org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java - sop/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 java.io.File;
 import java.io.IOException;
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
 import java.util.Date;
 import java.util.Locale;

 import org.eclipse.jgit.util.FS;

 /**
  * 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 {
 	/**
 	 * 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(-1, -1);

 	/**
 	 * 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(0, 0) {
 		@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) {
 		long read = System.currentTimeMillis();
 		long modified;
 		try {
 			modified = FS.DETECTED.lastModified(path);
 		} catch (IOException e) {
 			modified = path.lastModified();
 		}
 		return new FileSnapshot(read, modified);
 	}

 	/**
 	 * 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.
 	 *
 	 * @param modified
 	 *            the last modification time of the file
 	 *
 	 * @return the snapshot.
 	 */
 	public static FileSnapshot save(long modified) {
 		final long read = System.currentTimeMillis();
 		return new FileSnapshot(read, modified);
 	}

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

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

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

 	private FileSnapshot(long read, long modified) {
 		this.lastRead = read;
 		this.lastModified = modified;
 		this.cannotBeRacilyClean = notRacyClean(read);
 	}

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

 	/**
 	 * 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) {
 		long currLastModified;
 		try {
 			currLastModified = FS.DETECTED.lastModified(path);
 		} catch (IOException e) {
 			currLastModified = path.lastModified();
 		}
 		return isModified(currLastModified);
 	}

 	/**
 	 * 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 long now = other.lastRead;
 		if (notRacyClean(now))
 			cannotBeRacilyClean = true;
 		lastRead = now;
 	}

 	/**
 	 * 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.
 	 */
 	public boolean equals(FileSnapshot other) {
 		return lastModified == other.lastModified;
 	}

 	@Override
 	public boolean equals(Object other) {
 		if (other instanceof FileSnapshot)
 			return equals((FileSnapshot) other);
 		return false;
 	}

 	@Override
 	public int hashCode() {
 		// This is pretty pointless, but override hashCode to ensure that
 		// x.hashCode() == y.hashCode() when x.equals(y) is true.
 		//
 		return (int) lastModified;
 	}

 	@Override
 	public String toString() {
 		if (this == DIRTY)
 			return "DIRTY"; //$NON-NLS-1$
 		if (this == MISSING_FILE)
 			return "MISSING_FILE"; //$NON-NLS-1$
 		DateFormat f = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS", //$NON-NLS-1$
 				Locale.US);
 		return "FileSnapshot[modified: " + f.format(new Date(lastModified)) //$NON-NLS-1$
 				+ ", read: " + f.format(new Date(lastRead)) + "]"; //$NON-NLS-1$ //$NON-NLS-2$
 	}

 	private boolean notRacyClean(final long read) {
 		// The last modified time granularity of FAT filesystems is 2 seconds.
 		// Using 2.5 seconds here provides a reasonably high assurance that
 		// a modification was not missed.
 		//
 		return read - lastModified > 2500;
 	}

 	private boolean isModified(final long currLastModified) {
 		// Any difference indicates the path was modified.
 		//
 		if (lastModified != 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)
 			return false;

 		if (notRacyClean(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.
 			//
 			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.
 		//
 		return true;
 	}
 }
	/*
	* 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 java.io.File;
	import java.io.IOException;
	import java.text.DateFormat;
	import java.text.SimpleDateFormat;
	import java.util.Date;
	import java.util.Locale;

	import org.eclipse.jgit.util.FS;

	/**
	* 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 {
	/**
	* 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(-1, -1);

	/**
	* 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(0, 0) {
	@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) {
	long read = System.currentTimeMillis();
	long modified;
	try {
	modified = FS.DETECTED.lastModified(path);
	} catch (IOException e) {
	modified = path.lastModified();
	}
	return new FileSnapshot(read, modified);
	}

	/**
	* 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.
	*
	* @param modified
	* the last modification time of the file
	*
	* @return the snapshot.
	*/
	public static FileSnapshot save(long modified) {
	final long read = System.currentTimeMillis();
	return new FileSnapshot(read, modified);
	}

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

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

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

	private FileSnapshot(long read, long modified) {
	this.lastRead = read;
	this.lastModified = modified;
	this.cannotBeRacilyClean = notRacyClean(read);
	}

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

	/**
	* 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) {
	long currLastModified;
	try {
	currLastModified = FS.DETECTED.lastModified(path);
	} catch (IOException e) {
	currLastModified = path.lastModified();
	}
	return isModified(currLastModified);
	}

	/**
	* 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 long now = other.lastRead;
	if (notRacyClean(now))
	cannotBeRacilyClean = true;
	lastRead = now;
	}

	/**
	* 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.
	*/
	public boolean equals(FileSnapshot other) {
	return lastModified == other.lastModified;
	}

	@Override
	public boolean equals(Object other) {
	if (other instanceof FileSnapshot)
	return equals((FileSnapshot) other);
	return false;
	}

	@Override
	public int hashCode() {
	// This is pretty pointless, but override hashCode to ensure that
	// x.hashCode() == y.hashCode() when x.equals(y) is true.
	//
	return (int) lastModified;
	}

	@Override
	public String toString() {
	if (this == DIRTY)
	return "DIRTY"; //$NON-NLS-1$
	if (this == MISSING_FILE)
	return "MISSING_FILE"; //$NON-NLS-1$
	DateFormat f = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS", //$NON-NLS-1$
	Locale.US);
	return "FileSnapshot[modified: " + f.format(new Date(lastModified)) //$NON-NLS-1$
	+ ", read: " + f.format(new Date(lastRead)) + "]"; //$NON-NLS-1$ //$NON-NLS-2$
	}

	private boolean notRacyClean(final long read) {
	// The last modified time granularity of FAT filesystems is 2 seconds.
	// Using 2.5 seconds here provides a reasonably high assurance that
	// a modification was not missed.
	//
	return read - lastModified > 2500;
	}

	private boolean isModified(final long currLastModified) {
	// Any difference indicates the path was modified.
	//
	if (lastModified != 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)
	return false;

	if (notRacyClean(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.
	//
	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.
	//
	return true;
	}
	}