS.DETECTED.fileAttributes(path);
 			currLastModified = fileAttributes.lastModifiedTime().toInstant();
 			currSize = fileAttributes.size();
 			currFileKey = getFileKey(fileAttributes);
 		} catch (IOException e) {
 			currLastModified = Instant.ofEpochMilli(path.lastModified());
 			currSize = path.length();
 			currFileKey = MISSING_FILEKEY;
 		}
 		sizeChanged = isSizeChanged(currSize);
 		if (sizeChanged) {
 			return true;
 		}
 		fileKeyChanged = isFileKeyChanged(currFileKey);
 		if (fileKeyChanged) {
 			return true;
 		}
 		lastModifiedChanged = isModified(currLastModified);
 		if (lastModifiedChanged) {
 			return true;
 		}
 		return false;
 	}

 	/**
 	 * Update this snapshot when the content hasn't changed.
 	 * <p>
 	 * If the caller gets true from {@link #isModified(File)}, re-reads the
 	 * content, discovers the content is identical, and
 	 * {@link #equals(FileSnapshot)} is true, it can use
 	 * {@link #setClean(FileSnapshot)} to make a future
 	 * {@link #isModified(File)} return false. The logic goes something like
 	 * this:
 	 *
 	 * <pre>
 	 * if (snapshot.isModified(path)) {
 	 *  FileSnapshot other = FileSnapshot.save(path);
 	 *  Content newContent = ...;
 	 *  if (oldContent.equals(newContent) &amp;&amp; snapshot.equals(other))
 	 *      snapshot.setClean(other);
 	 * }
 	 * </pre>
 	 *
 	 * @param other
 	 *            the other snapshot.
 	 */
 	public void setClean(FileSnapshot other) {
 		final Instant now = other.lastRead;
 		if (!isRacyClean(now)) {
 			cannotBeRacilyClean = true;
 		}
 		lastRead = now;
 	}

 	/**
 	 * Wait until this snapshot's file can't be racy anymore
 	 *
 	 * @throws InterruptedException
 	 *             if sleep was interrupted
 	 */
 	public void waitUntilNotRacy() throws InterruptedException {
 		long timestampResolution = fileStoreAttributeCache
 				.getFsTimestampResolution().toNanos();
 		while (isRacyClean(Instant.now())) {
 			TimeUnit.NANOSECONDS.sleep(timestampResolution);
 		}
 	}

 	/**
 	 * Compare two snapshots to see if they cache the same information.
 	 *
 	 * @param other
 	 *            the other snapshot.
 	 * @return true if the two snapshots share the same information.
 	 */
 	@SuppressWarnings("NonOverridingEquals")
 	public boolean equals(FileSnapshot other) {
 		boolean sizeEq = size == UNKNOWN_SIZE || other.size == UNKNOWN_SIZE || size == other.size;
 		return lastModified.equals(other.lastModified) && sizeEq
 				&& Objects.equals(fileKey, other.fileKey);
 	}

 	/** {@inheritDoc} */
 	@Override
 	public boolean equals(Object obj) {
 		if (this == obj) {
 			return true;
 		}
 		if (obj == null) {
 			return false;
 		}
 		if (!(obj instanceof FileSnapshot)) {
 			return false;
 		}
 		FileSnapshot other = (FileSnapshot) obj;
 		return equals(other);
 	}

 	/** {@inheritDoc} */
 	@Override
 	public int hashCode() {
 		return Objects.hash(lastModified, Long.valueOf(size), fileKey);
 	}

 	/**
 	 * @return {@code true} if FileSnapshot.isModified(File) found the file size
 	 *         changed
 	 */
 	boolean wasSizeChanged() {
 		return sizeChanged;
 	}

 	/**
 	 * @return {@code true} if FileSnapshot.isModified(File) found the file key
 	 *         changed
 	 */
 	boolean wasFileKeyChanged() {
 		return fileKeyChanged;
 	}

 	/**
 	 * @return {@code true} if FileSnapshot.isModified(File) found the file's
 	 *         lastModified changed
 	 */
 	boolean wasLastModifiedChanged() {
 		return lastModifiedChanged;
 	}

 	/**
 	 * @return {@code true} if FileSnapshot.isModified(File) detected that
 	 *         lastModified is racily clean
 	 */
 	boolean wasLastModifiedRacilyClean() {
 		return wasRacyClean;
 	}

 	/**
 	 * @return the delta in nanoseconds between lastModified and lastRead during
 	 *         last racy check
 	 */
 	public long lastDelta() {
 		return delta;
 	}

