RevWalk support for shallow clones

StartGenerator now processes .git/shallow to have the
RevWalk stop for shallow commits.

See RevWalkShallowTest for tests.

Bug: 394543
CQ: 6908
Change-Id: Ia5af1dab3fe9c7888f44eeecab1e1bcf2e8e48fe
Signed-off-by: Chris Aniszczyk <zx@twitter.com>

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

@@ -48,7 +48,7 @@ import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
-
+import java.io.IOException;
 import java.util.List;
 
 import org.eclipse.jgit.lib.Constants;
@@ -58,7 +58,7 @@ import org.junit.Test;
 
 public class FooterLineTest extends RepositoryTestCase {
 	@Test
-	public void testNoFooters_EmptyBody() {
+	public void testNoFooters_EmptyBody() throws IOException {
 		final RevCommit commit = parse("");
 		final List<FooterLine> footers = commit.getFooterLines();
 		assertNotNull(footers);
@@ -66,7 +66,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testNoFooters_NewlineOnlyBody1() {
+	public void testNoFooters_NewlineOnlyBody1() throws IOException {
 		final RevCommit commit = parse("\n");
 		final List<FooterLine> footers = commit.getFooterLines();
 		assertNotNull(footers);
@@ -74,7 +74,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testNoFooters_NewlineOnlyBody5() {
+	public void testNoFooters_NewlineOnlyBody5() throws IOException {
 		final RevCommit commit = parse("\n\n\n\n\n");
 		final List<FooterLine> footers = commit.getFooterLines();
 		assertNotNull(footers);
@@ -82,7 +82,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testNoFooters_OneLineBodyNoLF() {
+	public void testNoFooters_OneLineBodyNoLF() throws IOException {
 		final RevCommit commit = parse("this is a commit");
 		final List<FooterLine> footers = commit.getFooterLines();
 		assertNotNull(footers);
@@ -90,7 +90,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testNoFooters_OneLineBodyWithLF() {
+	public void testNoFooters_OneLineBodyWithLF() throws IOException {
 		final RevCommit commit = parse("this is a commit\n");
 		final List<FooterLine> footers = commit.getFooterLines();
 		assertNotNull(footers);
@@ -98,7 +98,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testNoFooters_ShortBodyNoLF() {
+	public void testNoFooters_ShortBodyNoLF() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit");
 		final List<FooterLine> footers = commit.getFooterLines();
 		assertNotNull(footers);
@@ -106,7 +106,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testNoFooters_ShortBodyWithLF() {
+	public void testNoFooters_ShortBodyWithLF() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n");
 		final List<FooterLine> footers = commit.getFooterLines();
 		assertNotNull(footers);
@@ -114,7 +114,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testSignedOffBy_OneUserNoLF() {
+	public void testSignedOffBy_OneUserNoLF() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n" + "\n"
 				+ "Signed-off-by: A. U. Thor <a@example.com>");
 		final List<FooterLine> footers = commit.getFooterLines();
@@ -130,7 +130,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testSignedOffBy_OneUserWithLF() {
+	public void testSignedOffBy_OneUserWithLF() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n" + "\n"
 				+ "Signed-off-by: A. U. Thor <a@example.com>\n");
 		final List<FooterLine> footers = commit.getFooterLines();
@@ -146,7 +146,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testSignedOffBy_IgnoreWhitespace() {
+	public void testSignedOffBy_IgnoreWhitespace() throws IOException {
 		// We only ignore leading whitespace on the value, trailing
 		// is assumed part of the value.
 		//
@@ -165,7 +165,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testEmptyValueNoLF() {
+	public void testEmptyValueNoLF() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n" + "\n"
 				+ "Signed-off-by:");
 		final List<FooterLine> footers = commit.getFooterLines();
@@ -181,7 +181,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testEmptyValueWithLF() {
+	public void testEmptyValueWithLF() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n" + "\n"
 				+ "Signed-off-by:\n");
 		final List<FooterLine> footers = commit.getFooterLines();
@@ -197,7 +197,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testShortKey() {
+	public void testShortKey() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n" + "\n"
 				+ "K:V\n");
 		final List<FooterLine> footers = commit.getFooterLines();
@@ -213,7 +213,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testNonDelimtedEmail() {
+	public void testNonDelimtedEmail() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n" + "\n"
 				+ "Acked-by: re@example.com\n");
 		final List<FooterLine> footers = commit.getFooterLines();
@@ -229,7 +229,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testNotEmail() {
+	public void testNotEmail() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n" + "\n"
 				+ "Acked-by: Main Tain Er\n");
 		final List<FooterLine> footers = commit.getFooterLines();
@@ -245,7 +245,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testSignedOffBy_ManyUsers() {
+	public void testSignedOffBy_ManyUsers() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n"
 				+ "Not-A-Footer-Line: this line must not be read as a footer\n"
 				+ "\n" // paragraph break, now footers appear in final block
@@ -281,7 +281,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testSignedOffBy_SkipNonFooter() {
+	public void testSignedOffBy_SkipNonFooter() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n"
 				+ "Not-A-Footer-Line: this line must not be read as a footer\n"
 				+ "\n" // paragraph break, now footers appear in final block
@@ -314,7 +314,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testFilterFootersIgnoreCase() {
+	public void testFilterFootersIgnoreCase() throws IOException {
 		final RevCommit commit = parse("subject\n\nbody of commit\n"
 				+ "Not-A-Footer-Line: this line must not be read as a footer\n"
 				+ "\n" // paragraph break, now footers appear in final block
@@ -332,7 +332,7 @@ public class FooterLineTest extends RepositoryTestCase {
 	}
 
