package uk.ac.vamsas.client.simpleclient;

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;
import java.nio.channels.ReadableByteChannel;
/**
 * File based Locking mechanism to get around some bizarre limitations of JarEntry seeking.
 * Abstract locks have a target file, to which access is controlled when a lock is held. Native locks on WindowsXP seem to conflict with Jar seek operations, so a file lock creates an advisory lock.
 * Method:
 * A lock file is created, if it doesn't already exist - the naming convention is TargetFile+suffixSeparator+_LockSuffix.
 * A lock is obtained by locking the lock file with a native lock. The NativeLock is used for this.
 * @author JimP
 *
 */
public class FileLock extends Lock {
  private File _lock = null;
  protected static String _LockSuffix="lck";
  private NativeLock advisory=null;
  /**
   * ensure that the _lock file exists
   * and create a lock
   */
  private boolean ensureLockFile(boolean block) {
    if (_lock==null)
      return false;
    if (advisory!=null && advisory.isLocked())
      return true;
    try { 
      advisory=new NativeLock(_lock, block);
    } catch (Exception e) {
      if (!_lock.exists()) {
        // advisory cannot be created. this is serious.
        log.fatal("Failed to create advisory lock file "+_lock,e);
        throw new Error("Failed to create advisory lock file "+_lock);
      }
    }
    return (advisory!=null) && advisory.isLocked();
  }
  /**
   * call to clear up a filelock file after its been made
   *
   */
  private void tidy() {
    if (_lock!=null) { 
      if ( advisory!=null) {
        // TODO: fix occasional exceptions raised here (usually on JVM shutdown)
        if (advisory.target!=null) {
           advisory.target.deleteOnExit(); // release will null the target
        }
        advisory.release(true);
      }
      advisory=null;
      _lock=null;
    }
  }
  /**
   * @param lockfile
   * @param block true means thread blocks until FileLock is obtained.
   */
  public FileLock(File lockfile, boolean block) {
    super(lockfile);
    // try and get a lock.
    try {
      _lock = make_lockForTarget(lockfile);
      if (!ensureLockFile(block)) {
        log.debug("Couldn't get lock on "+_lock);
        tidy();
        return;
      }
      // create target file ready to be written to if necessary.
      if (!lockfile.exists())
        if (!lockfile.createNewFile()) {
          log.warn("Failed to create locked file "+lockfile);
          return;
        }
      //openRaFile();
    } catch (FileNotFoundException e) {
      //
      log.debug("FileLock failed with target="+lockfile+" and lockfile suffix of "+_LockSuffix);
      //log.error("Error! Couldn't create a lockfile at "
      //  + lockfile.getAbsolutePath(), e);
    } catch (IOException e) {
      log.error("Error! Problems with IO when creating a lock on "
          + lockfile.getAbsolutePath(),e);
    }
  }
  /**
   * 
   * @param lockfile target of lock
   * @return file object that will be locked for lockfile
   */
  private File make_lockForTarget(File lockfile) {
    return new File(lockfile.getParentFile(), lockfile.getName()+"."+_LockSuffix);
  }
  private boolean openRaFile() throws IOException {
    if (target==null)
      return false;
    if (advisory==null || !advisory.isLocked())
      return false;
    if (rafile==null || rafile.getFD()==null || !rafile.getFD().valid()) {
      rafile=new RandomAccessFile(target,"rw");
    } else {
      if (log.isDebugEnabled())
        log.debug("Reusing existing RandomAccessFile on "+target);
    }
    return (rafile.getChannel()!=null) && rafile.getChannel().isOpen();
  }
  
  public boolean isLocked() {
    if (advisory != null) {
      if (advisory.isLocked())
        return true;
      advisory=null;
      if (log.isDebugEnabled())
        log.debug("Lockfile "+_lock+" unexpectedly deleted ?");
    }
    return false;
  }
  
  public void release() {
    release(true);
  }
  
  public void release(boolean closeChannel) {
    if (!isLocked())
      return;
    if (rafile!=null) {
      if (closeChannel) {
        try {
          rafile.close();
        } catch (Exception e) {
          log.debug("Unexpected exception whilst closing RandomAccessFile on "+target, e);
        }
        rafile=null; // do not hold reference to rafile anymore either
      }
      if (log.isDebugEnabled())
        log.debug("Releasing advisory lock on "+target);
      // TODO: LATER: verify this change in closeChannel semantics really is correct - ArchiveClient works correctly
    }
    tidy();
  }
  
  public FileInputStream getFileInputStream(boolean atStart) throws IOException {
    if (!isLocked()) {
      log.debug("Don't hold lock on "+target+" to get locked FileInputStream.");
      return null;
    }
    openRaFile();
    if (atStart)
      rafile.seek(0);
    return new FileInputStream(rafile.getFD());
  }
  
  
  public FileOutputStream getFileOutputStream(boolean clear) throws IOException {
    if (!isLocked()) {
      log.debug("Don't hold lock on "+target+" to get locked FileOutputStream.");
      return null;
    }
    openRaFile();
    if (clear) {
      rafile.seek(0);
      rafile.setLength(0);
    } else
      rafile.seek(rafile.length());
    return new LockedFileOutputStream(rafile.getFD());
  }
  
  
  public BufferedOutputStream getBufferedOutputStream(boolean clear) throws IOException {
    log.debug("Getting BufferedOutputStream (clear="+clear+")");
    FileOutputStream fos = getFileOutputStream(clear);
    if (fos!=null)
      return new BufferedOutputStream(fos);
    return null;
  }
  
  /* (non-Javadoc)
   * @see uk.ac.vamsas.client.simpleclient.Lock#getLength()
   */
  public long length() {
    if (isLocked()) {
      if (!target.exists()) {
        try {
          target.createNewFile();
        } catch (Exception e) {
          log.error("Invalid lock:Failed to create target file "+target);
          tidy();
          return -1;
        }
        return 0;
      }
      return target.length();
    }
    return -1;
  }
  protected void finalize() throws Throwable {
    release(true); // we explicitly lose the lock here.
    super.finalize();
  }
  public RandomAccessFile getRaFile() throws IOException {
    if (isLocked() && openRaFile()) {
      return rafile;
    }
    log.debug("Failed to getRaFile on target "+target);
    return null;
  }
  public FileChannel getRaChannel() throws IOException {
    if (isLocked() && openRaFile()) {
      return rafile.getChannel();
    }
    log.debug("Failed to getRaChannel on target "+target);
    return null;
  }
  public boolean isTargetLockFile(File afile) {
    if (isLocked())
    {
      if (target.equals(afile) || make_lockForTarget(target).equals(afile))
        return true;
    }
    return false;
  }
}