/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.reef.runtime.local.process;

import org.apache.reef.runtime.common.evaluator.PIDStoreStartHandler;
import org.apache.reef.util.OSUtils;

import java.io.*;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * A runnable class that encapsulates a process.
 */
public final class RunnableProcess implements Runnable {

  private static final Logger LOG = Logger.getLogger(RunnableProcess.class.getName());

  private static final long DESTROY_WAIT_TIME = 100;

  /**
   * Name of the file used for STDERR redirection.
   */
  private final String standardErrorFileName;

  /**
   * Name of the file used for STDOUT redirection.
   */
  private final String standardOutFileName;

  /**
   * Command to execute.
   */
  private final List<String> command;

  /**
   * User supplied ID of this process.
   */
  private final String id;

  /**
   * The working folder in which the process runs. It is also where STDERR and STDOUT files will be deposited.
   */
  private final File folder;

  /**
   * The coarse-grained lock for state transition.
   */
  private final Lock stateLock = new ReentrantLock();

  private final Condition doneCond = stateLock.newCondition();

  /**
   * This will be informed of process start and stop.
   */
  private final RunnableProcessObserver processObserver;

  /**
   * The process.
   */
  private Process process;

  /**
   * The state of the process.
   */
  private RunnableProcessState state = RunnableProcessState.INIT;   // synchronized on stateLock

  /**
   * @param command the command to execute.
   * @param id The ID of the process. This is used to name files and in the logs created by this process.
   * @param folder The folder in which this will store its stdout and stderr output
   * @param processObserver will be informed of process state changes.
   * @param standardOutFileName The name of the file used for redirecting STDOUT
   * @param standardErrorFileName The name of the file used for redirecting STDERR
   */
  public RunnableProcess(
      final List<String> command,
      final String id,
      final File folder,
      final RunnableProcessObserver processObserver,
      final String standardOutFileName,
      final String standardErrorFileName) {

    this.processObserver = processObserver;
    this.command = Collections.unmodifiableList(expandEnvironmentVariables(command));
    this.id = id;
    this.folder = folder;

    assert this.folder.isDirectory();
    if (!this.folder.exists() && !this.folder.mkdirs()) {
      LOG.log(Level.WARNING, "Failed to create [{0}]", this.folder.getAbsolutePath());
    }

    this.standardOutFileName = standardOutFileName;
    this.standardErrorFileName = standardErrorFileName;

    LOG.log(Level.FINEST, "RunnableProcess ready.");
  }

  private static final Pattern ENV_REGEX = Pattern.compile("\\{\\{(\\w+)}}");

  /**
   * Replace {{ENV_VAR}} placeholders with the values of the corresponding environment variables.
   * @param command An input string with {{ENV_VAR}} placeholders
   * to be replaced with the values of the corresponding environment variables.
   * Replace unknown/unset variables with an empty string.
   * @return A new string with all the placeholders expanded.
   */
  public static String expandEnvironmentVariables(final String command) {

    final Matcher match = ENV_REGEX.matcher(command);
    final StringBuilder res = new StringBuilder(command.length());

    int i = 0;
    while (match.find()) {
      final String var = System.getenv(match.group(1));
      res.append(command.substring(i, match.start())).append(var == null ? "" : var);
      i = match.end();
    }

    return res.append(command.substring(i, command.length())).toString();
  }

  /**
   * Replace {{ENV_VAR}} placeholders with the values of the corresponding environment variables.
   * @param command An input list of strings with {{ENV_VAR}} placeholders
   * to be replaced with the values of the corresponding environment variables.
   * Replace unknown/unset variables with an empty string.
   * @return A new list of strings with all the placeholders expanded.
   */
  public static List<String> expandEnvironmentVariables(final List<String> command) {
    final ArrayList<String> res = new ArrayList<>(command.size());
    for (final String cmd : command) {
      res.add(expandEnvironmentVariables(cmd));
    }
    return res;
  }

  /**
   * Runs the configured process.
   * @throws IllegalStateException if the process is already running or has been running before.
   */
  @Override
  public void run() {

    this.stateLock.lock();

    try {

      if (this.state != RunnableProcessState.INIT) {
        throw new IllegalStateException("The RunnableProcess can't be reused");
      }