 	/**
 	 * @return the racyLimitNanos threshold in nanoseconds during last racy
 	 *         check
 	 */
 	public long lastRacyThreshold() {
 		return racyThreshold;
 	}

 	/** {@inheritDoc} */
 	@SuppressWarnings({ "nls", "ReferenceEquality" })
 	@Override
 	public String toString() {
 		if (this == DIRTY) {
 			return "DIRTY";
 		}
 		if (this == MISSING_FILE) {
 			return "MISSING_FILE";
 		}
 		return "FileSnapshot[modified: " + dateFmt.format(lastModified)
 				+ ", read: " + dateFmt.format(lastRead) + ", size:" + size
 				+ ", fileKey: " + fileKey + "]";
 	}

 	private boolean isRacyClean(Instant read) {
 		racyThreshold = getEffectiveRacyThreshold();
 		delta = Duration.between(lastModified, read).toNanos();
 		wasRacyClean = delta <= racyThreshold;
 		if (LOG.isDebugEnabled()) {
 			LOG.debug(
 					"file={}, isRacyClean={}, read={}, lastModified={}, delta={} ns, racy<={} ns", //$NON-NLS-1$
 					file, Boolean.valueOf(wasRacyClean), dateFmt.format(read),
 					dateFmt.format(lastModified), Long.valueOf(delta),
 					Long.valueOf(racyThreshold));
 		}
 		return wasRacyClean;
 	}

 	private long getEffectiveRacyThreshold() {
 		long timestampResolution = fileStoreAttributeCache
 				.getFsTimestampResolution().toNanos();
 		long minRacyInterval = fileStoreAttributeCache.getMinimalRacyInterval()
 				.toNanos();
 		long max = Math.max(timestampResolution, minRacyInterval);
 		// safety margin: factor 2.5 below 100ms otherwise 1.25
 		return max < 100_000_000L ? max * 5 / 2 : max * 5 / 4;
 	}

 	private boolean isModified(Instant currLastModified) {
 		// Any difference indicates the path was modified.

 		lastModifiedChanged = !lastModified.equals(currLastModified);
 		if (lastModifiedChanged) {
 			if (LOG.isDebugEnabled()) {
 				LOG.debug(
 						"file={}, lastModified changed from {} to {}", //$NON-NLS-1$
 						file, dateFmt.format(lastModified),
 						dateFmt.format(currLastModified));
 			}
 			return true;
 		}

 		// We have already determined the last read was far enough
 		// after the last modification that any new modifications
 		// are certain to change the last modified time.
 		if (cannotBeRacilyClean) {
 			LOG.debug("file={}, cannot be racily clean", file); //$NON-NLS-1$
 			return false;
 		}
 		if (!isRacyClean(lastRead)) {
 			// Our last read should have marked cannotBeRacilyClean,
 			// but this thread may not have seen the change. The read
 			// of the volatile field lastRead should have fixed that.
 			LOG.debug("file={}, is unmodified", file); //$NON-NLS-1$
 			return false;
 		}

 		// We last read this path too close to its last observed
 		// modification time. We may have missed a modification.
 		// Scan again, to ensure we still see the same state.
 		LOG.debug("file={}, is racily clean", file); //$NON-NLS-1$
 		return true;
 	}

 	private boolean isFileKeyChanged(Object currFileKey) {
 		boolean changed = currFileKey != MISSING_FILEKEY
 				&& !currFileKey.equals(fileKey);
 		if (changed) {
 			LOG.debug("file={}, FileKey changed from {} to {}", //$NON-NLS-1$
 					file, fileKey, currFileKey);
 		}
 		return changed;
 	}

 	private boolean isSizeChanged(long currSize) {
 		boolean changed = (currSize != UNKNOWN_SIZE) && (currSize != size);
 		if (changed) {
 			LOG.debug("file={}, size changed from {} to {} bytes", //$NON-NLS-1$
 					file, Long.valueOf(size), Long.valueOf(currSize));
 		}
 		return changed;
 	}
 }
	/*
	* Copyright (C) 2010, Google Inc.
	* and other copyright owners as documented in the project's IP log.
	*
	* This program and the accompanying materials are made available
	* under the terms of the Eclipse Distribution License v1.0 which
	* accompanies this distribution, is reproduced below, and is
	* available at http://www.eclipse.org/org/documents/edl-v10.php
	*
	* All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or
	* without modification, are permitted provided that the following
	* conditions are met:
	*
	* - Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	*
	* - Redistributions in binary form must reproduce the above
	* copyright notice, this list of conditions and the following
	* disclaimer in the documentation and/or other materials provided
	* with the distribution.
	*
	* - Neither the name of the Eclipse Foundation, Inc. nor the
	* names of its contributors may be used to endorse or promote
	* products derived from this software without specific prior
	* written permission.
	*
	* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
	* CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
	* INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
	* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
	* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
	* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
	* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
	* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
	* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
	* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
	* ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

