Allow PackWriter callers to manage the thread pool

By permitting the caller of PackWriter to select the Executor it
uses for task execution, we give the caller the ability to manage
the lifecycle of the thread pool, including reusing it across
concurrent pack generators.

This is the first step to supporting application thread pools
within Daemon or another managed service like Gerrit Code Review.

Change-Id: I96bee7b9c30ff9885f2bd261d0b6daaac713b5a4
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/DeltaTask.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright (C) 2010, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.storage.pack;
+
+import java.util.concurrent.Callable;
+
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.lib.ProgressMonitor;
+
+final class DeltaTask implements Callable<Object> {
+	private final PackWriter writer;
+
+	private final ObjectReader templateReader;
+
+	private final DeltaCache dc;
+
+	private final ProgressMonitor pm;
+
+	private final int batchSize;
+
+	private final int start;
+
+	private final ObjectToPack[] list;
+
+	DeltaTask(PackWriter writer, ObjectReader reader, DeltaCache dc,
+			ProgressMonitor pm, int batchSize, int start, ObjectToPack[] list) {
+		this.writer = writer;
+		this.templateReader = reader;
+		this.dc = dc;
+		this.pm = pm;
+		this.batchSize = batchSize;
+		this.start = start;
+		this.list = list;
+	}
+
+	public Object call() throws Exception {
+		final ObjectReader or = templateReader.newReader();
+		try {
+			DeltaWindow dw;
+			dw = new DeltaWindow(writer, dc, or);
+			dw.search(pm, list, start, batchSize);
+		} finally {
+			or.release();
+		}
+		return null;
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackWriter.java

@@ -58,8 +58,12 @@ import java.util.Collections;
 import java.util.Comparator;
 import java.util.Iterator;
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Executor;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import java.util.zip.Deflater;
 import java.util.zip.DeflaterOutputStream;
@@ -218,6 +222,8 @@ public class PackWriter {
 
 	private int threads = 1;
 
+	private Executor executor;
+
 	private boolean thin;
 
 	private boolean ignoreMissingUninteresting = true;
@@ -603,6 +609,10 @@ public class PackWriter {
 	 * During delta compression, if there are enough objects to be considered
 	 * the writer will start up concurrent threads and allow them to compress
 	 * different sections of the repository concurrently.
+	 * <p>
+	 * An application thread pool can be set by {@link #setExecutor(Executor)}.
+	 * If not set a temporary pool will be created by the writer, and torn down
+	 * automatically when compression is over.
 	 *
 	 * @param threads
 	 *            number of threads to use. If <= 0 the number of available
@@ -612,6 +622,22 @@ public class PackWriter {
 		this.threads = threads;
 	}
 
+	/**
+	 * Set the executor to use when using threads.
+	 * <p>
+	 * During delta compression if the executor is non-null jobs will be queued
+	 * up on it to perform delta compression in parallel. Aside from setting the
+	 * executor, the caller must set {@link #setThread(int)} to enable threaded
+	 * delta search.
+	 *
+	 * @param executor
+	 *            executor to use for threads. Set to null to create a temporary
+	 *            executor just for this writer.
+	 */
+	public void setExecutor(Executor executor) {
+		this.executor = executor;
+	}
+
 	/** @return true if this writer is producing a thin pack. */
 	public boolean isThin() {
 		return thin;
@@ -980,11 +1006,8 @@ public class PackWriter {
 			return;
 		}
 
-		final List<Throwable> errors = Collections
-				.synchronizedList(new ArrayList<Throwable>());
 		final DeltaCache dc = new ThreadSafeDeltaCache(this);
 		final ProgressMonitor pm = new ThreadSafeProgressMonitor(monitor);
-		final ExecutorService pool = Executors.newFixedThreadPool(threads);
 
