Class FileSnapshot

java.lang.Object
org.eclipse.jgit.internal.storage.file.FileSnapshot
Direct Known Subclasses:
PackFileSnapshot

public class FileSnapshot extends Object
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.

  • Field Details

    • LOG

      private static final org.slf4j.Logger LOG
    • UNKNOWN_SIZE

      public static final long UNKNOWN_SIZE
      An unknown file size. This value is used when a comparison needs to happen purely on the lastUpdate.
      See Also:
    • UNKNOWN_TIME

      private static final Instant UNKNOWN_TIME
    • MISSING_FILEKEY

      private static final Object MISSING_FILEKEY
    • dateFmt

      private static final DateTimeFormatter dateFmt
    • DIRTY

      public static final FileSnapshot DIRTY
      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.

    • MISSING_FILE

      public static final FileSnapshot MISSING_FILE
      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.

    • lastModified

      private final Instant lastModified
      Last observed modification time of the path.
    • lastRead

      private volatile Instant lastRead
      Last wall-clock time the path was read.
    • cannotBeRacilyClean

      private boolean cannotBeRacilyClean
      True once lastRead is far later than lastModified.
    • size

      private final long size
      Underlying file-system size in bytes. When set to UNKNOWN_SIZE the size is not considered for modification checks.
    • fileStoreAttributeCache

      private FS.FileStoreAttributes fileStoreAttributeCache
      measured FileStore attributes
    • fileKey

      private final Object fileKey
      Object that uniquely identifies the given file, or null if a file key is not available
    • file

      private final File file
    • sizeChanged

      private boolean sizeChanged
    • fileKeyChanged

      private boolean fileKeyChanged
    • lastModifiedChanged

      private boolean lastModifiedChanged
    • wasRacyClean

      private boolean wasRacyClean
    • delta

      private long delta
    • racyThreshold

      private long racyThreshold
  • Constructor Details

    • FileSnapshot

      protected FileSnapshot(File file)
      Record a snapshot for a specific file path.

      This method should be invoked before the file is accessed.

      Parameters:
      file - the path to remember meta data for. The path's current status information is saved.
    • FileSnapshot

      protected FileSnapshot(File file, boolean useConfig)
      Record a snapshot for a specific file path.

      This method should be invoked before the file is accessed.

      Parameters:
      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
    • FileSnapshot

      private FileSnapshot(Instant read, Instant modified, long size, @NonNull Duration fsTimestampResolution, @NonNull Object fileKey)
  • Method Details

    • save

      public static FileSnapshot save(File path)
      Record a snapshot for a specific file path.

      This method should be invoked before the file is accessed.

      Parameters:
      path - the path to later remember. The path's current status information is saved.
      Returns:
      the snapshot.
    • saveNoConfig

      public static FileSnapshot saveNoConfig(File 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.

      Parameters:
      path - the path to later remember. The path's current status information is saved.
      Returns:
      the snapshot.
    • getFileKey

      private static Object getFileKey(BasicFileAttributes fileAttributes)
    • save

      @Deprecated public static FileSnapshot save(long modified)
      Deprecated.
      use save(Instant) instead.
      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.

      Parameters:
      modified - the last modification time of the file
      Returns:
      the snapshot.
    • save

      public static FileSnapshot save(Instant modified)
      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.

      Parameters:
      modified - the last modification time of the file
      Returns:
      the snapshot.
    • lastModified

      @Deprecated public long lastModified()
      Deprecated.
      Get time of last snapshot update
      Returns:
      time of last snapshot update
    • lastModifiedInstant

      public Instant lastModifiedInstant()
      Get time of last snapshot update
      Returns:
      time of last snapshot update
    • size

      public long size()
      Returns:
      file size in bytes of last snapshot update
    • isModified

      public boolean isModified(File path)
      Check if the path may have been modified since the snapshot was saved.
      Parameters:
      path - the path the snapshot describes.
      Returns:
      true if the path needs to be read again.
    • setClean

      public void setClean(FileSnapshot other)
      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);
       }
       
      Parameters:
      other - the other snapshot.
    • waitUntilNotRacy

      public void waitUntilNotRacy() throws InterruptedException
      Wait until this snapshot's file can't be racy anymore
      Throws:
      InterruptedException - if sleep was interrupted
    • equals

      public boolean equals(FileSnapshot other)
      Compare two snapshots to see if they cache the same information.
      Parameters:
      other - the other snapshot.
      Returns:
      true if the two snapshots share the same information.
    • equals

      public boolean equals(Object obj)
      Overrides:
      equals in class Object
    • hashCode

      public int hashCode()
      Overrides:
      hashCode in class Object
    • wasSizeChanged

      boolean wasSizeChanged()
      Returns:
      true if FileSnapshot.isModified(File) found the file size changed
    • wasFileKeyChanged

      boolean wasFileKeyChanged()
      Returns:
      true if FileSnapshot.isModified(File) found the file key changed
    • wasLastModifiedChanged

      boolean wasLastModifiedChanged()
      Returns:
      true if FileSnapshot.isModified(File) found the file's lastModified changed
    • wasLastModifiedRacilyClean

      boolean wasLastModifiedRacilyClean()
      Returns:
      true if FileSnapshot.isModified(File) detected that lastModified is racily clean
    • lastDelta

      public long lastDelta()
      Returns:
      the delta in nanoseconds between lastModified and lastRead during last racy check
    • lastRacyThreshold

      public long lastRacyThreshold()
      Returns:
      the racyLimitNanos threshold in nanoseconds during last racy check
    • toString

      public String toString()
      Overrides:
      toString in class Object
    • isRacyClean

      private boolean isRacyClean(Instant read)
    • getEffectiveRacyThreshold

      private long getEffectiveRacyThreshold()
    • isModified

      private boolean isModified(Instant currLastModified)
    • isFileKeyChanged

      private boolean isFileKeyChanged(Object currFileKey)
    • isSizeChanged

      private boolean isSizeChanged(long currSize)