RefTree: Store references in a Git tree

A group of updates can be applied by updating the tree in one step,
writing out a new root tree, and storing its SHA-1. If references
are stored in RefTrees, comparing two repositories is a matter of
checking if two SHA-1s are identical. Without RefTrees comparing two
repositories requires listing all references and comparing the sets.

Track the "refs/" directory as a root tree by storing references
that point directly at an object as a GITLINK entry in the tree.
For example "refs/heads/master" is written as "heads/master".

Annotated tags also store their peeled value with ^{} suffix, using
"tags/v1.0" and "tags/v1.0^{}" GITLINK entries.

Symbolic references are written as SYMLINK entries with the blob of
the symlink carrying the name of the symbolic reference target.

HEAD is outside of "refs/" namespace so it is stored as a special
"..HEAD" entry. This name is chosen because ".." is not valid in
a reference name and it almost looks like "../HEAD" which names
HEAD if the reader was inside of the "refs/" directory.

A new Command type is required to handle symbolic references and
peeled references.

Change-Id: Id47e5d4d32149a9e500854147edd7d93c1041a39

org.eclipse.jgit.test/META-INF/MANIFEST.MF

@@ -26,6 +26,7 @@ Import-Package: com.googlecode.javaewah;version="[0.7.9,0.8.0)",
  org.eclipse.jgit.internal.storage.dfs;version="[4.2.0,4.3.0)",
  org.eclipse.jgit.internal.storage.file;version="[4.2.0,4.3.0)",
  org.eclipse.jgit.internal.storage.pack;version="[4.2.0,4.3.0)",
+ org.eclipse.jgit.internal.storage.reftree;version="[4.2.0,4.3.0)",
  org.eclipse.jgit.junit;version="[4.2.0,4.3.0)",
  org.eclipse.jgit.lib;version="[4.2.0,4.3.0)",
  org.eclipse.jgit.merge;version="[4.2.0,4.3.0)",

