src/main/java/com/google/devtools/build/lib/util/PersistentMap.java

// Copyright 2014 The Bazel Authors. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package com.google.devtools.build.lib.util;

import com.google.common.collect.ForwardingMap;
import com.google.devtools.build.lib.vfs.FileSystemUtils;
import com.google.devtools.build.lib.vfs.Path;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.LinkedHashMap;
import java.util.Map;

/**
 * A map that is backed by persistent storage. It uses two files on disk for
 * this: The first file contains all the entries and gets written when invoking
 * the {@link #save()} method. The second file contains a journal of all entries
 * that were added to or removed from the map since constructing the instance of
 * the map or the last invocation of {@link #save()} and gets written after each
 * update of the map although sub-classes are free to implement their own
 * journal update strategy.
 *
 * <p><b>Ceci n'est pas un Map</b>.  Strictly speaking, the {@link Map}
 * interface doesn't permit the possibility of failure.  This class uses
 * persistence; persistence means I/O, and I/O means the possibility of
 * failure.  Therefore the semantics of this may deviate from the Map contract
 * in failure cases.  In particular, updates are not guaranteed to succeed.
 * However, I/O failures are guaranteed to be reported upon the subsequent call
 * to a method that throws {@code IOException} such as {@link #save}.
 *
 * <p>To populate the map entries using the previously persisted entries call
 * {@link #load()} prior to invoking any other map operation.
 * <p>
 * Like {@link Hashtable} but unlike {@link HashMap}, this class does
 * <em>not</em> allow <tt>null</tt> to be used as a key or a value.
 * <p>
 * IO failures during reading or writing the map entries to disk may result in
 * {@link AssertionError} getting thrown from the failing method.
 * <p>
 * The implementation of the map is not synchronized. If access from multiple
 * threads is required it must be synchronized using an external object.
 * <p>
 * The constructor allows passing in a version number that gets written to the
 * files on disk and checked before reading from disk. Files with an
 * incompatible version number will be ignored. This allows the client code to
 * change the persistence format without polluting the file system name space.
 */
public abstract class PersistentMap<K, V> extends ForwardingMap<K, V> {

  private static final int MAGIC = 0x20071105;

  private static final int ENTRY_MAGIC = 0xfe;

  private final int version;
  private final Path mapFile;
  private final Path journalFile;
  private final Map<K, V> journal;
  private DataOutputStream journalOut;

  /**
   * 'dirty' is true when the in-memory representation of the map is more recent
   * than the on-disk representation.
   */
  private boolean dirty;

  /**
   * If non-null, contains the message from an {@code IOException} thrown by a
   * previously failed write.  This error is deferred until the next call to a
   * method which is able to throw an exception.
   */
  private String deferredIOFailure = null;

  /**
   * 'loaded' is true when the in-memory representation is at least as recent as
   * the on-disk representation.
   */
  private boolean loaded;

  private final Map<K, V> delegate;

  /**
   * Creates a new PersistentMap instance using the specified backing map.
   *
   * @param version the version tag. Changing the version tag allows updating
   *        the on disk format. The map will never read from a file that was
   *        written using a different version tag.
   * @param map the backing map to use for this PersistentMap.
   * @param mapFile the file to save the map entries to.
   * @param journalFile the journal file to write entries between invocations of
   *        {@link #save()}.
   */
  public PersistentMap(int version, Map<K, V> map, Path mapFile, Path journalFile) {
    this.version = version;
    journal = new LinkedHashMap<>();
    this.mapFile = mapFile;
    this.journalFile = journalFile;
    delegate = map;
  }


  @Override
  protected Map<K, V> delegate() {
    return delegate;
  }

  @Override
  public V put(K key, V value) {
    if (key == null) {
      throw new NullPointerException();
    }
    if (value == null) {
      throw new NullPointerException();
    }
    V previous = delegate().put(key, value);
    journal.put(key, value);
    markAsDirty();
    return previous;
  }

  /**
   * Marks the map as dirty and potentially writes updated entries to the
   * journal.
   */
  protected void markAsDirty() {
    dirty = true;
    if (updateJournal()) {
      writeJournal();
    }
  }

  /**
   * Determines if the journal should be updated. The default implementation
   * always returns 'true', but subclasses are free to override this to
   * implement their own journal updating strategy. For example it is possible
   * to implement an update at most every five seconds using the following code:
   *
   * <pre>
   * private long nextUpdate;
   * protected boolean updateJournal() {
   *   long time = System.currentTimeMillis();
   *   if (time &gt; nextUpdate) {
   *     nextUpdate = time + 5 * 1000;
   *     return true;
   *   }
   *   return false;
   * }
   * </pre>
   */
  protected boolean updateJournal() {
    return true;
  }