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

	import static org.eclipse.jgit.util.FS.FileStoreAttributes.FALLBACK_FILESTORE_ATTRIBUTES;
	import static org.eclipse.jgit.util.FS.FileStoreAttributes.FALLBACK_TIMESTAMP_RESOLUTION;
	import java.io.File;
	import java.io.IOException;
	import java.nio.file.attribute.BasicFileAttributes;
	import java.time.Duration;
	import java.time.Instant;
	import java.time.ZoneId;
	import java.time.format.DateTimeFormatter;
	import java.util.Locale;
	import java.util.Objects;
	import java.util.concurrent.TimeUnit;

	import org.eclipse.jgit.annotations.NonNull;
	import org.eclipse.jgit.util.FS;
	import org.eclipse.jgit.util.FS.FileStoreAttributes;
	import org.slf4j.Logger;
	import org.slf4j.LoggerFactory;

	/**
	* Caches when a file was last read, making it possible to detect future edits.
	* <p>
	* This object tracks the last modified time of a file. Later during an
	* invocation of {@link #isModified(File)} the object will return true if the
	* file may have been modified and should be re-read from disk.
	* <p>
	* A snapshot does not "live update" when the underlying filesystem changes.
	* Callers must poll for updates by periodically invoking
	* {@link #isModified(File)}.
	* <p>
	* To work around the "racy git" problem (where a file may be modified multiple
	* times within the granularity of the filesystem modification clock) this class
	* may return true from isModified(File) if the last modification time of the
	* file is less than 3 seconds ago.
	*/
	public class FileSnapshot {
	private static final Logger LOG = LoggerFactory
	.getLogger(FileSnapshot.class);
	/**
	* An unknown file size.
	*
	* This value is used when a comparison needs to happen purely on the lastUpdate.
	*/
	public static final long UNKNOWN_SIZE = -1;

	private static final Instant UNKNOWN_TIME = Instant.ofEpochMilli(-1);

	private static final Object MISSING_FILEKEY = new Object();

	private static final DateTimeFormatter dateFmt = DateTimeFormatter
	.ofPattern("yyyy-MM-dd HH:mm:ss.nnnnnnnnn") //$NON-NLS-1$
	.withLocale(Locale.getDefault()).withZone(ZoneId.systemDefault());

	/**
	* A FileSnapshot that is considered to always be modified.
	* <p>
	* This instance is useful for application code that wants to lazily read a
	* file, but only after {@link #isModified(File)} gets invoked. The returned
	* snapshot contains only invalid status information.
	*/
	public static final FileSnapshot DIRTY = new FileSnapshot(UNKNOWN_TIME,
	UNKNOWN_TIME, UNKNOWN_SIZE, Duration.ZERO, MISSING_FILEKEY);

	/**
	* A FileSnapshot that is clean if the file does not exist.
	* <p>
	* This instance is useful if the application wants to consider a missing
	* file to be clean. {@link #isModified(File)} will return false if the file
	* path does not exist.
	*/
	public static final FileSnapshot MISSING_FILE = new FileSnapshot(
	Instant.EPOCH, Instant.EPOCH, 0, Duration.ZERO, MISSING_FILEKEY) {
	@Override
	public boolean isModified(File path) {
	return FS.DETECTED.exists(path);
	}
	};

	/**
	* Record a snapshot for a specific file path.
	* <p>
	* This method should be invoked before the file is accessed.
	*
	* @param path
	* the path to later remember. The path's current status
	* information is saved.
	* @return the snapshot.
	*/
	public static FileSnapshot save(File path) {
	return new FileSnapshot(path);
	}

	/**
	* Record a snapshot for a specific file path without using config file to
	* get filesystem timestamp resolution.
	* <p>
	* This method should be invoked before the file is accessed. It is used by
	* FileBasedConfig to avoid endless recursion.
	*
	* @param path
	* the path to later remember. The path's current status
	* information is saved.
	* @return the snapshot.
	*/
	public static FileSnapshot saveNoConfig(File path) {
	return new FileSnapshot(path, false);
	}

	private static Object getFileKey(BasicFileAttributes fileAttributes) {
	Object fileKey = fileAttributes.fileKey();
	return fileKey == null ? MISSING_FILEKEY : fileKey;
	}

