src/main/java/com/google/devtools/build/lib/actions/FileContentsProxy.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.actions;

import com.google.devtools.build.lib.util.BigIntegerFingerprint;
import com.google.devtools.build.lib.vfs.FileStatus;
import java.io.IOException;
import java.io.Serializable;
import java.util.Objects;

/**
 * In case we can't get a fast digest from the filesystem, we store this metadata as a proxy to the
 * file contents. Currently it is a pair of a relevant timestamp and a "node id". On Linux the
 * former is the ctime and the latter is the inode number. We might want to add the device number in
 * the future.
 *
 * <p>For a Linux example of why mtime alone is insufficient, note that 'mv' preserves timestamps.
 * So if files 'a' and 'b' initially have the same timestamp, then we would think 'b' is unchanged
 * after the user executes `mv a b` between two builds.
 */
public final class FileContentsProxy implements Serializable {
  private final long ctime;
  private final long nodeId;

  /**
   * Visible for serialization / deserialization. Do not use this method, but call {@link #create}
   * instead.
   */
  public FileContentsProxy(long ctime, long nodeId) {
    this.ctime = ctime;
    this.nodeId = nodeId;
  }

  public static FileContentsProxy create(FileStatus stat) throws IOException {
    // Note: there are file systems that return mtime for this call instead of ctime, such as the
    // WindowsFileSystem.
    return new FileContentsProxy(stat.getLastChangeTime(), stat.getNodeId());
  }

  /** Visible for serialization / deserialization. Do not use this method; use {@link #equals}. */
  public long getCTime() {
    return ctime;
  }

  /** Visible for serialization / deserialization. Do not use this method; use {@link #equals}. */
  public long getNodeId() {
    return nodeId;
  }

  @Override
  public boolean equals(Object other) {
    if (other == this) {
      return true;
    }

    if (!(other instanceof FileContentsProxy)) {
      return false;
    }

    FileContentsProxy that = (FileContentsProxy) other;
    return ctime == that.ctime && nodeId == that.nodeId;
  }

  @Override
  public int hashCode() {
    return Objects.hash(ctime, nodeId);
  }

  void addToFingerprint(BigIntegerFingerprint fp) {
    fp.addLong(ctime);
    fp.addLong(nodeId);
  }

  @Override
  public String toString() {
    return prettyPrint();
  }

  public String prettyPrint() {
    return String.format("ctime of %d and nodeId of %d", ctime, nodeId);
  }
}