  @Override
  @SuppressWarnings("unchecked")
  public V remove(Object object) {
    V previous = delegate().remove(object);
    if (previous != null) {
      // we know that 'object' must be an instance of K, because the
      // remove call succeeded, i.e. 'object' was mapped to 'previous'.
      journal.put((K) object, null); // unchecked
      markAsDirty();
    }
    return previous;
  }

  /**
   * Updates the persistent journal by writing all entries to the
   * {@link #journalOut} stream and clearing the in memory journal.
   */
  private void writeJournal() {
    try {
      if (journalOut == null) {
        journalOut = createMapFile(journalFile);
      }
      writeEntries(journalOut, journal);
      journalOut.flush();
      journal.clear();
    } catch (IOException e) {
      this.deferredIOFailure = e.getMessage() + " during journal append";
    }
  }

  protected void forceFlush() {
    if (dirty) {
      writeJournal();
    }
  }

  /**
   * Load the previous written map entries from disk.
   *
   * @param failFast if true, throw IOException rather than silently ignoring.
   * @throws IOException
   */
  public void load(boolean failFast) throws IOException {
    if (!loaded) {
      loadEntries(mapFile, failFast);
      if (journalFile.exists()) {
        try {
          loadEntries(journalFile, failFast);
        } catch (IOException e) {
          if (failFast) {
            throw e;
          }
          //Else: ignore any errors reading the journal file as it may contain
          //partial entries.
        }
        // Force the map to be dirty, so that we can save it to disk.
        dirty = true;
        save(/*fullSave=*/ true);
      } else {
        dirty = false;
      }
      loaded = true;
    }
  }

  /**
   * Load the previous written map entries from disk.
   *
   * @throws IOException
   */
  public void load() throws IOException {
    load(/*throwOnLoadFailure=*/ false);
  }

  @Override
  public void clear() {
    super.clear();
    markAsDirty();
    try {
      save();
    } catch (IOException e) {
      this.deferredIOFailure = e.getMessage() + " during map write";
    }
  }

  /**
   * Saves all the entries of this map to disk and deletes the journal file.
   *
   * @throws IOException if there was an I/O error during this call, or any previous call since the
   *                     last save().
   */
  public long save() throws IOException {
    return save(false);
  }

  /**
   * Saves all the entries of this map to disk and deletes the journal file.
   *
   * @param fullSave if true, always write the full cache to disk, without the
   *        journal.
   * @throws IOException if there was an I/O error during this call, or any
   *   previous call since the last save().
   */
  private long save(boolean fullSave) throws IOException {
    /* Report a previously failing I/O operation. */
    if (deferredIOFailure != null) {
      try {
        throw new IOException(deferredIOFailure);
      } finally {
        deferredIOFailure = null;
      }
    }
    if (dirty) {
      if (!fullSave && keepJournal()) {
        forceFlush();
        journalOut.close();
        journalOut = null;
        return journalSize() + cacheSize();
      } else {
        dirty = false;
        Path mapTemp =
            mapFile.getRelative(FileSystemUtils.replaceExtension(mapFile.asFragment(), ".tmp"));
        try {
          saveEntries(delegate(), mapTemp);
          mapFile.delete();
          mapTemp.renameTo(mapFile);
        } finally {
          mapTemp.delete();
        }
        clearJournal();
        journalFile.delete();
        return cacheSize();
      }
    } else {
      return cacheSize();
    }
  }

  protected final long journalSize() throws IOException {
    return journalFile.exists() ? journalFile.getFileSize() : 0;
  }

  protected final long cacheSize() throws IOException {
    return mapFile.exists() ? mapFile.getFileSize() : 0;
  }