	/**
	* Record a snapshot for a file for which the last modification time is
	* already known.
	* <p>
	* This method should be invoked before the file is accessed.
	* <p>
	* Note that this method cannot rely on measuring file timestamp resolution
	* to avoid racy git issues caused by finite file timestamp resolution since
	* it's unknown in which filesystem the file is located. Hence the worst
	* case fallback for timestamp resolution is used.
	*
	* @param modified
	* the last modification time of the file
	* @return the snapshot.
	* @deprecated use {@link #save(Instant)} instead.
	*/
	@Deprecated
	public static FileSnapshot save(long modified) {
	final Instant read = Instant.now();
	return new FileSnapshot(read, Instant.ofEpochMilli(modified),
	UNKNOWN_SIZE, FALLBACK_TIMESTAMP_RESOLUTION, MISSING_FILEKEY);
	}

	/**
	* Record a snapshot for a file for which the last modification time is
	* already known.
	* <p>
	* This method should be invoked before the file is accessed.
	* <p>
	* Note that this method cannot rely on measuring file timestamp resolution
	* to avoid racy git issues caused by finite file timestamp resolution since
	* it's unknown in which filesystem the file is located. Hence the worst
	* case fallback for timestamp resolution is used.
	*
	* @param modified
	* the last modification time of the file
	* @return the snapshot.
	*/
	public static FileSnapshot save(Instant modified) {
	final Instant read = Instant.now();
	return new FileSnapshot(read, modified, UNKNOWN_SIZE,
	FALLBACK_TIMESTAMP_RESOLUTION, MISSING_FILEKEY);
	}

	/** Last observed modification time of the path. */
	private final Instant lastModified;

	/** Last wall-clock time the path was read. */
	private volatile Instant lastRead;

	/** True once {@link #lastRead} is far later than {@link #lastModified}. */
	private boolean cannotBeRacilyClean;

	/** Underlying file-system size in bytes.
	*
	* When set to {@link #UNKNOWN_SIZE} the size is not considered for modification checks. */
	private final long size;

	/** measured FileStore attributes */
	private FileStoreAttributes fileStoreAttributeCache;

	/**
	* Object that uniquely identifies the given file, or {@code
	* null} if a file key is not available
	*/
	private final Object fileKey;

	private final File file;

	/**
	* Record a snapshot for a specific file path.
	* <p>
	* This method should be invoked before the file is accessed.
	*
	* @param file
	* the path to remember meta data for. The path's current status
	* information is saved.
	*/
	protected FileSnapshot(File file) {
	this(file, true);
	}

	/**
	* Record a snapshot for a specific file path.
	* <p>
	* This method should be invoked before the file is accessed.
	*
	* @param file
	* the path to remember meta data for. The path's current status
	* information is saved.
	* @param useConfig
	* if {@code true} read filesystem time resolution from
	* configuration file otherwise use fallback resolution
	*/
	protected FileSnapshot(File file, boolean useConfig) {
	this.file = file;
	this.lastRead = Instant.now();
	this.fileStoreAttributeCache = useConfig
	? FS.getFileStoreAttributes(file.toPath().getParent())
	: FALLBACK_FILESTORE_ATTRIBUTES;
	BasicFileAttributes fileAttributes = null;
	try {
	fileAttributes = FS.DETECTED.fileAttributes(file);
	} catch (IOException e) {
	this.lastModified = Instant.ofEpochMilli(file.lastModified());
	this.size = file.length();
	this.fileKey = MISSING_FILEKEY;
	return;
	}
	this.lastModified = fileAttributes.lastModifiedTime().toInstant();
	this.size = fileAttributes.size();
	this.fileKey = getFileKey(fileAttributes);
	if (LOG.isDebugEnabled()) {
	LOG.debug("file={}, create new FileSnapshot: lastRead={}, lastModified={}, size={}, fileKey={}", //$NON-NLS-1$
	file, dateFmt.format(lastRead),
	dateFmt.format(lastModified), Long.valueOf(size),
	fileKey.toString());
	}
	}

	private boolean sizeChanged;

	private boolean fileKeyChanged;

	private boolean lastModifiedChanged;

	private boolean wasRacyClean;

	private long delta;

	private long racyThreshold;

	private FileSnapshot(Instant read, Instant modified, long size,
	@NonNull Duration fsTimestampResolution, @NonNull Object fileKey) {
	this.file = null;
	this.lastRead = read;
	this.lastModified = modified;
	this.fileStoreAttributeCache = new FileStoreAttributes(
	fsTimestampResolution);
	this.size = size;
	this.fileKey = fileKey;
	}

