FileRepository: Add pack-based inserter implementation

Applications that use ObjectInserters to create lots of individual
objects may prefer to avoid cluttering up the object directory with
loose objects. Add a specialized inserter implementation that produces a
single pack file no matter how many objects. This inserter is loosely
based on the existing DfsInserter implementation, but is simpler since
we don't need to buffer blocks in memory before writing to storage.

An alternative for such applications would be to write out the loose
objects and then repack just those objects later. This operation is not
currently supported with the GC class, which always repacks existing
packs when compacting loose objects. This in turn requires more
CPU-intensive reachability checks and extra I/O to copy objects from old
packs to new packs.

So, the choice was between implementing a new variant of repack, or not
writing loose objects in the first place. The latter approach is likely
less code overall, and avoids unnecessary I/O at runtime.

The current implementation does not yet support newReader() for reading
back objects.

Change-Id: I2074418f4e65853b7113de5eaced3a6b037d1a17

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackInserterTest.java

@@ -0,0 +1,421 @@
+/*
+ * Copyright (C) 2017, 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.internal.storage.file;
+
+import static java.util.Comparator.comparing;
+import static java.util.stream.Collectors.toList;
+import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;
+import static org.eclipse.jgit.lib.Constants.OBJ_COMMIT;
+import static org.hamcrest.Matchers.greaterThan;
+import static org.hamcrest.Matchers.lessThan;
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertThat;
+import static org.junit.Assert.fail;
+
+import java.io.ByteArrayInputStream;
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.FileVisitResult;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.SimpleFileVisitor;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.function.Predicate;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.eclipse.jgit.dircache.DirCache;
+import org.eclipse.jgit.dircache.DirCacheBuilder;
+import org.eclipse.jgit.dircache.DirCacheEntry;
+import org.eclipse.jgit.junit.RepositoryTestCase;
+import org.eclipse.jgit.lib.CommitBuilder;
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.FileMode;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectLoader;
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.treewalk.CanonicalTreeParser;
+import org.junit.Before;
+import org.junit.Test;
+
+@SuppressWarnings("boxing")
+public class PackInserterTest extends RepositoryTestCase {
+	@Before
+	public void emptyAtSetUp() throws Exception {
+		assertEquals(0, listPacks().size());
+		assertNoObjects();
+	}
+
+	@Test
+	public void noFlush() throws Exception {
+		try (PackInserter ins = newInserter()) {
+			ins.insert(OBJ_BLOB, Constants.encode("foo contents"));
+			// No flush.
+		}
+		assertNoObjects();
+	}
+
+	@Test
+	public void flushEmptyPack() throws Exception {
+		try (PackInserter ins = newInserter()) {
+			ins.flush();
+		}
+		assertNoObjects();
+	}
+
+	@Test
+	public void singlePack() throws Exception {
+		ObjectId blobId;
+		byte[] blob = Constants.encode("foo contents");
+		ObjectId treeId;
+		ObjectId commitId;
+		byte[] commit;
+		try (PackInserter ins = newInserter()) {
+			blobId = ins.insert(OBJ_BLOB, blob);
+
+			DirCache dc = DirCache.newInCore();
+			DirCacheBuilder b = dc.builder();
+			DirCacheEntry dce = new DirCacheEntry("foo");
+			dce.setFileMode(FileMode.REGULAR_FILE);
+			dce.setObjectId(blobId);
+			b.add(dce);
+			b.finish();
+			treeId = dc.writeTree(ins);
+
+			CommitBuilder cb = new CommitBuilder();
+			cb.setTreeId(treeId);
+			cb.setAuthor(author);
+			cb.setCommitter(committer);
+			cb.setMessage("Commit message");
+			commit = cb.toByteArray();
+			commitId = ins.insert(cb);
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		List<PackFile> packs = listPacks();