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftree/RefTreeTest.java

@@ -0,0 +1,303 @@
+/*
+ * Copyright (C) 2016, 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.reftree;
+
+import static org.eclipse.jgit.lib.Constants.HEAD;
+import static org.eclipse.jgit.lib.Constants.R_HEADS;
+import static org.eclipse.jgit.lib.Constants.R_TAGS;
+import static org.eclipse.jgit.lib.Ref.Storage.LOOSE;
+import static org.eclipse.jgit.lib.Ref.Storage.NEW;
+import static org.eclipse.jgit.transport.ReceiveCommand.Result.LOCK_FAILURE;
+import static org.eclipse.jgit.transport.ReceiveCommand.Result.NOT_ATTEMPTED;
+import static org.eclipse.jgit.transport.ReceiveCommand.Result.REJECTED_OTHER_REASON;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Collections;
+
+import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.internal.JGitText;
+import org.eclipse.jgit.internal.storage.dfs.DfsRepositoryDescription;
+import org.eclipse.jgit.internal.storage.dfs.InMemoryRepository;
+import org.eclipse.jgit.junit.TestRepository;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectIdRef;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.SymbolicRef;
+import org.eclipse.jgit.revwalk.RevBlob;
+import org.eclipse.jgit.revwalk.RevTag;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.transport.ReceiveCommand;
+import org.junit.Before;
+import org.junit.Test;
+
+public class RefTreeTest {
+	private static final String R_MASTER = R_HEADS + "master";
+	private InMemoryRepository repo;
+	private TestRepository<InMemoryRepository> git;
+
+	@Before
+	public void setUp() throws IOException {
+		repo = new InMemoryRepository(new DfsRepositoryDescription("RefTree"));
+		git = new TestRepository<>(repo);
+	}
+
+	@Test
+	public void testEmptyTree() throws IOException {
+		RefTree tree = RefTree.newEmptyTree();
+		try (ObjectReader reader = repo.newObjectReader()) {
+			assertNull(HEAD, tree.exactRef(reader, HEAD));
+			assertNull("master", tree.exactRef(reader, R_MASTER));
+		}
+	}
+
+	@Test
+	public void testApplyThenReadMaster() throws Exception {
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob id = git.blob("A");
+		Command cmd = new Command(null, ref(R_MASTER, id));
+		assertTrue(tree.apply(Collections.singletonList(cmd)));
+		assertSame(NOT_ATTEMPTED, cmd.getResult());
+
+		try (ObjectReader reader = repo.newObjectReader()) {
+			Ref m = tree.exactRef(reader, R_MASTER);
+			assertNotNull(R_MASTER, m);
+			assertEquals(R_MASTER, m.getName());
+			assertEquals(id, m.getObjectId());
+			assertTrue("peeled", m.isPeeled());
+		}
+	}
+
+	@Test
+	public void testUpdateMaster() throws Exception {
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob id1 = git.blob("A");
+		Command cmd1 = new Command(null, ref(R_MASTER, id1));
+		assertTrue(tree.apply(Collections.singletonList(cmd1)));
+		assertSame(NOT_ATTEMPTED, cmd1.getResult());
+
+		RevBlob id2 = git.blob("B");
+		Command cmd2 = new Command(ref(R_MASTER, id1), ref(R_MASTER, id2));
+		assertTrue(tree.apply(Collections.singletonList(cmd2)));
+		assertSame(NOT_ATTEMPTED, cmd2.getResult());
+
+		try (ObjectReader reader = repo.newObjectReader()) {
+			Ref m = tree.exactRef(reader, R_MASTER);
+			assertNotNull(R_MASTER, m);
+			assertEquals(R_MASTER, m.getName());
+			assertEquals(id2, m.getObjectId());
+			assertTrue("peeled", m.isPeeled());
+		}
+	}
+
+	@Test
+	public void testHeadSymref() throws Exception {
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob id = git.blob("A");
+		Command cmd1 = new Command(null, ref(R_MASTER, id));
+		Command cmd2 = new Command(null, symref(HEAD, R_MASTER));
+		assertTrue(tree.apply(Arrays.asList(new Command[] { cmd1, cmd2 })));
+		assertSame(NOT_ATTEMPTED, cmd1.getResult());
+		assertSame(NOT_ATTEMPTED, cmd2.getResult());
+
+		try (ObjectReader reader = repo.newObjectReader()) {
+			Ref m = tree.exactRef(reader, HEAD);
+			assertNotNull(HEAD, m);
+			assertEquals(HEAD, m.getName());
+			assertTrue("symbolic", m.isSymbolic());
+			assertNotNull(m.getTarget());
+			assertEquals(R_MASTER, m.getTarget().getName());
+			assertEquals(id, m.getTarget().getObjectId());
+		}
+
+		// Writing flushes some buffers, re-read from blob.
+		ObjectId newId = write(tree);
+		try (ObjectReader reader = repo.newObjectReader();
+				RevWalk rw = new RevWalk(reader)) {
+			tree = RefTree.read(reader, rw.parseTree(newId));
+			Ref m = tree.exactRef(reader, HEAD);
+			assertEquals(R_MASTER, m.getTarget().getName());
+		}
+	}
+
