src/main/java/com/google/devtools/build/lib/util/io/FileWatcher.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.io;

import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
import com.google.devtools.build.lib.vfs.Path;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

/**
 * The FileWatcher dumps the contents of a files into an OutErr.
 * It then stays active and dumps any content to the OutErr that is
 * added to the file, until it is told to stop and all output has
 * been dumped.
 *
 * This is useful to emulate streaming test output.
 */
@ThreadSafe
public class FileWatcher extends Thread {

  // How often we check for updates in the file we watch. (in ms)
  private static final int WATCH_INTERVAL = 100;

  private final Path outputFile;
  private final OutErr output;
  private volatile boolean finishPumping;
  private long toSkip = 0;

  /**
   * Creates a FileWatcher that will dump any output that is appended to
   * outputFile onto output. If skipExisting is true, the watcher will not dump
   * any output that is in outputFile when we construct the watcher. If
   * skipExisting is false, already existing output will be dumped, too.
   *
   * @param outputFile the File to watch
   * @param output the outErr to dump the files contents to
   * @param skipExisting whether to dump already existing output or not.
   */
  public FileWatcher(Path outputFile, OutErr output, boolean skipExisting) throws IOException {
    super("Streaming Test Output Pump");
    this.outputFile = outputFile;
    this.output = output;
    finishPumping = false;

    if (outputFile.exists() && skipExisting) {
      toSkip = outputFile.getFileSize();
    }
  }

  /**
   * Tells the FileWatcher to stop pumping output and finish.
   * The FileWatcher will only finish until there is no data left to display.
   * This means that it is rarely a good idea to unconditionally wait for the
   * FileWatcher thread to terminate -- Instead, it is better to have a timeout.
   */
  @ThreadSafe
  public void stopPumping() {
    finishPumping = true;
  }

  @Override
  public void run() {

    try {

      // Wait until the file exists, or we have to abort.
      while (!outputFile.exists() && !finishPumping) {
        Thread.sleep(WATCH_INTERVAL);
      }

      // Check that we did not have abort before the file was created.
      if (outputFile.exists()) {
        try (InputStream inputStream = outputFile.getInputStream();
             OutputStream outputStream = output.getOutputStream();) {
          byte[] buffer = new byte[1024];
          while (!finishPumping || (inputStream.available() != 0)) {
            if (inputStream.available() != 0) {
              if (toSkip > 0) {
                toSkip -= inputStream.skip(toSkip);
              } else {
                int read = inputStream.read(buffer);
                if (read > 0) {
                  outputStream.write(buffer, 0, read);
                }
              }
            } else {
              Thread.sleep(WATCH_INTERVAL);
            }
          }
        }
      }
    } catch (IOException ex) {
      output.printOutLn("Failure reading or writing: " + ex.getMessage());
    } catch (InterruptedException ex) {
      // Don't do anything.
    }
  }
}