+		assertEquals(1, packs.size());
+		assertEquals(3, packs.get(0).getObjectCount());
+
+		try (ObjectReader reader = db.newObjectReader()) {
+			assertBlob(reader, blobId, blob);
+
+			CanonicalTreeParser treeParser =
+					new CanonicalTreeParser(null, reader, treeId);
+			assertEquals("foo", treeParser.getEntryPathString());
+			assertEquals(blobId, treeParser.getEntryObjectId());
+
+			ObjectLoader commitLoader = reader.open(commitId);
+			assertEquals(OBJ_COMMIT, commitLoader.getType());
+			assertArrayEquals(commit, commitLoader.getBytes());
+		}
+	}
+
+	@Test
+	public void multiplePacks() throws Exception {
+		ObjectId blobId1;
+		ObjectId blobId2;
+		byte[] blob1 = Constants.encode("blob1");
+		byte[] blob2 = Constants.encode("blob2");
+
+		try (PackInserter ins = newInserter()) {
+			blobId1 = ins.insert(OBJ_BLOB, blob1);
+			ins.flush();
+			blobId2 = ins.insert(OBJ_BLOB, blob2);
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		List<PackFile> packs = listPacks();
+		assertEquals(2, packs.size());
+		assertEquals(1, packs.get(0).getObjectCount());
+		assertEquals(1, packs.get(1).getObjectCount());
+
+		try (ObjectReader reader = db.newObjectReader()) {
+			assertBlob(reader, blobId1, blob1);
+			assertBlob(reader, blobId2, blob2);
+		}
+	}
+
+	@Test
+	public void largeBlob() throws Exception {
+		ObjectId blobId;
+		byte[] blob = newLargeBlob();
+		try (PackInserter ins = newInserter()) {
+			assertThat(blob.length, greaterThan(ins.getBufferSize()));
+			blobId =
+					ins.insert(OBJ_BLOB, blob.length, new ByteArrayInputStream(blob));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		Collection<PackFile> packs = listPacks();
+		assertEquals(1, packs.size());
+		PackFile p = packs.iterator().next();
+		assertEquals(1, p.getObjectCount());
+
+		try (ObjectReader reader = db.newObjectReader()) {
+			assertBlob(reader, blobId, blob);
+		}
+	}
+
+	@Test
+	public void overwriteExistingPack() throws Exception {
+		ObjectId blobId;
+		byte[] blob = Constants.encode("foo contents");
+
+		try (PackInserter ins = newInserter()) {
+			blobId = ins.insert(OBJ_BLOB, blob);
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		List<PackFile> packs = listPacks();
+		assertEquals(1, packs.size());
+		PackFile pack = packs.get(0);
+		assertEquals(1, pack.getObjectCount());
+
+		String inode = getInode(pack.getPackFile());
+
+		try (PackInserter ins = newInserter()) {
+			ins.checkExisting(false);
+			assertEquals(blobId, ins.insert(OBJ_BLOB, blob));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		packs = listPacks();
+		assertEquals(1, packs.size());
+		pack = packs.get(0);
+		assertEquals(1, pack.getObjectCount());
+
+		if (inode != null) {
+			// Old file was overwritten with new file, although objects were
+			// equivalent.
+			assertNotEquals(inode, getInode(pack.getPackFile()));
+		}
+	}
+
+	@Test
+	public void checkExisting() throws Exception {
+		ObjectId blobId;
+		byte[] blob = Constants.encode("foo contents");
+
+		try (PackInserter ins = newInserter()) {
+			blobId = ins.insert(OBJ_BLOB, blob);
+			ins.insert(OBJ_BLOB, Constants.encode("another blob"));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		assertEquals(1, listPacks().size());
+
+		try (PackInserter ins = newInserter()) {
+			assertEquals(blobId, ins.insert(OBJ_BLOB, blob));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		assertEquals(1, listPacks().size());
+
+		try (PackInserter ins = newInserter()) {
+			ins.checkExisting(false);
+			assertEquals(blobId, ins.insert(OBJ_BLOB, blob));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		assertEquals(2, listPacks().size());
+
+		try (ObjectReader reader = db.newObjectReader()) {
+			assertBlob(reader, blobId, blob);
+		}
+	}
+
+	@Test
+	public void insertSmallInputStreamRespectsCheckExisting() throws Exception {
+		ObjectId blobId;
+		byte[] blob = Constants.encode("foo contents");
+		try (PackInserter ins = newInserter()) {
+			assertThat(blob.length, lessThan(ins.getBufferSize()));
