BitmappedReachabilityChecker: Reachability check using bitmaps

The "basic" reachability check walks the graph starting from the tips
marking things as "uninteresting". If the target commit is marked as
"uninteresting" it was reached; it is reachable from those tips.

This requires a lot of walking and can be solved directly with bitmaps.
Most of the time the bitmaps are already calculated or a short walk
away.

This should improve the performance of reachability checks, for example
in Gitiles.

Change-Id: I83d33271f58d95d2dc9ed151967b3eda513c99f7
Signed-off-by: Ivan Frade <ifrade@google.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/BitmappedReachabilityCheckerTest.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright (C) 2019, Google LLC
+ * 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.revwalk;
+
+import static org.junit.Assert.assertNotNull;
+
+import org.eclipse.jgit.internal.storage.file.FileRepository;
+import org.eclipse.jgit.internal.storage.file.GC;
+import org.eclipse.jgit.junit.TestRepository;
+
+public class BitmappedReachabilityCheckerTest
+		extends ReachabilityCheckerTestCase {
+
+	@Override
+	protected ReachabilityChecker getChecker(
+			TestRepository<FileRepository> repository) throws Exception {
+		// GC generates the bitmaps
+		GC gc = new GC(repo.getRepository());
+		gc.setAuto(false);
+		gc.gc();
+
+		// This is null when the test didn't create any branch
+		assertNotNull("Probably the test didn't define any ref",
+				repo.getRevWalk().getObjectReader().getBitmapIndex());
+
+		return new BitmappedReachabilityChecker(repository.getRevWalk());
+	}
+
+}

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/BitmappedReachabilityChecker.java

@@ -0,0 +1,122 @@
+/*
+ * Copyright (C) 2019, Google LLC
+ * 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.revwalk;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.Optional;
+
+import org.eclipse.jgit.errors.IncorrectObjectTypeException;
+import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
+import org.eclipse.jgit.lib.NullProgressMonitor;
+
+/**
+ * Checks the reachability using bitmaps.
+ *
+ * @since 5.5
+ */
+public class BitmappedReachabilityChecker implements ReachabilityChecker {
+
+	private final RevWalk walk;
+
+	/**
+	 * @param walk
+	 *            walk on the repository to get or create the bitmaps for the
+	 *            commits. It must have bitmaps.
+	 * @throws AssertionError
+	 *             runtime exception if walk is over a repository without
+	 *             bitmaps
+	 * @throws IOException
+	 *             if the index or the object reader cannot be opened.
+	 */
+	public BitmappedReachabilityChecker(RevWalk walk)
+			throws IOException {
+		this.walk = walk;
+		if (walk.getObjectReader().getBitmapIndex() == null) {
+			throw new AssertionError(
+					"Trying to use bitmapped reachability check " //$NON-NLS-1$
+							+ "on a repository without bitmaps"); //$NON-NLS-1$
+		}
+	}
+
+	/**
+	 * Check all targets are reachable from the starters.
+	 * <p>
+	 * In this implementation, it is recommended to put the most popular
+	 * starters (e.g. refs/heads tips) at the beginning of the collection
+	 */
+	@Override
+	public Optional<RevCommit> areAllReachable(Collection<RevCommit> targets,
+			Collection<RevCommit> starters) throws MissingObjectException,
+			IncorrectObjectTypeException, IOException {
+		BitmapCalculator calculator = new BitmapCalculator(walk);
+
+		/**
+		 * Iterate over starters bitmaps and remove targets as they become
+		 * reachable.
+		 *
+		 * Building the total starters bitmap has the same cost (iterating over
+		 * all starters adding the bitmaps) and this gives us the chance to
+		 * shorcut the loop.
+		 *
+		 * This is based on the assuption that most of the starters will have
+		 * the reachability bitmap precalculated. If many require a walk, the
+		 * walk.reset() could start to take too much time.
+		 */
+		List<RevCommit> remainingTargets = new ArrayList<>(targets);
+		for (RevCommit starter : starters) {
+			BitmapBuilder starterBitmap = calculator.getBitmap(starter,
+					NullProgressMonitor.INSTANCE);
+			remainingTargets.removeIf(starterBitmap::contains);
+			if (remainingTargets.isEmpty()) {
+				return Optional.empty();
+			}
+		}
+
+		return Optional.of(remainingTargets.get(0));
+	}
+
+}