 	@Test
-	public void testMatchesBugId() {
+	public void testMatchesBugId() throws IOException {
 		final RevCommit commit = parse("this is a commit subject for test\n"
 				+ "\n" // paragraph break, now footers appear in final block
 				+ "Simple-Bug-Id: 42\n");
@@ -352,7 +352,7 @@ public class FooterLineTest extends RepositoryTestCase {
 		assertFalse("not CC", line.matches(FooterKey.CC));
 	}
 
-	private RevCommit parse(final String msg) {
+	private RevCommit parse(final String msg) throws IOException {
 		final StringBuilder buf = new StringBuilder();
 		buf.append("tree " + ObjectId.zeroId().name() + "\n");
 		buf.append("author A. U. Thor <a@example.com> 1 +0000\n");

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

@@ -0,0 +1,195 @@
+/*
+ * Copyright (C) 2012, Marc Strapetz <marc.strapetz@syntevo.com>
+ * 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.File;
+import java.io.IOException;
+
+import org.eclipse.jgit.junit.JGitTestUtil;
+import org.eclipse.jgit.lib.*;
+import org.junit.*;
+
+import static org.junit.Assert.*;
+
+public class RevWalkShallowTest extends RevWalkTestCase {
+
+	// Accessing ==============================================================
+
+	@Test
+	public void testDepth1() throws Exception {
+		final RevCommit a = commit();
+		final RevCommit b = commit(a);
+		final RevCommit c = commit(b);
+		final RevCommit d = commit(c);
+
+		createShallowFile(d);
+
+		rw.reset();
+		markStart(d);
+		assertCommit(d, rw.next());
+		assertNull(rw.next());
+	}
+
+	@Test
+	public void testDepth2() throws Exception {
+		final RevCommit a = commit();
+		final RevCommit b = commit(a);
+		final RevCommit c = commit(b);
+		final RevCommit d = commit(c);
+
+		createShallowFile(c);
+
+		rw.reset();
+		markStart(d);
+		assertCommit(d, rw.next());
+		assertCommit(c, rw.next());
+		assertNull(rw.next());
+	}
+
+	@Test
+	public void testDepth3() throws Exception {
+		final RevCommit a = commit();
+		final RevCommit b = commit(a);
+		final RevCommit c = commit(b);
+		final RevCommit d = commit(c);
+
+		createShallowFile(b);
+
+		rw.reset();
+		markStart(d);
+		assertCommit(d, rw.next());
+		assertCommit(c, rw.next());
+		assertCommit(b, rw.next());
+		assertNull(rw.next());
+	}
+
+	@Test
+	public void testMergeCommitOneParentShallow() throws Exception {
+		final RevCommit a = commit();
+		final RevCommit b = commit(a);
+		final RevCommit c = commit(b);
+		final RevCommit d = commit(b);
+		final RevCommit e = commit(d);
+		final RevCommit merge = commit(c, e);
+
+		createShallowFile(e);
+
+		rw.reset();
+		markStart(merge);
+		assertCommit(merge, rw.next());
+		assertCommit(e, rw.next());
+		assertCommit(c, rw.next());
+		assertCommit(b, rw.next());
+		assertCommit(a, rw.next());
+		assertNull(rw.next());
+	}
+
+	@Test
+	public void testMergeCommitEntirelyShallow() throws Exception {
+		final RevCommit a = commit();
+		final RevCommit b = commit(a);
+		final RevCommit c = commit(b);
+		final RevCommit d = commit(b);
+		final RevCommit e = commit(d);
+		final RevCommit merge = commit(c, e);
+
+		createShallowFile(c, e);
+
+		rw.reset();
+		markStart(merge);
+		assertCommit(merge, rw.next());
+		assertCommit(e, rw.next());
+		assertCommit(c, rw.next());