+			blobId = ins.insert(OBJ_BLOB, blob);
+			ins.insert(OBJ_BLOB, Constants.encode("another blob"));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		assertEquals(1, listPacks().size());
+
+		try (PackInserter ins = newInserter()) {
+			assertEquals(blobId,
+					ins.insert(OBJ_BLOB, blob.length, new ByteArrayInputStream(blob)));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		assertEquals(1, listPacks().size());
+	}
+
+	@Test
+	public void insertLargeInputStreamBypassesCheckExisting() throws Exception {
+		ObjectId blobId;
+		byte[] blob = newLargeBlob();
+
+		try (PackInserter ins = newInserter()) {
+			assertThat(blob.length, greaterThan(ins.getBufferSize()));
+			blobId = ins.insert(OBJ_BLOB, blob);
+			ins.insert(OBJ_BLOB, Constants.encode("another blob"));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		assertEquals(1, listPacks().size());
+
+		try (PackInserter ins = newInserter()) {
+			assertEquals(blobId,
+					ins.insert(OBJ_BLOB, blob.length, new ByteArrayInputStream(blob)));
+			ins.flush();
+		}
+
+		assertPacksOnly();
+		assertEquals(2, listPacks().size());
+	}
+
+	private List<PackFile> listPacks() throws Exception {
+		List<PackFile> fromOpenDb = listPacks(db);
+		List<PackFile> reopened;
+		try (FileRepository db2 = new FileRepository(db.getDirectory())) {
+			reopened = listPacks(db2);
+		}
+		assertEquals(fromOpenDb.size(), reopened.size());
+		for (int i = 0 ; i < fromOpenDb.size(); i++) {
+			PackFile a = fromOpenDb.get(i);
+			PackFile b = reopened.get(i);
+			assertEquals(a.getPackName(), b.getPackName());
+			assertEquals(
+					a.getPackFile().getAbsolutePath(), b.getPackFile().getAbsolutePath());
+			assertEquals(a.getObjectCount(), b.getObjectCount());
+			a.getObjectCount();
+		}
+		return fromOpenDb;
+	}
+
+	private static List<PackFile> listPacks(FileRepository db) throws Exception {
+		return db.getObjectDatabase().getPacks().stream()
+				.sorted(comparing(PackFile::getPackName)).collect(toList());
+	}
+
+	private PackInserter newInserter() {
+		return db.getObjectDatabase().newPackInserter();
+	}
+
+	private static byte[] newLargeBlob() {
+		byte[] blob = new byte[10240];
+		for (int i = 0; i < blob.length; i++) {
+			blob[i] = (byte) ('0' + (i % 10));
+		}
+		return blob;
+	}
+
+	private static String getInode(File f) throws Exception {
+		BasicFileAttributes attrs = Files.readAttributes(
+				f.toPath(), BasicFileAttributes.class);
+		Object k = attrs.fileKey();
+		if (k == null) {
+			return null;
+		}
+		Pattern p = Pattern.compile("^\\(dev=[^,]*,ino=(\\d+)\\)$");
+		Matcher m = p.matcher(k.toString());
+		return m.matches() ? m.group(1) : null;
+	}
+
+	private static void assertBlob(ObjectReader reader, ObjectId id,
+			byte[] expected) throws Exception {
+		ObjectLoader loader = reader.open(id);
+		assertEquals(OBJ_BLOB, loader.getType());
+		assertEquals(expected.length, loader.getSize());
+		assertArrayEquals(expected, loader.getBytes());
+	}
+
+	private void assertPacksOnly() throws Exception {
+		new BadFileCollector(f -> !f.endsWith(".pack") && !f.endsWith(".idx"))
+				.assertNoBadFiles(db.getObjectDatabase().getDirectory());
+	}
+
+	private void assertNoObjects() throws Exception {
+		new BadFileCollector(f -> true)
+				.assertNoBadFiles(db.getObjectDatabase().getDirectory());
+	}
+
+	private static class BadFileCollector extends SimpleFileVisitor<Path> {
+		private final Predicate<String> badName;
+		private List<String> bad;
+
+		BadFileCollector(Predicate<String> badName) {
+			this.badName = badName;
+		}
+
+		void assertNoBadFiles(File f) throws IOException {
+			bad = new ArrayList<>();
+			Files.walkFileTree(f.toPath(), this);
+			if (!bad.isEmpty()) {
+				fail("unexpected files in object directory: " + bad);
+			}
+		}
+
+		@Override
+		public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) {
+			String name = file.getFileName().toString();
+			if (!attrs.isDirectory() && badName.test(name)) {
+				bad.add(name);
+			}
+			return FileVisitResult.CONTINUE;
+		}
+	}
+}