 		// Guess at the size of batch we want. Because we don't really
 		// have a way for a thread to steal work from another thread if
@@ -995,6 +1018,7 @@ public class PackWriter {
 		if (estSize < 2 * getDeltaSearchWindowSize())
 			estSize = 2 * getDeltaSearchWindowSize();
 
+		final List<DeltaTask> myTasks = new ArrayList<DeltaTask>(threads * 2);
 		for (int i = 0; i < cnt;) {
 			final int start = i;
 			final int batchSize;
@@ -1019,27 +1043,24 @@ public class PackWriter {
 				batchSize = end - start;
 			}
 			i += batchSize;
-
-			pool.submit(new Runnable() {
-				public void run() {
-					try {
-						final ObjectReader or = reader.newReader();
-						try {
-							DeltaWindow dw;
-							dw = new DeltaWindow(PackWriter.this, dc, or);
-							dw.search(pm, list, start, batchSize);
-						} finally {
-							or.release();
-						}
-					} catch (Throwable err) {
-						errors.add(err);
-					}
-				}
-			});
+			myTasks.add(new DeltaTask(this, reader, dc, pm, batchSize, start, list));
 		}
 
-		// Tell the pool to stop.
+		final List<Throwable> errors = Collections
+				.synchronizedList(new ArrayList<Throwable>());
+		if (executor instanceof ExecutorService) {
+			// Caller supplied us a service, use it directly.
+			//
+			runTasks((ExecutorService) executor, myTasks, errors);
+
+		} else if (executor == null) {
+			// Caller didn't give us a way to run the tasks, spawn up a
+			// temporary thread pool and make sure it tears down cleanly.
 			//
+			ExecutorService pool = Executors.newFixedThreadPool(threads);
+			try {
+				runTasks(pool, myTasks, errors);
+			} finally {
 				pool.shutdown();
 				for (;;) {
 					try {
@@ -1050,8 +1071,38 @@ public class PackWriter {
 								JGitText.get().packingCancelledDuringObjectsWriting);
 					}
 				}
+			}
+		} else {
+			// The caller gave us an executor, but it might not do
+			// asynchronous execution.  Wrap everything and hope it
+			// can schedule these for us.
+			//
+			final CountDownLatch done = new CountDownLatch(myTasks.size());
+			for (final DeltaTask task : myTasks) {
+				executor.execute(new Runnable() {
+					public void run() {
+						try {
+							task.call();
+						} catch (Throwable failure) {
+							errors.add(failure);
+						} finally {
+							done.countDown();
+						}
+					}
+				});
+			}
+			try {
+				done.await();
+			} catch (InterruptedException ie) {
+				// We can't abort the other tasks as we have no handle.
+				// Cross our fingers and just break out anyway.
+				//
+				throw new IOException(
+						JGitText.get().packingCancelledDuringObjectsWriting);
+			}
+		}
 
-		// If any thread threw an error, try to report it back as
+		// If any task threw an error, try to report it back as
 		// though we weren't using a threaded search algorithm.
 		//
 		if (!errors.isEmpty()) {
@@ -1069,6 +1120,28 @@ public class PackWriter {
 		}
 	}
 
+	private void runTasks(ExecutorService pool, List<DeltaTask> tasks,
+			List<Throwable> errors) throws IOException {
+		List<Future<?>> futures = new ArrayList<Future<?>>(tasks.size());
+		for (DeltaTask task : tasks)
+			futures.add(pool.submit(task));
+
+		try {
+			for (Future<?> f : futures) {
+				try {
+					f.get();
+				} catch (ExecutionException failed) {
+					errors.add(failed.getCause());
+				}
+			}
+		} catch (InterruptedException ie) {
+			for (Future<?> f : futures)
+				f.cancel(true);
+			throw new IOException(
+					JGitText.get().packingCancelledDuringObjectsWriting);
+		}
+	}
+
 	private void writeObjects(ProgressMonitor writeMonitor, PackOutputStream out)
 			throws IOException {
 		for (List<ObjectToPack> list : objectsLists) {