+	@Test
+	public void testTagIsPeeled() throws Exception {
+		String name = "v1.0";
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob id = git.blob("A");
+		RevTag tag = git.tag(name, id);
+
+		String ref = R_TAGS + name;
+		Command cmd = create(ref, tag);
+		assertTrue(tree.apply(Collections.singletonList(cmd)));
+		assertSame(NOT_ATTEMPTED, cmd.getResult());
+
+		try (ObjectReader reader = repo.newObjectReader()) {
+			Ref m = tree.exactRef(reader, ref);
+			assertNotNull(ref, m);
+			assertEquals(ref, m.getName());
+			assertEquals(tag, m.getObjectId());
+			assertTrue("peeled", m.isPeeled());
+			assertEquals(id, m.getPeeledObjectId());
+		}
+	}
+
+	@Test
+	public void testApplyAlreadyExists() throws Exception {
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob a = git.blob("A");
+		Command cmd = new Command(null, ref(R_MASTER, a));
+		assertTrue(tree.apply(Collections.singletonList(cmd)));
+		ObjectId treeId = write(tree);
+
+		RevBlob b = git.blob("B");
+		Command cmd1 = create(R_MASTER, b);
+		Command cmd2 = create(R_MASTER, b);
+		assertFalse(tree.apply(Arrays.asList(new Command[] { cmd1, cmd2 })));
+		assertSame(LOCK_FAILURE, cmd1.getResult());
+		assertSame(REJECTED_OTHER_REASON, cmd2.getResult());
+		assertEquals(JGitText.get().transactionAborted, cmd2.getMessage());
+		assertEquals(treeId, write(tree));
+	}
+
+	@Test
+	public void testApplyWrongOldId() throws Exception {
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob a = git.blob("A");
+		Command cmd = new Command(null, ref(R_MASTER, a));
+		assertTrue(tree.apply(Collections.singletonList(cmd)));
+		ObjectId treeId = write(tree);
+
+		RevBlob b = git.blob("B");
+		RevBlob c = git.blob("C");
+		Command cmd1 = update(R_MASTER, b, c);
+		Command cmd2 = create(R_MASTER, b);
+		assertFalse(tree.apply(Arrays.asList(new Command[] { cmd1, cmd2 })));
+		assertSame(LOCK_FAILURE, cmd1.getResult());
+		assertSame(REJECTED_OTHER_REASON, cmd2.getResult());
+		assertEquals(JGitText.get().transactionAborted, cmd2.getMessage());
+		assertEquals(treeId, write(tree));
+	}
+
+	@Test
+	public void testApplyWrongOldIdButAlreadyCurrentIsNoOp() throws Exception {
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob a = git.blob("A");
+		Command cmd = new Command(null, ref(R_MASTER, a));
+		assertTrue(tree.apply(Collections.singletonList(cmd)));
+		ObjectId treeId = write(tree);
+
+		RevBlob b = git.blob("B");
+		cmd = update(R_MASTER, b, a);
+		assertTrue(tree.apply(Collections.singletonList(cmd)));
+		assertEquals(treeId, write(tree));
+	}
+
+	@Test
+	public void testApplyCannotCreateSubdirectory() throws Exception {
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob a = git.blob("A");
+		Command cmd = new Command(null, ref(R_MASTER, a));
+		assertTrue(tree.apply(Collections.singletonList(cmd)));
+		ObjectId treeId = write(tree);
+
+		RevBlob b = git.blob("B");
+		Command cmd1 = create(R_MASTER + "/fail", b);
+		assertFalse(tree.apply(Collections.singletonList(cmd1)));
+		assertSame(LOCK_FAILURE, cmd1.getResult());
+		assertEquals(treeId, write(tree));
+	}
+
+	@Test
+	public void testApplyCannotCreateParentRef() throws Exception {
+		RefTree tree = RefTree.newEmptyTree();
+		RevBlob a = git.blob("A");
+		Command cmd = new Command(null, ref(R_MASTER, a));
+		assertTrue(tree.apply(Collections.singletonList(cmd)));
+		ObjectId treeId = write(tree);
+
+		RevBlob b = git.blob("B");
+		Command cmd1 = create("refs/heads", b);
+		assertFalse(tree.apply(Collections.singletonList(cmd1)));
+		assertSame(LOCK_FAILURE, cmd1.getResult());
+		assertEquals(treeId, write(tree));
+	}
+
+	private static Ref ref(String name, ObjectId id) {
+		return new ObjectIdRef.PeeledNonTag(LOOSE, name, id);
+	}
+
+	private static Ref symref(String name, String dest) {
+		Ref d = new ObjectIdRef.PeeledNonTag(NEW, dest, null);
+		return new SymbolicRef(name, d);
+	}
+
+	private Command create(String name, ObjectId id)
+			throws MissingObjectException, IOException {
+		return update(name, ObjectId.zeroId(), id);
+	}
+
+	private Command update(String name, ObjectId oldId, ObjectId newId)
+			throws MissingObjectException, IOException {
+		try (RevWalk rw = new RevWalk(repo)) {
+			return new Command(rw, new ReceiveCommand(oldId, newId, name));
+		}
+	}
+
+	private ObjectId write(RefTree tree) throws IOException {
+		try (ObjectInserter ins = repo.newObjectInserter()) {
+			ObjectId id = tree.writeTree(ins);
+			ins.flush();
+			return id;
+		}
+	}
+}

