jgit/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java

/*
 * Copyright (C) 2010, Google Inc.
 * and other copyright owners as documented in the project's IP log.
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 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Distribution License v1.0 which
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package org.eclipse.jgit.internal.storage.file;

import java.io.File;

/**
 * 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 path.exists();
		}
	};

	/**
	 * 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) &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;
	}

	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;
	}
}