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

@@ -220,6 +220,16 @@ public class ObjectDirectory extends FileObjectDatabase {
 		return new ObjectDirectoryInserter(this, config);
 	}
 
+	/**
+	 * Create a new inserter that inserts all objects as pack files, not loose
+	 * objects.
+	 *
+	 * @return new inserter.
+	 */
+	public PackInserter newPackInserter() {
+		return new PackInserter(this);
+	}
+
 	@Override
 	public void close() {
 		unpackedObjectCache.clear();

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackInserter.java

@@ -0,0 +1,408 @@
+/*
+ * Copyright (C) 2017, 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.internal.storage.file;
+
+import static java.nio.file.StandardCopyOption.ATOMIC_MOVE;
+import static org.eclipse.jgit.lib.Constants.OBJECT_ID_LENGTH;
+
+import java.io.EOFException;
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.io.RandomAccessFile;
+import java.text.MessageFormat;
+import java.util.Collections;
+import java.util.List;
+import java.util.zip.CRC32;
+import java.util.zip.Deflater;
+import java.util.zip.DeflaterOutputStream;
+
+import org.eclipse.jgit.internal.JGitText;
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectIdOwnerMap;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.transport.PackParser;
+import org.eclipse.jgit.transport.PackedObjectInfo;
+import org.eclipse.jgit.util.BlockList;
+import org.eclipse.jgit.util.FileUtils;
+import org.eclipse.jgit.util.IO;
+import org.eclipse.jgit.util.NB;
+import org.eclipse.jgit.util.io.CountingOutputStream;
+import org.eclipse.jgit.util.sha1.SHA1;
+
+/**
+ * Object inserter that inserts one pack per call to {@link #flush()}, and never
+ * inserts loose objects.
+ */
+class PackInserter extends ObjectInserter {
+	/** Always produce version 2 indexes, to get CRC data. */
+	private static final int INDEX_VERSION = 2;
+
+	private final ObjectDirectory db;
+
+	private List<PackedObjectInfo> objectList;
+	private ObjectIdOwnerMap<PackedObjectInfo> objectMap;
+	private boolean rollback;
+	private boolean checkExisting = true;
+
+	private int compression = Deflater.BEST_COMPRESSION;
+	private File tmpPack;
+	private PackStream packOut;
+
+	PackInserter(ObjectDirectory db) {
+		this.db = db;
+	}
+
+	/**
+	 * @param check
+	 *            if false, will write out possibly-duplicate objects without
+	 *            first checking whether they exist in the repo; default is true.
+	 */
+	public void checkExisting(boolean check) {
+		checkExisting = check;
+	}
+
+	/**
+	 * @param compression
+	 *            compression level for zlib deflater.
+	 */
+	public void setCompressionLevel(int compression) {
+		this.compression = compression;
+	}
+
+	int getBufferSize() {
+		return buffer().length;
+	}
+
+	@Override
+	public ObjectId insert(int type, byte[] data, int off, int len)
+			throws IOException {
+		ObjectId id = idFor(type, data, off, len);
+		if (objectMap != null && objectMap.contains(id)) {
+			return id;
+		}
+		// Ignore loose objects, which are potentially unreachable.
+		if (checkExisting && db.hasPackedObject(id)) {
+			return id;
+		}
+
+		long offset = beginObject(type, len);
+		packOut.compress.write(data, off, len);
+		packOut.compress.finish();
+		return endObject(id, offset);
+	}
+
+	@Override
+	public ObjectId insert(int type, long len, InputStream in)
+			throws IOException {
+		byte[] buf = buffer();
+		if (len <= buf.length) {