+		assertNull(rw.next());
+	}
+
+	@Test
+	public void testObjectDirectorySnapshot() throws Exception {
+		RevCommit a = commit();
+		RevCommit b = commit(a);
+		RevCommit c = commit(b);
+		RevCommit d = commit(c);
+
+		createShallowFile(d);
+
+		rw.reset();
+		markStart(d);
+		assertCommit(d, rw.next());
+		assertNull(rw.next());
+
+		rw = createRevWalk();
+		a = rw.lookupCommit(a);
+		b = rw.lookupCommit(b);
+		c = rw.lookupCommit(c);
+		d = rw.lookupCommit(d);
+
+		rw.reset();
+		markStart(d);
+		assertCommit(d, rw.next());
+		assertNull(rw.next());
+
+		createShallowFile(c);
+
+		rw = createRevWalk();
+		a = rw.lookupCommit(a);
+		b = rw.lookupCommit(b);
+		c = rw.lookupCommit(c);
+		d = rw.lookupCommit(d);
+
+		rw.reset();
+		markStart(d);
+		assertCommit(d, rw.next());
+		assertCommit(c, rw.next());
+		assertNull(rw.next());
+	}
+
+	private void createShallowFile(ObjectId... shallowCommits)
+			throws IOException {
+		final StringBuilder builder = new StringBuilder();
+		for (ObjectId commit : shallowCommits)
+			builder.append(commit.getName() + "\n");
+		JGitTestUtil.write(new File(rw.repository.getDirectory(), "shallow"),
+				builder.toString());
+	}
+}

org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties

@@ -423,6 +423,7 @@ sequenceTooLargeForDiffAlgorithm=Sequence too large for difference algorithm.
 serviceNotEnabledNoName=Service not enabled
 serviceNotPermitted={0} not permitted
 serviceNotPermittedNoName=Service not permitted
+shallowCommitsAlreadyInitialized=Shallow commits have already been initialized
 shortCompressedStreamAt=Short compressed stream at {0}
 shortReadOfBlock=Short read of block.
 shortReadOfOptionalDIRCExtensionExpectedAnotherBytes=Short read of optional DIRC extension {0}; expected another {1} bytes within the section.

org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java

@@ -484,6 +484,7 @@ public class JGitText extends TranslationBundle {
 	/***/ public String serviceNotEnabledNoName;
 	/***/ public String serviceNotPermitted;
 	/***/ public String serviceNotPermittedNoName;
+	/***/ public String shallowCommitsAlreadyInitialized;
 	/***/ public String shortCompressedStreamAt;
 	/***/ public String shortReadOfBlock;
 	/***/ public String shortReadOfOptionalDIRCExtensionExpectedAnotherBytes;

org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java

@@ -340,6 +340,9 @@ public final class Constants {
 	/** Name of the submodules file */
 	public static final String DOT_GIT_MODULES = ".gitmodules";
 
+	/** Name of the .git/shallow file */
+	public static final String SHALLOW = "shallow";
+
 	/**
 	 * Create a new digest function for objects.
 	 *

org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectReader.java

@@ -48,6 +48,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Set;
 
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
@@ -250,6 +251,14 @@ public abstract class ObjectReader {
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException;
 
+	/**
+	 * Returns IDs for those commits which should be considered as shallow.
+	 *
+	 * @return IDs of shallow commits
+	 * @throws IOException
+	 */
+	public abstract Set<ObjectId> getShallowCommits() throws IOException;
+
 	/**
 	 * Asynchronous object opening.
 	 *

