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- /*
- * 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.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) {
- final long read = System.currentTimeMillis();
- final long 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) {
- return isModified(path.lastModified());
- }
-
- /**
- * 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;
- }
-
- // Our lastRead flag may be old, refresh and retry
- lastRead = System.currentTimeMillis();
- if (notRacyClean(lastRead)) {
- 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;
- }
- }
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