+			IO.readFully(in, buf, 0, (int) len);
+			return insert(type, buf, 0, (int) len);
+		}
+
+		long offset = beginObject(type, len);
+		SHA1 md = digest();
+		md.update(Constants.encodedTypeString(type));
+		md.update((byte) ' ');
+		md.update(Constants.encodeASCII(len));
+		md.update((byte) 0);
+
+		while (0 < len) {
+			int n = in.read(buf, 0, (int) Math.min(buf.length, len));
+			if (n <= 0) {
+				throw new EOFException();
+			}
+			md.update(buf, 0, n);
+			packOut.compress.write(buf, 0, n);
+			len -= n;
+		}
+		packOut.compress.finish();
+		return endObject(md.toObjectId(), offset);
+	}
+
+	private long beginObject(int type, long len) throws IOException {
+		if (packOut == null) {
+			beginPack();
+		}
+		long offset = packOut.getOffset();
+		packOut.beginObject(type, len);
+		return offset;
+	}
+
+	private ObjectId endObject(ObjectId id, long offset) {
+		PackedObjectInfo obj = new PackedObjectInfo(id);
+		obj.setOffset(offset);
+		obj.setCRC((int) packOut.crc32.getValue());
+		objectList.add(obj);
+		objectMap.addIfAbsent(obj);
+		return id;
+	}
+
+	private static File idxFor(File packFile) {
+		String p = packFile.getName();
+		return new File(
+				packFile.getParentFile(),
+				p.substring(0, p.lastIndexOf('.')) + ".idx"); //$NON-NLS-1$
+	}
+
+	private void beginPack() throws IOException {
+		objectList = new BlockList<>();
+		objectMap = new ObjectIdOwnerMap<>();
+
+		rollback = true;
+		tmpPack = File.createTempFile("insert_", ".pack", db.getDirectory()); //$NON-NLS-1$ //$NON-NLS-2$
+		packOut = new PackStream(tmpPack);
+
+		// Write the header as though it were a single object pack.
+		packOut.write(packOut.hdrBuf, 0, writePackHeader(packOut.hdrBuf, 1));
+	}
+
+	private static int writePackHeader(byte[] buf, int objectCount) {
+		System.arraycopy(Constants.PACK_SIGNATURE, 0, buf, 0, 4);
+		NB.encodeInt32(buf, 4, 2); // Always use pack version 2.
+		NB.encodeInt32(buf, 8, objectCount);
+		return 12;
+	}
+
+	@Override
+	public PackParser newPackParser(InputStream in) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public ObjectReader newReader() {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void flush() throws IOException {
+		if (tmpPack == null) {
+			return;
+		}
+
+		if (packOut == null) {
+			throw new IOException();
+		}
+
+		byte[] packHash;
+		try {
+			packHash = packOut.finishPack();
+		} finally {
+			packOut.close();
+			packOut = null;
+		}
+
+		Collections.sort(objectList);
+		File tmpIdx = idxFor(tmpPack);
+		writePackIndex(tmpIdx, packHash, objectList);
+
+		File realPack = new File(
+				new File(db.getDirectory(), "pack"), //$NON-NLS-1$
+				"pack-" + computeName(objectList).name() + ".pack"); //$NON-NLS-1$ //$NON-NLS-2$
+		db.closeAllPackHandles(realPack);
+		tmpPack.setReadOnly();
+		FileUtils.rename(tmpPack, realPack, ATOMIC_MOVE);
+
+		File realIdx = idxFor(realPack);
+		tmpIdx.setReadOnly();
+		try {
+			FileUtils.rename(tmpIdx, realIdx, ATOMIC_MOVE);
+		} catch (IOException e) {
+			File newIdx = new File(
+					realIdx.getParentFile(), realIdx.getName() + ".new"); //$NON-NLS-1$
+			try {
+				FileUtils.rename(tmpIdx, newIdx, ATOMIC_MOVE);
+			} catch (IOException e2) {
+				newIdx = tmpIdx;
+				e = e2;
+			}
+			throw new IOException(MessageFormat.format(
+					JGitText.get().panicCantRenameIndexFile, newIdx,
+					realIdx), e);
+		}
+
+		db.openPack(realPack);
+		rollback = false;
+		clear();