	/**
	* Get time of last snapshot update
	*
	* @return time of last snapshot update
	* @deprecated use {@link #lastModifiedInstant()} instead
	*/
	@Deprecated
	public long lastModified() {
	return lastModified.toEpochMilli();
	}

	/**
	* Get time of last snapshot update
	*
	* @return time of last snapshot update
	*/
	public Instant lastModifiedInstant() {
	return lastModified;
	}

	/**
	* @return file size in bytes of last snapshot update
	*/
	public long size() {
	return size;
	}

	/**
	* Check if the path may have been modified since the snapshot was saved.
	*
	* @param path
	* the path the snapshot describes.
	* @return true if the path needs to be read again.
	*/
	public boolean isModified(File path) {
	Instant currLastModified;
	long currSize;
	Object currFileKey;
	try {
	BasicFileAttributes fileAttributes = FS.DETECTED.fileAttributes(path);
	currLastModified = fileAttributes.lastModifiedTime().toInstant();
	currSize = fileAttributes.size();
	currFileKey = getFileKey(fileAttributes);
	} catch (IOException e) {
	currLastModified = Instant.ofEpochMilli(path.lastModified());
	currSize = path.length();
	currFileKey = MISSING_FILEKEY;
	}
	sizeChanged = isSizeChanged(currSize);
	if (sizeChanged) {
	return true;
	}
	fileKeyChanged = isFileKeyChanged(currFileKey);
	if (fileKeyChanged) {
	return true;
	}
	lastModifiedChanged = isModified(currLastModified);
	if (lastModifiedChanged) {
	return true;
	}
	return false;
	}

	/**
	* Update this snapshot when the content hasn't changed.
	* <p>
	* If the caller gets true from {@link #isModified(File)}, re-reads the
	* content, discovers the content is identical, and
	* {@link #equals(FileSnapshot)} is true, it can use
	* {@link #setClean(FileSnapshot)} to make a future
	* {@link #isModified(File)} return false. The logic goes something like
	* this:
	*
	* <pre>
	* if (snapshot.isModified(path)) {
	* FileSnapshot other = FileSnapshot.save(path);
	* Content newContent = ...;
	* if (oldContent.equals(newContent) && snapshot.equals(other))
	* snapshot.setClean(other);
	* }
	* </pre>
	*
	* @param other
	* the other snapshot.
	*/
	public void setClean(FileSnapshot other) {
	final Instant now = other.lastRead;
	if (!isRacyClean(now)) {
	cannotBeRacilyClean = true;
	}
	lastRead = now;
	}

	/**
	* Wait until this snapshot's file can't be racy anymore
	*
	* @throws InterruptedException
	* if sleep was interrupted
	*/
	public void waitUntilNotRacy() throws InterruptedException {
	long timestampResolution = fileStoreAttributeCache
	.getFsTimestampResolution().toNanos();
	while (isRacyClean(Instant.now())) {
	TimeUnit.NANOSECONDS.sleep(timestampResolution);
	}
	}

	/**
	* Compare two snapshots to see if they cache the same information.
	*
	* @param other
	* the other snapshot.
	* @return true if the two snapshots share the same information.
	*/
	@SuppressWarnings("NonOverridingEquals")
	public boolean equals(FileSnapshot other) {
	boolean sizeEq = size == UNKNOWN_SIZE \|\| other.size == UNKNOWN_SIZE \|\| size == other.size;
	return lastModified.equals(other.lastModified) && sizeEq
	&& Objects.equals(fileKey, other.fileKey);
	}

	/** {@inheritDoc} */
	@Override
	public boolean equals(Object obj) {
	if (this == obj) {
	return true;
	}
	if (obj == null) {
	return false;
	}
	if (!(obj instanceof FileSnapshot)) {
	return false;
	}
	FileSnapshot other = (FileSnapshot) obj;
	return equals(other);
	}

	/** {@inheritDoc} */
	@Override
	public int hashCode() {
	return Objects.hash(lastModified, Long.valueOf(size), fileKey);
	}

	/**
	* @return {@code true} if FileSnapshot.isModified(File) found the file size
	* changed
	*/
	boolean wasSizeChanged() {
	return sizeChanged;
	}

