/*
    * Copyright (C) 2007, Dave Watson <dwatson@mimvista.com>
    * Copyright (C) 2009-2010, Google Inc.
    * Copyright (C) 2007, Robin Rosenberg <robin.rosenberg@dewire.com>
    * Copyright (C) 2006, Shawn O. Pearce <spearce@spearce.org> and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
   *
   * SPDX-License-Identifier: BSD-3-Clause
   */
  
  package org.eclipse.jgit.internal.storage.file;
  
  import static java.nio.charset.StandardCharsets.UTF_8;
  import static org.eclipse.jgit.lib.Constants.HEAD;
  import static org.eclipse.jgit.lib.Constants.LOGS;
  import static org.eclipse.jgit.lib.Constants.OBJECT_ID_STRING_LENGTH;
  import static org.eclipse.jgit.lib.Constants.PACKED_REFS;
  import static org.eclipse.jgit.lib.Constants.R_HEADS;
  import static org.eclipse.jgit.lib.Constants.R_REFS;
  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.lib.Ref.Storage.PACKED;
  
  import java.io.BufferedReader;
  import java.io.File;
  import java.io.FileInputStream;
  import java.io.FileNotFoundException;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.InterruptedIOException;
  import java.nio.file.DirectoryNotEmptyException;
  import java.nio.file.Files;
  import java.nio.file.Path;
  import java.security.DigestInputStream;
  import java.security.MessageDigest;
  import java.text.MessageFormat;
  import java.util.Arrays;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Map;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.atomic.AtomicReference;
  import java.util.concurrent.locks.ReentrantLock;
  import java.util.stream.Stream;
  
  import org.eclipse.jgit.annotations.NonNull;
  import org.eclipse.jgit.annotations.Nullable;
  import org.eclipse.jgit.errors.InvalidObjectIdException;
  import org.eclipse.jgit.errors.LockFailedException;
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.errors.ObjectWritingException;
  import org.eclipse.jgit.events.RefsChangedEvent;
  import org.eclipse.jgit.internal.JGitText;
  import org.eclipse.jgit.lib.ConfigConstants;
  import org.eclipse.jgit.lib.Constants;
  import org.eclipse.jgit.lib.ObjectId;
  import org.eclipse.jgit.lib.ObjectIdRef;
  import org.eclipse.jgit.lib.Ref;
  import org.eclipse.jgit.lib.RefComparator;
  import org.eclipse.jgit.lib.RefDatabase;
  import org.eclipse.jgit.lib.RefUpdate;
  import org.eclipse.jgit.lib.RefWriter;
  import org.eclipse.jgit.lib.Repository;
  import org.eclipse.jgit.lib.SymbolicRef;
  import org.eclipse.jgit.revwalk.RevObject;
  import org.eclipse.jgit.revwalk.RevTag;
  import org.eclipse.jgit.revwalk.RevWalk;
  import org.eclipse.jgit.util.FS;
  import org.eclipse.jgit.util.FileUtils;
  import org.eclipse.jgit.util.IO;
  import org.eclipse.jgit.util.RawParseUtils;
  import org.eclipse.jgit.util.RefList;
  import org.eclipse.jgit.util.RefMap;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  /**
   * Traditional file system based {@link org.eclipse.jgit.lib.RefDatabase}.
   * <p>
   * This is the classical reference database representation for a Git repository.
   * References are stored in two formats: loose, and packed.
   * <p>
   * Loose references are stored as individual files within the {@code refs/}
   * directory. The file name matches the reference name and the file contents is
   * the current {@link org.eclipse.jgit.lib.ObjectId} in string form.
   * <p>
   * Packed references are stored in a single text file named {@code packed-refs}.
   * In the packed format, each reference is stored on its own line. This file
   * reduces the number of files needed for large reference spaces, reducing the
   * overall size of a Git repository on disk.
   */
  public class RefDirectory extends RefDatabase {
 	private static final Logger LOG = LoggerFactory
 			.getLogger(RefDirectory.class);
 
 	/** Magic string denoting the start of a symbolic reference file. */
 	public static final String SYMREF = "ref: "; //$NON-NLS-1$
 
 	/** Magic string denoting the header of a packed-refs file. */
 	public static final String PACKED_REFS_HEADER = "# pack-refs with:"; //$NON-NLS-1$
 
 	/** If in the header, denotes the file has peeled data. */
 	public static final String PACKED_REFS_PEELED = " peeled"; //$NON-NLS-1$
 