+	}
+
+	private static void writePackIndex(File idx, byte[] packHash,
+			List<PackedObjectInfo> list) throws IOException {
+		try (OutputStream os = new FileOutputStream(idx)) {
+			PackIndexWriter w = PackIndexWriter.createVersion(os, INDEX_VERSION);
+			w.write(list, packHash);
+		}
+	}
+
+	private ObjectId computeName(List<PackedObjectInfo> list) {
+		SHA1 md = digest().reset();
+		byte[] buf = buffer();
+		for (PackedObjectInfo otp : list) {
+			otp.copyRawTo(buf, 0);
+			md.update(buf, 0, OBJECT_ID_LENGTH);
+		}
+		return ObjectId.fromRaw(md.digest());
+	}
+
+	@Override
+	public void close() {
+		try {
+			if (packOut != null) {
+				try {
+					packOut.close();
+				} catch (IOException err) {
+					// Ignore a close failure, the pack should be removed.
+				}
+			}
+			if (rollback && tmpPack != null) {
+				try {
+					FileUtils.delete(tmpPack);
+				} catch (IOException e) {
+					// Still delete idx.
+				}
+				try {
+					FileUtils.delete(idxFor(tmpPack));
+				} catch (IOException e) {
+					// Ignore error deleting temp idx.
+				}
+				rollback = false;
+			}
+		} finally {
+			clear();
+		}
+	}
+
+	private void clear() {
+		objectList = null;
+		objectMap = null;
+		tmpPack = null;
+		packOut = null;
+	}
+
+	private class PackStream extends OutputStream {
+		final byte[] hdrBuf;
+		final CRC32 crc32;
+		final DeflaterOutputStream compress;
+
+		private final RandomAccessFile file;
+		private final CountingOutputStream out;
+		private final Deflater deflater;
+
+		PackStream(File pack) throws IOException {
+			file = new RandomAccessFile(pack, "rw"); //$NON-NLS-1$
+			out = new CountingOutputStream(new FileOutputStream(file.getFD()));
+			deflater = new Deflater(compression);
+			compress = new DeflaterOutputStream(this, deflater, 8192);
+			hdrBuf = new byte[32];
+			crc32 = new CRC32();
+		}
+
+		long getOffset() {
+			return out.getCount();
+		}
+
+		void beginObject(int objectType, long length) throws IOException {
+			crc32.reset();
+			deflater.reset();
+			write(hdrBuf, 0, encodeTypeSize(objectType, length));
+		}
+
+		private int encodeTypeSize(int type, long rawLength) {
+			long nextLength = rawLength >>> 4;
+			hdrBuf[0] = (byte) ((nextLength > 0 ? 0x80 : 0x00) | (type << 4) | (rawLength & 0x0F));
+			rawLength = nextLength;
+			int n = 1;
+			while (rawLength > 0) {
+				nextLength >>>= 7;
+				hdrBuf[n++] = (byte) ((nextLength > 0 ? 0x80 : 0x00) | (rawLength & 0x7F));
+				rawLength = nextLength;
+			}
+			return n;
+		}
+
+		@Override
+		public void write(final int b) throws IOException {
+			hdrBuf[0] = (byte) b;
+			write(hdrBuf, 0, 1);
+		}
+
+		@Override
+		public void write(byte[] data, int off, int len) throws IOException {
+			crc32.update(data, off, len);
+			out.write(data, off, len);
+		}
+
+		byte[] finishPack() throws IOException {
+			// Overwrite placeholder header with actual object count, then hash.
+			file.seek(0);
+			write(hdrBuf, 0, writePackHeader(hdrBuf, objectList.size()));
+
+			byte[] buf = buffer();
+			SHA1 md = digest().reset();
+			file.seek(0);
+			while (true) {
+				int r = file.read(buf);
+				if (r < 0) {
+					break;
+				}
+				md.update(buf, 0, r);
+			}
+			byte[] packHash = md.digest();
+			out.write(packHash, 0, packHash.length);
+			return packHash;
+		}
+
+		@Override
+		public void close() throws IOException {
+			deflater.end();
+			out.close();
+			file.close();
+		}
+	}
+}