/*
 * 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.
 This object tracks the last modified time of a file. Later during an invocation of isModified(File) the object will return true if the file may have been modified and should be re-read from disk. 
 A snapshot does not "live update" when the underlying filesystem changes. Callers must poll for updates by periodically invoking isModified(File). 

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.
/**
 * 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.
/**
	 * 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.
 This instance is useful for application code that wants to lazily read a file, but only after isModified(File) gets invoked. The returned snapshot contains only invalid status information. /**
	 * 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.
 This instance is useful if the application wants to consider a missing file to be clean. isModified(File) will return false if the file path does not exist. /**
	 * 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.

This method should be invoked before the file is accessed.
Params:
path – 
           the path to later remember. The path's current status
           information is saved.
Returns:
the snapshot./**
	 * 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.

This method should be invoked before the file is accessed. It is used by
FileBasedConfig to avoid endless recursion.
Params:
path – 
           the path to later remember. The path's current status
           information is saved.
Returns:
the snapshot./**
	 * 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.

This method should be invoked before the file is accessed.

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.
Params:
modified – 
           the last modification time of the file
Returns:
the snapshot.
Deprecated:
use save(Instant) instead./**
	 * 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.

This method should be invoked before the file is accessed.

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.
Params:
modified – 
           the last modification time of the file
Returns:
the snapshot./**
	 * 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. /** Last observed modification time of the path. */
	private final Instant lastModified;

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

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

	
Underlying file-system size in bytes. When set to UNKNOWN_SIZE the size is not considered for modification checks. /** 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 /** measured FileStore attributes */
	private FileStoreAttributes fileStoreAttributeCache;

	
Object that uniquely identifies the given file, or 
null if a file key is not available /**
	 * 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.

This method should be invoked before the file is accessed.
Params:
file – 
           the path to remember meta data for. The path's current status
           information is saved./**
	 * 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.

This method should be invoked before the file is accessed.
Params:
file – 
           the path to remember meta data for. The path's current status
           information is saved.
useConfig –  if true read filesystem time resolution from configuration file otherwise use fallback resolution/**
	 * 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
Returns:
time of last snapshot update
Deprecated:
use lastModifiedInstant() instead/**
	 * 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
Returns:
time of last snapshot update/**
	 * Get time of last snapshot update
	 *
	 * @return time of last snapshot update
	 */
	public Instant lastModifiedInstant() {
		return lastModified;
	}

	
Returns:
file size in bytes of last snapshot update/**
	 * @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.
Params:
path – 
           the path the snapshot describes.
Returns:
true if the path needs to be read again./**
	 * 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.
 If the caller gets true from isModified(File), re-reads the content, discovers the content is identical, and equals(FileSnapshot) is true, it can use setClean(FileSnapshot) to make a future isModified(File) return false. The logic goes something like this: 
if (snapshot.isModified(path)) {
 FileSnapshot other = FileSnapshot.save(path);
 Content newContent = ...;
 if (oldContent.equals(newContent) && snapshot.equals(other))
     snapshot.setClean(other);
}

Params:
other – 
           the other snapshot./**
	 * 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/**
	 * 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.
Params:
other – 
           the other snapshot.
Returns:
true if the two snapshots share the same information./**
	 * 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} /** {@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} /** {@inheritDoc} */
	@Override
	public int hashCode() {
		return Objects.hash(lastModified, Long.valueOf(size), fileKey);
	}

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

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

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

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

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

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

	
{@inheritDoc} /** {@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;
	}
}