 	/** The names of the additional refs supported by this class */
 	private static final String[] additionalRefsNames = new String[] {
 			Constants.MERGE_HEAD, Constants.FETCH_HEAD, Constants.ORIG_HEAD,
 			Constants.CHERRY_PICK_HEAD };
 
 	@SuppressWarnings("boxing")
 	private static final List<Integer> RETRY_SLEEP_MS =
 			Collections.unmodifiableList(Arrays.asList(0, 100, 200, 400, 800, 1600));
 
 	private final FileRepository parent;
 
 	private final File gitDir;
 
 	final File refsDir;
 
 	final File packedRefsFile;
 
 	final File logsDir;
 
 	final File logsRefsDir;
 
 	/**
 	 * Immutable sorted list of loose references.
 	 * <p>
 	 * Symbolic references in this collection are stored unresolved, that is
 	 * their target appears to be a new reference with no ObjectId. These are
 	 * converted into resolved references during a get operation, ensuring the
 	 * live value is always returned.
 	 */
 	private final AtomicReference<RefList<LooseRef>> looseRefs = new AtomicReference<>();
 
 	/** Immutable sorted list of packed references. */
 	final AtomicReference<PackedRefList> packedRefs = new AtomicReference<>();
 
 	/**
 	 * Lock for coordinating operations within a single process that may contend
 	 * on the {@code packed-refs} file.
 	 * <p>
 	 * All operations that write {@code packed-refs} must still acquire a
 	 * {@link LockFile} on {@link #packedRefsFile}, even after they have acquired
 	 * this lock, since there may be multiple {@link RefDirectory} instances or
 	 * other processes operating on the same repo on disk.
 	 * <p>
 	 * This lock exists so multiple threads in the same process can wait in a fair
 	 * queue without trying, failing, and retrying to acquire the on-disk lock. If
 	 * {@code RepositoryCache} is used, this lock instance will be used by all
 	 * threads.
 	 */
 	final ReentrantLock inProcessPackedRefsLock = new ReentrantLock(true);
 
 	/**
 	 * Number of modifications made to this database.
 	 * <p>
 	 * This counter is incremented when a change is made, or detected from the
 	 * filesystem during a read operation.
 	 */
 	private final AtomicInteger modCnt = new AtomicInteger();
 
 	/**
 	 * Last {@link #modCnt} that we sent to listeners.
 	 * <p>
 	 * This value is compared to {@link #modCnt}, and a notification is sent to
 	 * the listeners only when it differs.
 	 */
 	private final AtomicInteger lastNotifiedModCnt = new AtomicInteger();
 
 	private List<Integer> retrySleepMs = RETRY_SLEEP_MS;
 
 	RefDirectory(FileRepository db) {
 		final FS fs = db.getFS();
 		parent = db;
 		gitDir = db.getDirectory();
 		refsDir = fs.resolve(gitDir, R_REFS);
 		logsDir = fs.resolve(gitDir, LOGS);
 		logsRefsDir = fs.resolve(gitDir, LOGS + '/' + R_REFS);
 		packedRefsFile = fs.resolve(gitDir, PACKED_REFS);
 
 		looseRefs.set(RefList.<LooseRef> emptyList());
 		packedRefs.set(NO_PACKED_REFS);
 	}
 
 	Repository getRepository() {
 		return parent;
 	}
 
 	ReflogWriter newLogWriter(boolean force) {
 		return new ReflogWriter(this, force);
 	}
 
 	/**
 	 * Locate the log file on disk for a single reference name.
 	 *
 	 * @param name
 	 *            name of the ref, relative to the Git repository top level
 	 *            directory (so typically starts with refs/).
 	 * @return the log file location.
 	 */
 	public File logFor(String name) {
 		if (name.startsWith(R_REFS)) {
 			name = name.substring(R_REFS.length());
 			return new File(logsRefsDir, name);
 		}
 		return new File(logsDir, name);
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public void create() throws IOException {
 		FileUtils.mkdir(refsDir);
 		FileUtils.mkdir(new File(refsDir, R_HEADS.substring(R_REFS.length())));
 		FileUtils.mkdir(new File(refsDir, R_TAGS.substring(R_REFS.length())));
 		newLogWriter(false).create();
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public void close() {
 		clearReferences();
 	}
 
 	private void clearReferences() {
 		looseRefs.set(RefList.<LooseRef> emptyList());
 		packedRefs.set(NO_PACKED_REFS);
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public void refresh() {
 		super.refresh();
 		clearReferences();
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public boolean isNameConflicting(String name) throws IOException {
 		RefList<Ref> packed = getPackedRefs();
 		RefList<LooseRef> loose = getLooseRefs();
 