	/**
	* @return {@code true} if FileSnapshot.isModified(File) found the file key
	* changed
	*/
	boolean wasFileKeyChanged() {
	return fileKeyChanged;
	}

	/**
	* @return {@code true} if FileSnapshot.isModified(File) found the file's
	* lastModified changed
	*/
	boolean wasLastModifiedChanged() {
	return lastModifiedChanged;
	}

	/**
	* @return {@code true} if FileSnapshot.isModified(File) detected that
	* lastModified is racily clean
	*/
	boolean wasLastModifiedRacilyClean() {
	return wasRacyClean;
	}

	/**
	* @return the delta in nanoseconds between lastModified and lastRead during
	* last racy check
	*/
	public long lastDelta() {
	return delta;
	}

	/**
	* @return the racyLimitNanos threshold in nanoseconds during last racy
	* check
	*/
	public long lastRacyThreshold() {
	return racyThreshold;
	}

	/** {@inheritDoc} */
	@SuppressWarnings({ "nls", "ReferenceEquality" })
	@Override
	public String toString() {
	if (this == DIRTY) {
	return "DIRTY";
	}
	if (this == MISSING_FILE) {
	return "MISSING_FILE";
	}
	return "FileSnapshot[modified: " + dateFmt.format(lastModified)
	+ ", read: " + dateFmt.format(lastRead) + ", size:" + size
	+ ", fileKey: " + fileKey + "]";
	}

	private boolean isRacyClean(Instant read) {
	racyThreshold = getEffectiveRacyThreshold();
	delta = Duration.between(lastModified, read).toNanos();
	wasRacyClean = delta <= racyThreshold;
	if (LOG.isDebugEnabled()) {
	LOG.debug(
	"file={}, isRacyClean={}, read={}, lastModified={}, delta={} ns, racy<={} ns", //$NON-NLS-1$
	file, Boolean.valueOf(wasRacyClean), dateFmt.format(read),
	dateFmt.format(lastModified), Long.valueOf(delta),
	Long.valueOf(racyThreshold));
	}
	return wasRacyClean;
	}

	private long getEffectiveRacyThreshold() {
	long timestampResolution = fileStoreAttributeCache
	.getFsTimestampResolution().toNanos();
	long minRacyInterval = fileStoreAttributeCache.getMinimalRacyInterval()
	.toNanos();
	long max = Math.max(timestampResolution, minRacyInterval);
	// safety margin: factor 2.5 below 100ms otherwise 1.25
	return max < 100_000_000L ? max * 5 / 2 : max * 5 / 4;
	}

	private boolean isModified(Instant currLastModified) {
	// Any difference indicates the path was modified.

	lastModifiedChanged = !lastModified.equals(currLastModified);
	if (lastModifiedChanged) {
	if (LOG.isDebugEnabled()) {
	LOG.debug(
	"file={}, lastModified changed from {} to {}", //$NON-NLS-1$
	file, dateFmt.format(lastModified),
	dateFmt.format(currLastModified));
	}
	return true;
	}

	// We have already determined the last read was far enough
	// after the last modification that any new modifications
	// are certain to change the last modified time.
	if (cannotBeRacilyClean) {
	LOG.debug("file={}, cannot be racily clean", file); //$NON-NLS-1$
	return false;
	}
	if (!isRacyClean(lastRead)) {
	// Our last read should have marked cannotBeRacilyClean,
	// but this thread may not have seen the change. The read
	// of the volatile field lastRead should have fixed that.
	LOG.debug("file={}, is unmodified", file); //$NON-NLS-1$
	return false;
	}

	// We last read this path too close to its last observed
	// modification time. We may have missed a modification.
	// Scan again, to ensure we still see the same state.
	LOG.debug("file={}, is racily clean", file); //$NON-NLS-1$
	return true;
	}

	private boolean isFileKeyChanged(Object currFileKey) {
	boolean changed = currFileKey != MISSING_FILEKEY
	&& !currFileKey.equals(fileKey);
	if (changed) {
	LOG.debug("file={}, FileKey changed from {} to {}", //$NON-NLS-1$
	file, fileKey, currFileKey);
	}
	return changed;
	}

	private boolean isSizeChanged(long currSize) {
	boolean changed = (currSize != UNKNOWN_SIZE) && (currSize != size);
	if (changed) {
	LOG.debug("file={}, size changed from {} to {} bytes", //$NON-NLS-1$
	file, Long.valueOf(size), Long.valueOf(currSize));
	}
	return changed;
	}
	}