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

@@ -80,7 +80,11 @@ public class RevCommit extends RevObject {
 	 *         available to the caller.
 	 */
 	public static RevCommit parse(byte[] raw) {
-		return parse(new RevWalk((ObjectReader) null), raw);
+		try {
+			return parse(new RevWalk((ObjectReader) null), raw);
+		} catch (IOException ex) {
+			throw new RuntimeException(ex);
+		}
 	}
 
 	/**
@@ -88,7 +92,7 @@ public class RevCommit extends RevObject {
 	 *
 	 * This method inserts the commit directly into the caller supplied revision
 	 * pool, making it appear as though the commit exists in the repository,
-	 * even if it doesn't.  The repository under the pool is not affected.
+	 * even if it doesn't. The repository under the pool is not affected.
 	 *
 	 * @param rw
 	 *            the revision pool to allocate the commit within. The commit's
@@ -97,8 +101,10 @@ public class RevCommit extends RevObject {
 	 *            the canonical formatted commit to be parsed.
 	 * @return the parsed commit, in an isolated revision pool that is not
 	 *         available to the caller.
+	 * @throws IOException
+	 *             in case of RevWalk initialization fails
 	 */
-	public static RevCommit parse(RevWalk rw, byte[] raw) {
+	public static RevCommit parse(RevWalk rw, byte[] raw) throws IOException {
 		ObjectInserter.Formatter fmt = new ObjectInserter.Formatter();
 		boolean retain = rw.isRetainBody();
 		rw.setRetainBody(true);
@@ -146,7 +152,11 @@ public class RevCommit extends RevObject {
 		}
 	}
 
-	void parseCanonical(final RevWalk walk, final byte[] raw) {
+	void parseCanonical(final RevWalk walk, final byte[] raw)
+			throws IOException {
+		if (!walk.shallowCommitsInitialized)
+			walk.initializeShallowCommits();
+
 		final MutableObjectId idBuffer = walk.idBuffer;
 		idBuffer.fromString(raw, 5);
 		tree = walk.lookupTree(idBuffer);

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

@@ -192,6 +192,8 @@ public class RevWalk implements Iterable<RevCommit> {
 
 	private boolean retainBody;
 
+	boolean shallowCommitsInitialized;
+
 	/**
 	 * Create a new revision walker for a given repository.
 	 *
@@ -1209,6 +1211,7 @@ public class RevWalk implements Iterable<RevCommit> {
 		roots.clear();
 		queue = new DateRevQueue();
 		pending = new StartGenerator(this);
+		shallowCommitsInitialized = false;
 	}
 
 	/**
@@ -1312,4 +1315,18 @@ public class RevWalk implements Iterable<RevCommit> {
 		if (carry != 0)
 			RevCommit.carryFlags(c, carry);
 	}
+
+	void initializeShallowCommits() throws IOException {
+		if (shallowCommitsInitialized)
+			throw new IllegalStateException(
+					JGitText.get().shallowCommitsAlreadyInitialized);
+
+		shallowCommitsInitialized = true;
+
+		if (reader == null)
+			return;
+
+		for (ObjectId id : reader.getShallowCommits())
+			lookupCommit(id).parents = RevCommit.NO_PARENTS;
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/storage/dfs/DfsReader.java

@@ -61,6 +61,7 @@ import java.util.HashSet;
 import java.util.Iterator;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Set;
 import java.util.concurrent.ExecutionException;
 import java.util.zip.DataFormatException;
 import java.util.zip.Inflater;
@@ -192,6 +193,11 @@ public final class DfsReader extends ObjectReader implements ObjectReuseAsIs {
 		throw new MissingObjectException(objectId.copy(), typeHint);
 	}
 
+	@Override
+	public Set<ObjectId> getShallowCommits() {
+		return Collections.emptySet();
+	}
+
 	private static final Comparator<FoundObject<?>> FOUND_OBJECT_SORT = new Comparator<FoundObject<?>>() {
 		public int compare(FoundObject<?> a, FoundObject<?> b) {
 			int cmp = a.packIndex - b.packIndex;

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/CachedObjectDirectory.java

@@ -141,6 +141,11 @@ class CachedObjectDirectory extends FileObjectDatabase {
 		return wrapped.getFS();
 	}
 
+	@Override
+	Set<ObjectId> getShallowCommits() throws IOException {
+		return wrapped.getShallowCommits();
+	}
+
 	@Override
 	Collection<? extends CachedPack> getCachedPacks() throws IOException {
 		return wrapped.getCachedPacks();

