/*
    * Copyright (C) 2016, Google Inc. and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  
  package org.eclipse.jgit.util.io;
  
  import java.io.IOException;
  import java.io.InterruptedIOException;
  import java.io.OutputStream;
  import java.util.concurrent.ArrayBlockingQueue;
  import java.util.concurrent.Callable;
  import java.util.concurrent.ExecutionException;
  import java.util.concurrent.ExecutorService;
  import java.util.concurrent.Future;
  import java.util.concurrent.RejectedExecutionException;
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.ThreadPoolExecutor;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.TimeoutException;
  import java.util.concurrent.atomic.AtomicInteger;
  
  import org.eclipse.jgit.internal.JGitText;
  
  /**
   * OutputStream isolated from interrupts.
   * <p>
   * Wraps an OutputStream to prevent interrupts during writes from being made
   * visible to that stream instance. This works around buggy or difficult
   * OutputStream implementations like JSch that cannot gracefully handle an
   * interrupt during write.
   * <p>
   * Every write (or flush) requires a context switch to another thread. Callers
   * should wrap this stream with {@code BufferedOutputStream} using a suitable
   * buffer size to amortize the cost of context switches.
   *
   * @since 4.6
   */
  public class IsolatedOutputStream extends OutputStream {
  	private final OutputStream dst;
  	private final ExecutorService copier;
  	private Future<Void> pending;
  
  	/**
  	 * Wraps an OutputStream.
  	 *
  	 * @param out
  	 *            stream to send all writes to.
  	 */
  	public IsolatedOutputStream(OutputStream out) {
  		dst = out;
  		copier = new ThreadPoolExecutor(1, 1, 0, TimeUnit.MILLISECONDS,
  				new ArrayBlockingQueue<>(1), new NamedThreadFactory());
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public void write(int ch) throws IOException {
  		write(new byte[] { (byte) ch }, 0, 1);
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public void write(byte[] buf, int pos, int cnt)
  			throws IOException {
  		checkClosed();
  		execute(() -> {
  			dst.write(buf, pos, cnt);
  			return null;
  		});
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public void flush() throws IOException {
  		checkClosed();
  		execute(() -> {
  			dst.flush();
  			return null;
  		});
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public void close() throws IOException {
  		if (!copier.isShutdown()) {
  			try {
  				if (pending == null || tryCleanClose()) {
  					cleanClose();
  				} else {
  					dirtyClose();
  				}
  			} finally {
  				copier.shutdown();
 			}
 		}
 	}
 
 	private boolean tryCleanClose() {
 		/*
 		 * If the caller stopped waiting for a prior write or flush, they could
 		 * be trying to close a stream that is still in-use. Check if the prior
 		 * operation ended in a predictable way.
 		 */
 		try {
 			pending.get(0, TimeUnit.MILLISECONDS);
 			pending = null;
 			return true;
 		} catch (TimeoutException | InterruptedException e) {
 			return false;
 		} catch (ExecutionException e) {
 			pending = null;
 			return true;
 		}
 	}
 
 	private void cleanClose() throws IOException {
 		execute(() -> {
 			dst.close();
 			return null;
 		});
 	}
 
 	private void dirtyClose() throws IOException {
 		/*
 		 * Interrupt any still pending write or flush operation. This may cause
 		 * massive failures inside of the stream, but its going to be closed as
 		 * the next step.
 		 */
 		pending.cancel(true);
 
 		Future<Void> close;
 		try {
 			close = copier.submit(() -> {
 				dst.close();
 				return null;
 			});
 		} catch (RejectedExecutionException e) {
 			throw new IOException(e);
 		}
 		try {
 			close.get(200, TimeUnit.MILLISECONDS);
 		} catch (InterruptedException | TimeoutException e) {
 			close.cancel(true);
 			throw new IOException(e);
 		} catch (ExecutionException e) {
 			throw new IOException(e.getCause());
 		}
 	}
 
 	private void checkClosed() throws IOException {
 		if (copier.isShutdown()) {
 			throw new IOException(JGitText.get().closed);
 		}
 	}
 
 	private void execute(Callable<Void> task) throws IOException {
 		if (pending != null) {
 			// Check (and rethrow) any prior failed operation.
 			checkedGet(pending);
 		}
 		try {
 			pending = copier.submit(task);
 		} catch (RejectedExecutionException e) {
 			throw new IOException(e);
 		}
 		checkedGet(pending);
 		pending = null;
 	}
 
 	private static void checkedGet(Future<Void> future) throws IOException {
 		try {
 			future.get();
 		} catch (InterruptedException e) {
 			throw interrupted(e);
 		} catch (ExecutionException e) {
 			throw new IOException(e.getCause());
 		}
 	}
 
 	private static InterruptedIOException interrupted(InterruptedException c) {
 		InterruptedIOException e = new InterruptedIOException();
 		e.initCause(c);
 		return e;
 	}
 
 	private static class NamedThreadFactory implements ThreadFactory {
 		private static final AtomicInteger cnt = new AtomicInteger();
 
 		@Override
 		public Thread newThread(Runnable r) {
 			int n = cnt.incrementAndGet();
 			String name = IsolatedOutputStream.class.getSimpleName() + '-' + n;
 			return new Thread(r, name);
 		}
 	}
 }