      // Setup the stdout and stderr destinations.
      final File errFile = new File(folder, standardErrorFileName);
      final File outFile = new File(folder, standardOutFileName);

      // Launch the process
      try {

        LOG.log(Level.FINEST,
            "Launching process \"{0}\"\nSTDERR can be found in {1}\nSTDOUT can be found in {2}",
            new Object[] {this.id, errFile.getAbsolutePath(), outFile.getAbsolutePath()});

        this.process = new ProcessBuilder()
            .command(this.command)
            .directory(this.folder)
            .redirectError(errFile)
            .redirectOutput(outFile)
            .start();

        this.setState(RunnableProcessState.RUNNING);
        this.processObserver.onProcessStarted(this.id);

      } catch (final IOException ex) {
        LOG.log(Level.SEVERE, "Unable to spawn process " + this.id + " with command " + this.command, ex);
      }

    } finally {
      this.stateLock.unlock();
    }

    try {

      // Wait for its completion
      LOG.log(Level.FINER, "Wait for process completion: {0}", this.id);
      final int returnValue = this.process.waitFor();
      this.processObserver.onProcessExit(this.id, returnValue);

      this.stateLock.lock();
      try {
        this.setState(RunnableProcessState.ENDED);
        this.doneCond.signalAll();
      } finally {
        this.stateLock.unlock();
      }

      LOG.log(Level.FINER, "Process \"{0}\" returned {1}", new Object[] {this.id, returnValue});

    } catch (final InterruptedException ex) {
      LOG.log(Level.SEVERE,
          "Interrupted while waiting for the process \"{0}\" to complete. Exception: {1}",
          new Object[] {this.id, ex});
    }
  }

  /**
   * Cancels the running process if it is running.
   */
  public void cancel() {

    this.stateLock.lock();

    try {

      if (this.state == RunnableProcessState.RUNNING) {
        this.process.destroy();
        if (!this.doneCond.await(DESTROY_WAIT_TIME, TimeUnit.MILLISECONDS)) {
          LOG.log(Level.FINE, "{0} milliseconds elapsed", DESTROY_WAIT_TIME);
        }
      }

      if (this.state == RunnableProcessState.RUNNING) {
        LOG.log(Level.WARNING, "The child process survived Process.destroy()");
        if (OSUtils.isUnix() || OSUtils.isWindows()) {
          LOG.log(Level.WARNING, "Attempting to kill the process via the kill command line");
          try {
            final long pid = readPID();
            OSUtils.kill(pid);
          } catch (final IOException | InterruptedException | NumberFormatException e) {
            LOG.log(Level.SEVERE, "Unable to kill the process.", e);
          }
        }
      }

    } catch (final InterruptedException ex) {
      LOG.log(Level.SEVERE,
          "Interrupted while waiting for the process \"{0}\" to complete. Exception: {1}",
          new Object[] {this.id, ex});
    } finally {
      this.stateLock.unlock();
    }
  }

  /**
   * @return the PID stored in the PID file.
   * @throws IOException if the file can't be read.
   */
  private long readPID() throws IOException {
    final String pidFileName = this.folder.getAbsolutePath() + "/" + PIDStoreStartHandler.PID_FILE_NAME;
    try (final BufferedReader r = new BufferedReader(
        new InputStreamReader(new FileInputStream(pidFileName), StandardCharsets.UTF_8))) {
      return Long.parseLong(r.readLine());
    }
  }

  /**
   * @return the ID of the process.
   */
  public String getId() {
    return this.id;
  }

  /**
   * @return the command given to the process.
   */
  public List<String> getCommand() {
    return this.command;
  }

  /**
   * Sets a new state for the process.
   * @param newState a new process state to transition to.
   * @throws IllegalStateException if the new state is illegal.
   */
  private void setState(final RunnableProcessState newState) {
    if (!this.state.isLegal(newState)) {
      throw new IllegalStateException("Transition from " + this.state + " to " + newState + " is illegal");
    }
    this.state = newState;
  }
}