org.eclipse.jgit/META-INF/MANIFEST.MF

@@ -68,6 +68,7 @@ Export-Package: org.eclipse.jgit.annotations;version="4.2.0",
    org.eclipse.jgit.pgm.test,
    org.eclipse.jgit.pgm",
  org.eclipse.jgit.internal.storage.pack;version="4.2.0";x-friends:="org.eclipse.jgit.junit,org.eclipse.jgit.test,org.eclipse.jgit.pgm",
+ org.eclipse.jgit.internal.storage.reftree;version="4.2.0";x-friends:="org.eclipse.jgit.junit,org.eclipse.jgit.test,org.eclipse.jgit.pgm",
  org.eclipse.jgit.lib;version="4.2.0";
   uses:="org.eclipse.jgit.revwalk,
    org.eclipse.jgit.treewalk.filter,

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheEntry.java

@@ -294,6 +294,23 @@ public class DirCacheEntry {
 		NB.encodeInt16(info, infoOffset + P_FLAGS, flags);
 	}
 
+	/**
+	 * Duplicate DirCacheEntry with same path and copied info.
+	 * <p>
+	 * The same path buffer is reused (avoiding copying), however a new info
+	 * buffer is created and its contents are copied.
+	 *
+	 * @param src
+	 *            entry to clone.
+	 * @since 4.2
+	 */
+	public DirCacheEntry(DirCacheEntry src) {
+		path = src.path;
+		info = new byte[INFO_LEN];
+		infoOffset = 0;
+		System.arraycopy(src.info, src.infoOffset, info, 0, INFO_LEN);
+	}
+
 	void write(final OutputStream os) throws IOException {
 		final int len = isExtended() ? INFO_LEN_EXTENDED : INFO_LEN;
 		final int pathLen = path.length;

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftree/Command.java

@@ -0,0 +1,290 @@
+/*
+ * Copyright (C) 2016, 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.reftree;
+
+import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;
+import static org.eclipse.jgit.lib.Constants.encode;
+import static org.eclipse.jgit.lib.FileMode.TYPE_GITLINK;
+import static org.eclipse.jgit.lib.FileMode.TYPE_SYMLINK;
+import static org.eclipse.jgit.lib.Ref.Storage.NETWORK;
+import static org.eclipse.jgit.transport.ReceiveCommand.Result.NOT_ATTEMPTED;
+
+import java.io.IOException;
+
+import org.eclipse.jgit.annotations.Nullable;
+import org.eclipse.jgit.dircache.DirCacheEntry;
+import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectIdRef;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.revwalk.RevObject;
+import org.eclipse.jgit.revwalk.RevTag;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.transport.ReceiveCommand;
+import org.eclipse.jgit.transport.ReceiveCommand.Result;
+
+/**
+ * Command to create, update or delete an entry inside a {@link RefTree}.
+ * <p>
+ * Unlike {@link ReceiveCommand} (which can only update a reference to an
+ * {@link ObjectId}), a RefTree Command can also create, modify or delete
+ * symbolic references to a target reference.
+ * <p>
+ * RefTree Commands may wrap a {@code ReceiveCommand} to allow callers to
+ * process an existing ReceiveCommand against a RefTree.
+ * <p>
+ * Commands should be passed into {@link RefTree#apply(java.util.Collection)}
+ * for processing.
+ */
+public class Command {
+	private final Ref oldRef;
+	private final Ref newRef;
+	private final ReceiveCommand cmd;
+	private Result result;
+
+	/**
+	 * Create a command to create, update or delete a reference.
+	 * <p>
+	 * At least one of {@code oldRef} or {@code newRef} must be supplied.
+	 *
+	 * @param oldRef
+	 *            expected value. Null if the ref should not exist.
+	 * @param newRef
+	 *            desired value, must be peeled if not null and not symbolic.
+	 *            Null to delete the ref.
+	 */
+	public Command(@Nullable Ref oldRef, @Nullable Ref newRef) {
+		this.oldRef = oldRef;
+		this.newRef = newRef;
+		this.cmd = null;
+		this.result = NOT_ATTEMPTED;
+
+		if (oldRef == null && newRef == null) {
+			throw new IllegalArgumentException();
+		}
+		if (newRef != null && !newRef.isPeeled() && !newRef.isSymbolic()) {
+			throw new IllegalArgumentException();
+		}
+		if (oldRef != null && newRef != null
+				&& !oldRef.getName().equals(newRef.getName())) {
+			throw new IllegalArgumentException();
+		}
+	}
+
+	/**
+	 * Construct a RefTree command wrapped around a ReceiveCommand.
+	 *
+	 * @param rw
+	 *            walk instance to peel the {@code newId}.
+	 * @param cmd
+	 *            command received from a push client.
+	 * @throws MissingObjectException
+	 *             {@code oldId} or {@code newId} is missing.
+	 * @throws IOException
+	 *             {@code oldId} or {@code newId} cannot be peeled.
+	 */
+	public Command(RevWalk rw, ReceiveCommand cmd)
+			throws MissingObjectException, IOException {
+		this.oldRef = toRef(rw, cmd.getOldId(), cmd.getRefName(), false);
+		this.newRef = toRef(rw, cmd.getNewId(), cmd.getRefName(), true);
+		this.cmd = cmd;
+	}
+
+	private static Ref toRef(RevWalk rw, ObjectId id, String name,
+			boolean mustExist) throws MissingObjectException, IOException {
+		if (ObjectId.zeroId().equals(id)) {
+			return null;
+		}
+
+		try {
+			RevObject o = rw.parseAny(id);
+			if (o instanceof RevTag) {
+				RevObject p = rw.peel(o);
+				return new ObjectIdRef.PeeledTag(NETWORK, name, id, p.copy());
+			}
+			return new ObjectIdRef.PeeledNonTag(NETWORK, name, id);
+		} catch (MissingObjectException e) {
+			if (mustExist) {
+				throw e;
+			}
+			return new ObjectIdRef.Unpeeled(NETWORK, name, id);
+		}
+	}
+
+	/** @return name of the reference affected by this command. */
+	public String getRefName() {
+		if (cmd != null) {
+			return cmd.getRefName();
+		} else if (newRef != null) {
+			return newRef.getName();
+		}
+		return oldRef.getName();
+	}
+
+	/**
+	 * Set the result of this command.
+	 *
+	 * @param result
+	 *            the command result.
+	 */
+	public void setResult(Result result) {
+		setResult(result, null);
+	}
+
+	/**
+	 * Set the result of this command.
+	 *
+	 * @param result
+	 *            the command result.
+	 * @param why
+	 *            optional message explaining the result status.
+	 */
+	public void setResult(Result result, @Nullable String why) {
+		if (cmd != null) {
+			cmd.setResult(result, why);
+		} else {
+			this.result = result;
+		}
+	}
+
+	/** @return result of executing this command. */
+	public Result getResult() {
+		return cmd != null ? cmd.getResult() : result;
+	}
+
+	/** @return optional message explaining command failure. */
+	@Nullable
+	public String getMessage() {
+		return cmd != null ? cmd.getMessage() : null;
+	}
+
+	/**
+	 * Old peeled reference.
+	 *
+	 * @return the old reference; null if the command is creating the reference.
+	 */
+	@Nullable
+	public Ref getOldRef() {
+		return oldRef;
+	}
+
+	/**
+	 * New peeled reference.
+	 *
+	 * @return the new reference; null if the command is deleting the reference.
+	 */
+	@Nullable
+	public Ref getNewRef() {
+		return newRef;
+	}
+
+	@Override
+	public String toString() {
+		StringBuilder s = new StringBuilder();
+		append(s, oldRef, "CREATE"); //$NON-NLS-1$
+		s.append(' ');
+		append(s, newRef, "DELETE"); //$NON-NLS-1$
+		s.append(' ').append(getRefName());
+		s.append(' ').append(getResult());
+		if (getMessage() != null) {
+			s.append(' ').append(getMessage());
+		}
+		return s.toString();
+	}
+
+	private static void append(StringBuilder s, Ref r, String nullName) {
+		if (r == null) {
+			s.append(nullName);
+		} else if (r.isSymbolic()) {
+			s.append(r.getTarget().getName());
+		} else {
+			ObjectId id = r.getObjectId();
+			if (id != null) {
+				s.append(id.name());
+			}
+		}
+	}
+
+	/**
+	 * Check the entry is consistent with either the old or the new ref.
+	 *
+	 * @param entry
+	 *            current entry; null if the entry does not exist.
+	 * @return true if entry matches {@link #getOldRef()} or
+	 *         {@link #getNewRef()}; otherwise false.
+	 */
+	boolean checkRef(@Nullable DirCacheEntry entry) {
+		if (entry != null && entry.getRawMode() == 0) {
+			entry = null;
+		}
+		return check(entry, oldRef) || check(entry, newRef);
+	}
+
+	private static boolean check(@Nullable DirCacheEntry cur,
+			@Nullable Ref exp) {
+		if (cur == null) {
+			// Does not exist, ok if oldRef does not exist.
+			return exp == null;
+		} else if (exp == null) {
+			// Expected to not exist, but currently exists, fail.
+			return false;
+		}
+
+		if (exp.isSymbolic()) {
+			String dst = exp.getTarget().getName();
+			return cur.getRawMode() == TYPE_SYMLINK
+					&& cur.getObjectId().equals(symref(dst));
+		}
+
+		return cur.getRawMode() == TYPE_GITLINK
+				&& cur.getObjectId().equals(exp.getObjectId());
+	}
+
+	static ObjectId symref(String s) {
+		@SuppressWarnings("resource")
+		ObjectInserter.Formatter fmt = new ObjectInserter.Formatter();
+		return fmt.idFor(OBJ_BLOB, encode(s));
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftree/RefTree.java

@@ -0,0 +1,410 @@
+/*
+ * Copyright (C) 2016, 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.reftree;
+
+import static org.eclipse.jgit.lib.Constants.OBJ_BLOB;
+import static org.eclipse.jgit.lib.Constants.R_REFS;
+import static org.eclipse.jgit.lib.Constants.encode;
+import static org.eclipse.jgit.lib.FileMode.GITLINK;
+import static org.eclipse.jgit.lib.FileMode.SYMLINK;
+import static org.eclipse.jgit.lib.FileMode.TYPE_GITLINK;
+import static org.eclipse.jgit.lib.FileMode.TYPE_SYMLINK;
+import static org.eclipse.jgit.lib.Ref.Storage.NEW;
+import static org.eclipse.jgit.lib.Ref.Storage.PACKED;
+import static org.eclipse.jgit.lib.RefDatabase.MAX_SYMBOLIC_REF_DEPTH;
+import static org.eclipse.jgit.transport.ReceiveCommand.Result.LOCK_FAILURE;
+import static org.eclipse.jgit.transport.ReceiveCommand.Result.NOT_ATTEMPTED;
+import static org.eclipse.jgit.transport.ReceiveCommand.Result.REJECTED_OTHER_REASON;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.Map;
+
+import org.eclipse.jgit.annotations.Nullable;
+import org.eclipse.jgit.dircache.DirCache;
+import org.eclipse.jgit.dircache.DirCacheBuilder;
+import org.eclipse.jgit.dircache.DirCacheEditor;
+import org.eclipse.jgit.dircache.DirCacheEditor.DeletePath;
+import org.eclipse.jgit.dircache.DirCacheEditor.PathEdit;
+import org.eclipse.jgit.dircache.DirCacheEntry;
+import org.eclipse.jgit.errors.CorruptObjectException;
+import org.eclipse.jgit.errors.DirCacheNameConflictException;
+import org.eclipse.jgit.errors.IncorrectObjectTypeException;
+import org.eclipse.jgit.errors.MissingObjectException;
+import org.eclipse.jgit.internal.JGitText;
+import org.eclipse.jgit.lib.FileMode;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectIdRef;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.SymbolicRef;
+import org.eclipse.jgit.revwalk.RevTree;
+import org.eclipse.jgit.util.RawParseUtils;
+
+/**
+ * Tree of references in the reference graph.
+ * <p>
+ * The root corresponds to the {@code "refs/"} subdirectory, for example the
+ * default reference {@code "refs/heads/master"} is stored at path
+ * {@code "heads/master"} in a {@code RefTree}.
+ * <p>
+ * Normal references are stored as {@link FileMode#GITLINK} tree entries. The
+ * ObjectId in the tree entry is the ObjectId the reference refers to.
+ * <p>
+ * Symbolic references are stored as {@link FileMode#SYMLINK} entries, with the
+ * blob storing the name of the target reference.
+ * <p>
+ * Annotated tags also store the peeled object using a {@code GITLINK} entry
+ * with the suffix <code>"^{}"</code>, for example {@code "tags/v1.0"} stores
+ * the annotated tag object, while <code>"tags/v1.0^{}"</code> stores the commit
+ * the tag annotates.
+ * <p>
+ * {@code HEAD} is a special case and stored as {@code "..HEAD"}.
+ */
+public class RefTree {
+	/** Suffix applied to GITLINK to indicate its the peeled value of a tag. */
+	public static final String PEELED_SUFFIX = "^{}"; //$NON-NLS-1$
+	static final String ROOT_DOTDOT = ".."; //$NON-NLS-1$
+
+	/**
+	 * Create an empty reference tree.
+	 *
+	 * @return a new empty reference tree.
+	 */
+	public static RefTree newEmptyTree() {
+		return new RefTree(DirCache.newInCore());
+	}
+
+	/**
+	 * Load a reference tree.
+	 *
+	 * @param reader
+	 *            reader to scan the reference tree with.
+	 * @param tree
+	 *            the tree to read.
+	 * @return the ref tree read from the commit.
+	 * @throws IOException
+	 *             the repository cannot be accessed through the reader.
+	 * @throws CorruptObjectException
+	 *             a tree object is corrupt and cannot be read.
+	 * @throws IncorrectObjectTypeException
+	 *             a tree object wasn't actually a tree.
+	 * @throws MissingObjectException
+	 *             a reference tree object doesn't exist.
+	 */
+	public static RefTree read(ObjectReader reader, RevTree tree)
+			throws MissingObjectException, IncorrectObjectTypeException,
+			CorruptObjectException, IOException {
+		return new RefTree(DirCache.read(reader, tree));
+	}
+
+	private DirCache contents;
+	private Map<ObjectId, String> pendingBlobs;
+
+	private RefTree(DirCache dc) {
+		this.contents = dc;
+	}
+
+	/**
+	 * Read one reference.
+	 * <p>
+	 * References are always returned peeled ({@link Ref#isPeeled()} is true).
+	 * If the reference points to an annotated tag, the returned reference will
+	 * be peeled and contain {@link Ref#getPeeledObjectId()}.
+	 * <p>
+	 * If the reference is a symbolic reference and the chain depth is less than
+	 * {@link org.eclipse.jgit.lib.RefDatabase#MAX_SYMBOLIC_REF_DEPTH} the
+	 * returned reference is resolved. If the chain depth is longer, the
+	 * symbolic reference is returned without resolving.
+	 *
+	 * @param reader
+	 *            to access objects necessary to read the requested reference.
+	 * @param name
+	 *            name of the reference to read.
+	 * @return the reference; null if it does not exist.
+	 * @throws IOException
+	 *             cannot read a symbolic reference target.
+	 */
+	@Nullable
+	public Ref exactRef(ObjectReader reader, String name) throws IOException {
+		Ref r = readRef(reader, name);
+		if (r == null) {
+			return null;
+		} else if (r.isSymbolic()) {
+			return resolve(reader, r, 0);
+		}
+
+		DirCacheEntry p = contents.getEntry(peeledPath(name));
+		if (p != null && p.getRawMode() == TYPE_GITLINK) {
+			return new ObjectIdRef.PeeledTag(PACKED, r.getName(),
+					r.getObjectId(), p.getObjectId());
+		}
+		return r;
+	}
+
+	private Ref readRef(ObjectReader reader, String name) throws IOException {
+		DirCacheEntry e = contents.getEntry(refPath(name));
+		return e != null ? toRef(reader, e, name) : null;
+	}
+
+	private Ref toRef(ObjectReader reader, DirCacheEntry e, String name)
+			throws IOException {
+		int mode = e.getRawMode();
+		if (mode == TYPE_GITLINK) {
+			ObjectId id = e.getObjectId();
+			return new ObjectIdRef.PeeledNonTag(PACKED, name, id);
+		}
+
+		if (mode == TYPE_SYMLINK) {
+			ObjectId id = e.getObjectId();
+			String n = pendingBlobs != null ? pendingBlobs.get(id) : null;
+			if (n == null) {
+				byte[] bin = reader.open(id, OBJ_BLOB).getCachedBytes();
+				n = RawParseUtils.decode(bin);
+			}
+			Ref dst = new ObjectIdRef.Unpeeled(NEW, n, null);
+			return new SymbolicRef(name, dst);
+		}
+
+		return null; // garbage file or something; not a reference.
+	}
+
+	private Ref resolve(ObjectReader reader, Ref ref, int depth)
+			throws IOException {
+		if (ref.isSymbolic() && depth < MAX_SYMBOLIC_REF_DEPTH) {
+			Ref r = readRef(reader, ref.getTarget().getName());
+			if (r == null) {
+				return ref;
+			}
+			Ref dst = resolve(reader, r, depth + 1);
+			return new SymbolicRef(ref.getName(), dst);
+		}
+		return ref;
+	}
+
+	/**
+	 * Attempt a batch of commands against this RefTree.
+	 * <p>
+	 * The batch is applied atomically, either all commands apply at once, or
+	 * they all reject and the RefTree is left unmodified.
+	 * <p>
+	 * On success (when this method returns {@code true}) the command results
+	 * are left as-is (probably {@code NOT_ATTEMPTED}). Result fields are set
+	 * only when this method returns {@code false} to indicate failure.
+	 *
+	 * @param cmdList
+	 *            to apply. All commands should still have result NOT_ATTEMPTED.
+	 * @return true if the commands applied; false if they were rejected.
+	 */
+	public boolean apply(Collection<Command> cmdList) {
+		try {
+			DirCacheEditor ed = contents.editor();
+			for (Command cmd : cmdList) {
+				apply(ed, cmd);
+			}
+			ed.finish();
+			return true;
+		} catch (DirCacheNameConflictException e) {
+			String r1 = refName(e.getPath1());
+			String r2 = refName(e.getPath2());
+			for (Command cmd : cmdList) {
+				if (r1.equals(cmd.getRefName())
+						|| r2.equals(cmd.getRefName())) {
+					cmd.setResult(LOCK_FAILURE);
+					break;
+				}
+			}
+			return abort(cmdList);
+		} catch (LockFailureException e) {
+			return abort(cmdList);
+		}
+	}
+
+	private void apply(DirCacheEditor ed, final Command cmd) {
+		String path = refPath(cmd.getRefName());
+		Ref oldRef = cmd.getOldRef();
+		final Ref newRef = cmd.getNewRef();
+
+		if (newRef == null) {
+			checkRef(contents.getEntry(path), cmd);
+			ed.add(new DeletePath(path));
+			cleanupPeeledRef(ed, oldRef);
+			return;
+		}
+
+		if (newRef.isSymbolic()) {
+			final String dst = newRef.getTarget().getName();
+			ed.add(new PathEdit(path) {
+				@Override
+				public void apply(DirCacheEntry ent) {
+					checkRef(ent, cmd);
+					ObjectId id = Command.symref(dst);
+					ent.setFileMode(SYMLINK);
+					ent.setObjectId(id);
+					if (pendingBlobs == null) {
+						pendingBlobs = new HashMap<>(4);
+					}
+					pendingBlobs.put(id, dst);
+				}
+			}.setReplace(false));
+			cleanupPeeledRef(ed, oldRef);
+			return;
+		}
+
+		ed.add(new PathEdit(path) {
+			@Override
+			public void apply(DirCacheEntry ent) {
+				checkRef(ent, cmd);
+				ent.setFileMode(GITLINK);
+				ent.setObjectId(newRef.getObjectId());
+			}
+		}.setReplace(false));
+
+		if (newRef.getPeeledObjectId() != null) {
+			ed.add(new PathEdit(peeledPath(newRef.getName())) {
+				@Override
+				public void apply(DirCacheEntry ent) {
+					ent.setFileMode(GITLINK);
+					ent.setObjectId(newRef.getPeeledObjectId());
+				}
+			}.setReplace(false));
+		} else {
+			cleanupPeeledRef(ed, oldRef);
+		}
+	}
+
+	private static void checkRef(@Nullable DirCacheEntry ent, Command cmd) {
+		if (!cmd.checkRef(ent)) {
+			cmd.setResult(LOCK_FAILURE);
+			throw new LockFailureException();
+		}
+	}
+
+	private static void cleanupPeeledRef(DirCacheEditor ed, Ref ref) {
+		if (ref != null && !ref.isSymbolic()
+				&& (!ref.isPeeled() || ref.getPeeledObjectId() != null)) {
+			ed.add(new DeletePath(peeledPath(ref.getName())));
+		}
+	}
+
+	private static boolean abort(Iterable<Command> cmdList) {
+		for (Command cmd : cmdList) {
+			if (cmd.getResult() == NOT_ATTEMPTED) {
+				reject(cmd, JGitText.get().transactionAborted);
+			}
+		}
+		return false;
+	}
+
+	private static void reject(Command cmd, String msg) {
+		cmd.setResult(REJECTED_OTHER_REASON, msg);
+	}
+
+	/**
+	 * Convert a path name in a RefTree to the reference name known by Git.
+	 *
+	 * @param path
+	 *            name read from the RefTree structure, for example
+	 *            {@code "heads/master"}.
+	 * @return reference name for the path, {@code "refs/heads/master"}.
+	 */
+	public static String refName(String path) {
+		if (path.startsWith(ROOT_DOTDOT)) {
+			return path.substring(2);
+		}
+		return R_REFS + path;
+	}
+
+	private static String refPath(String name) {
+		if (name.startsWith(R_REFS)) {
+			return name.substring(R_REFS.length());
+		}
+		return ROOT_DOTDOT + name;
+	}
+
+	private static String peeledPath(String name) {
+		return refPath(name) + PEELED_SUFFIX;
+	}
+
+	/**
+	 * Write this reference tree.
+	 *
+	 * @param inserter
+	 *            inserter to use when writing trees to the object database.
+	 *            Caller is responsible for flushing the inserter before trying
+	 *            to read the objects, or exposing them through a reference.
+	 * @return the top level tree.
+	 * @throws IOException
+	 *             a tree could not be written.
+	 */
+	public ObjectId writeTree(ObjectInserter inserter) throws IOException {
+		if (pendingBlobs != null) {
+			for (String s : pendingBlobs.values()) {
+				inserter.insert(OBJ_BLOB, encode(s));
+			}
+			pendingBlobs = null;
+		}
+		return contents.writeTree(inserter);
+	}
+
+	/** @return a deep copy of this RefTree. */
+	public RefTree copy() {
+		RefTree r = new RefTree(DirCache.newInCore());
+		DirCacheBuilder b = r.contents.builder();
+		for (int i = 0; i < contents.getEntryCount(); i++) {
+			b.add(new DirCacheEntry(contents.getEntry(i)));
+		}
+		b.finish();
+		if (pendingBlobs != null) {
+			r.pendingBlobs = new HashMap<>(pendingBlobs);
+		}
+		return r;
+	}
+
+	private static class LockFailureException extends RuntimeException {
+		private static final long serialVersionUID = 1L;
+	}
+}

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

@@ -82,8 +82,10 @@ public abstract class RefDatabase {
 	 * <p>
 	 * If the reference is nested deeper than this depth, the implementation
 	 * should either fail, or at least claim the reference does not exist.
+	 *
+	 * @since 4.2
 	 */
-	protected static final int MAX_SYMBOLIC_REF_DEPTH = 5;
+	public static final int MAX_SYMBOLIC_REF_DEPTH = 5;
 
 	/** Magic value for {@link #getRefs(String)} to return all references. */
 	public static final String ALL = "";//$NON-NLS-1$