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/FileObjectDatabase.java

@@ -137,6 +137,8 @@ abstract class FileObjectDatabase extends ObjectDatabase {
 
 	abstract FS getFS();
 
+	abstract Set<ObjectId> getShallowCommits() throws IOException;
+
 	/**
 	 * Open an object from this database.
 	 * <p>

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/FileRepository.java

@@ -178,7 +178,8 @@ public class FileRepository extends Repository {
 		objectDatabase = new ObjectDirectory(repoConfig, //
 				options.getObjectDirectory(), //
 				options.getAlternateObjectDirectories(), //
-				getFS());
+				getFS(), //
+				new File(getDirectory(), Constants.SHALLOW));
 
 		if (objectDatabase.exists()) {
 			final long repositoryFormatVersion = getConfig().getLong(

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/ObjectDirectory.java

@@ -127,6 +127,12 @@ public class ObjectDirectory extends FileObjectDatabase {
 
 	private final UnpackedObjectCache unpackedObjectCache;
 
+	private final File shallowFile;
+
+	private FileSnapshot shallowFileSnapshot = FileSnapshot.DIRTY;
+
+	private Set<ObjectId> shallowCommitsIds;
+
 	/**
 	 * Initialize a reference to an on-disk object directory.
 	 *
@@ -139,11 +145,14 @@ public class ObjectDirectory extends FileObjectDatabase {
 	 * @param fs
 	 *            the file system abstraction which will be necessary to perform
 	 *            certain file system operations.
+	 * @param shallowFile
+	 *            file which contains IDs of shallow commits, null if shallow
+	 *            commits handling should be turned off
 	 * @throws IOException
 	 *             an alternate object cannot be opened.
 	 */
 	public ObjectDirectory(final Config cfg, final File dir,
-			File[] alternatePaths, FS fs) throws IOException {
+			File[] alternatePaths, FS fs, File shallowFile) throws IOException {
 		config = cfg;
 		objects = dir;
 		infoDirectory = new File(objects, "info");
@@ -154,6 +163,7 @@ public class ObjectDirectory extends FileObjectDatabase {
 		cachedPacks = new AtomicReference<CachedPackList>();
 		unpackedObjectCache = new UnpackedObjectCache();
 		this.fs = fs;
+		this.shallowFile = shallowFile;
 
 		alternates = new AtomicReference<AlternateHandle[]>();
 		if (alternatePaths != null) {
@@ -614,6 +624,30 @@ public class ObjectDirectory extends FileObjectDatabase {
 		return fs;
 	}
 
+	@Override
+	Set<ObjectId> getShallowCommits() throws IOException {
+		if (shallowFile == null || !shallowFile.isFile())
+			return Collections.emptySet();
+
+		if (shallowFileSnapshot == null
+				|| shallowFileSnapshot.isModified(shallowFile)) {
+			shallowCommitsIds = new HashSet<ObjectId>();
+
+			final BufferedReader reader = open(shallowFile);
+			try {
+				String line;
+				while ((line = reader.readLine()) != null)
+					shallowCommitsIds.add(ObjectId.fromString(line));
+			} finally {
+				reader.close();
+			}
+
+			shallowFileSnapshot = FileSnapshot.save(shallowFile);
+		}
+
+		return shallowCommitsIds;
+	}
+
 	private void insertPack(final PackFile pf) {
 		PackList o, n;
 		do {
@@ -829,7 +863,7 @@ public class ObjectDirectory extends FileObjectDatabase {
 			return new AlternateRepository(db);
 		}
 
-		ObjectDirectory db = new ObjectDirectory(config, objdir, null, fs);
+		ObjectDirectory db = new ObjectDirectory(config, objdir, null, fs, null);
 		return new AlternateHandle(db);
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/WindowCursor.java

@@ -52,6 +52,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 import java.util.zip.DataFormatException;
 import java.util.zip.Inflater;
 
@@ -130,6 +131,11 @@ final class WindowCursor extends ObjectReader implements ObjectReuseAsIs {
 		return ldr;
 	}
 
+	@Override
+	public Set<ObjectId> getShallowCommits() throws IOException {
+		return db.getShallowCommits();
+	}
+
 	public long getObjectSize(AnyObjectId objectId, int typeHint)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {