/*
    * Copyright (C) 2010, Google Inc. and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  
  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.NoSuchFileException;
  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;
 
 	/**
 	 * if {@code true} read filesystem time resolution from configuration file
 	 * otherwise use fallback resolution
 	 */
 	private boolean useConfig;
 
 	/**
 	 * 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.useConfig = useConfig;
 		BasicFileAttributes fileAttributes = null;
 		try {
 			fileAttributes = FS.DETECTED.fileAttributes(file);
 		} catch (NoSuchFileException e) {
 			this.lastModified = Instant.EPOCH;
 			this.size = 0L;
 			this.fileKey = MISSING_FILEKEY;
 			return;
 		} catch (IOException e) {
 			LOG.error(e.getMessage(), e);
 			this.lastModified = Instant.EPOCH;
 			this.size = 0L;
 			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 (NoSuchFileException e) {
 			currLastModified = Instant.EPOCH;
 			currSize = 0L;
 			currFileKey = MISSING_FILEKEY;
 		} catch (IOException e) {
 			LOG.error(e.getMessage(), e);
 			currLastModified = Instant.EPOCH;
 			currSize = 0L;
 			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;
 	}
 
 	private FileStoreAttributes fileStoreAttributeCache() {
 		if (fileStoreAttributeCache == null) {
 			fileStoreAttributeCache = useConfig
 					? FS.getFileStoreAttributes(file.toPath().getParent())
 					: FALLBACK_FILESTORE_ATTRIBUTES;
 		}
 		return fileStoreAttributeCache;
 	}
 }