  /**
   * If true, keep the journal during the save(). The journal is flushed, but
   * the map file is not touched. This may be useful in cases where the journal
   * is much smaller than the map.
   */
  protected boolean keepJournal() {
    return false;
  }

  private void clearJournal() throws IOException {
    journal.clear();
    if (journalOut != null) {
      journalOut.close();
      journalOut = null;
    }
  }

  private void loadEntries(Path mapFile, boolean failFast) throws IOException {
    if (!mapFile.exists()) {
      return;
    }
    DataInputStream in = new DataInputStream(new BufferedInputStream(mapFile.getInputStream()));
    try {
      long fileSize = mapFile.getFileSize();
      if (fileSize < (16)) {
        if (failFast) {
          throw new IOException(mapFile + " is too short: Only " + fileSize + " bytes");
        } else {
          return;
        }
      }
      if (in.readLong() != MAGIC) { // not a PersistentMap
        if (failFast) {
          throw new IOException("Unexpected format");
        }
        return;
      }
      if (in.readLong() != version) { // PersistentMap version incompatible
        if (failFast) {
          throw new IOException("Unexpected format");
        }
        return;
      }
      readEntries(in, failFast);
    } finally {
      in.close();
    }
  }

  /**
   * Saves the entries in the specified map into the specified file.
   *
   * @param map the map to be written into the file.
   * @param mapFile the file the map is written to.
   * @throws IOException
   */
  private void saveEntries(Map<K, V> map, Path mapFile) throws IOException {
    try (DataOutputStream out = createMapFile(mapFile)) {
      writeEntries(out, map);
    }
  }

  /**
   * Creates the specified file and returns the DataOuputStream suitable for writing entries.
   *
   * @param mapFile the file the map is written to.
   * @return the DataOutputStream that was can be used for saving the map to the file.
   * @throws IOException
   */
  private DataOutputStream createMapFile(Path mapFile) throws IOException {
    FileSystemUtils.createDirectoryAndParents(mapFile.getParentDirectory());
    DataOutputStream out =
        new DataOutputStream(new BufferedOutputStream(mapFile.getOutputStream()));
    out.writeLong(MAGIC);
    out.writeLong(version);
    return out;
  }

  /**
   * Writes the Map entries to the specified DataOutputStream.
   *
   * @param out the DataOutputStream to write the Map entries to.
   * @param map the Map containing the entries to be written to the
   *        DataOutputStream.
   * @throws IOException
   */
  private void writeEntries(DataOutputStream out, Map<K, V> map) throws IOException {
    for (Map.Entry<K, V> entry : map.entrySet()) {
      out.writeByte(ENTRY_MAGIC);
      writeKey(entry.getKey(), out);
      V value = entry.getValue();
      boolean isEntry = (value != null);
      out.writeBoolean(isEntry);
      if (isEntry) {
        writeValue(value, out);
      }
    }
  }

  /**
   * Reads the Map entries from the specified DataInputStream.
   *
   * @param failFast if true, throw IOException if entries are in an unexpected
   *                 format.
   * @param in the DataInputStream to read the Map entries from.
   * @throws IOException
   */
  private void readEntries(DataInputStream in, boolean failFast) throws IOException {
    Map<K, V> map = delegate();
    while (hasEntries(in, failFast)) {
      K key = readKey(in);
      boolean isEntry = in.readBoolean();
      if (isEntry) {
        V value = readValue(in);
        map.put(key, value);
      } else {
        map.remove(key);
      }
    }
  }

  private boolean hasEntries(DataInputStream in, boolean failFast) throws IOException {
    if (in.available() <= 0) {
      return false;
    } else if (in.readUnsignedByte() != ENTRY_MAGIC) {
      if (failFast) {
        throw new IOException("Corrupted entry separator");
      } else {
        return false;
      }
    }
    return true;
  }

  /**
   * Writes a key of this map into the specified DataOutputStream.
   *
   * @param key the key to write to the DataOutputStream.
   * @param out the DataOutputStream to write the entry to.
   * @throws IOException
   */
  protected abstract void writeKey(K key, DataOutputStream out) throws IOException;

  /**
   * Writes a value of this map into the specified DataOutputStream.
   *
   * @param value the value to write to the DataOutputStream.
   * @param out the DataOutputStream to write the entry to.
   * @throws IOException
   */
  protected abstract void writeValue(V value, DataOutputStream out) throws IOException;

  /**
   * Reads an entry of this map from the specified DataInputStream.
   *
   * @param in the DataOutputStream to read the entry from.
   * @return the entry that was read from the DataInputStream.
   * @throws IOException
   */
  protected abstract K readKey(DataInputStream in) throws IOException;

  /**
   * Reads an entry of this map from the specified DataInputStream.
   *
   * @param in the DataOutputStream to read the entry from.
   * @return the entry that was read from the DataInputStream.
   * @throws IOException
   */
  protected abstract V readValue(DataInputStream in) throws IOException;
}