 		// Cannot be nested within an existing reference.
 		int lastSlash = name.lastIndexOf('/');
 		while (0 < lastSlash) {
 			String needle = name.substring(0, lastSlash);
 			if (loose.contains(needle) || packed.contains(needle))
 				return true;
 			lastSlash = name.lastIndexOf('/', lastSlash - 1);
 		}
 
 		// Cannot be the container of an existing reference.
 		String prefix = name + '/';
 		int idx;
 
 		idx = -(packed.find(prefix) + 1);
 		if (idx < packed.size() && packed.get(idx).getName().startsWith(prefix))
 			return true;
 
 		idx = -(loose.find(prefix) + 1);
 		if (idx < loose.size() && loose.get(idx).getName().startsWith(prefix))
 			return true;
 
 		return false;
 	}
 
 	private RefList<LooseRef> getLooseRefs() {
 		final RefList<LooseRef> oldLoose = looseRefs.get();
 
 		LooseScanner scan = new LooseScanner(oldLoose);
 		scan.scan(ALL);
 
 		RefList<LooseRef> loose;
 		if (scan.newLoose != null) {
 			loose = scan.newLoose.toRefList();
 			if (looseRefs.compareAndSet(oldLoose, loose))
 				modCnt.incrementAndGet();
 		} else
 			loose = oldLoose;
 		return loose;
 	}
 
 	@Nullable
 	private Ref readAndResolve(String name, RefList<Ref> packed) throws IOException {
 		try {
 			Ref ref = readRef(name, packed);
 			if (ref != null) {
 				ref = resolve(ref, 0, null, null, packed);
 			}
 			return ref;
 		} catch (IOException e) {
 			if (name.contains("/") //$NON-NLS-1$
 					|| !(e.getCause() instanceof InvalidObjectIdException)) {
 				throw e;
 			}
 
 			// While looking for a ref outside of refs/ (e.g., 'config'), we
 			// found a non-ref file (e.g., a config file) instead.  Treat this
 			// as a ref-not-found condition.
 			return null;
 		}
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public Ref exactRef(String name) throws IOException {
 		try {
 			return readAndResolve(name, getPackedRefs());
 		} finally {
 			fireRefsChanged();
 		}
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	@NonNull
 	public Map<String, Ref> exactRef(String... refs) throws IOException {
 		try {
 			RefList<Ref> packed = getPackedRefs();
 			Map<String, Ref> result = new HashMap<>(refs.length);
 			for (String name : refs) {
 				Ref ref = readAndResolve(name, packed);
 				if (ref != null) {
 					result.put(name, ref);
 				}
 			}
 			return result;
 		} finally {
 			fireRefsChanged();
 		}
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	@Nullable
 	public Ref firstExactRef(String... refs) throws IOException {
 		try {
 			RefList<Ref> packed = getPackedRefs();
 			for (String name : refs) {
 				Ref ref = readAndResolve(name, packed);
 				if (ref != null) {
 					return ref;
 				}
 			}
 			return null;
 		} finally {
 			fireRefsChanged();
 		}
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public Map<String, Ref> getRefs(String prefix) throws IOException {
 		final RefList<LooseRef> oldLoose = looseRefs.get();
 		LooseScanner scan = new LooseScanner(oldLoose);
 		scan.scan(prefix);
 		final RefList<Ref> packed = getPackedRefs();
 
 		RefList<LooseRef> loose;
 		if (scan.newLoose != null) {
 			scan.newLoose.sort();
 			loose = scan.newLoose.toRefList();
 			if (looseRefs.compareAndSet(oldLoose, loose))
 				modCnt.incrementAndGet();
 		} else
 			loose = oldLoose;
 		fireRefsChanged();
 
 		RefList.Builder<Ref> symbolic = scan.symbolic;
 		for (int idx = 0; idx < symbolic.size();) {
 			final Ref symbolicRef = symbolic.get(idx);
 			final Ref resolvedRef = resolve(symbolicRef, 0, prefix, loose, packed);
 			if (resolvedRef != null && resolvedRef.getObjectId() != null) {
 				symbolic.set(idx, resolvedRef);
 				idx++;
 			} else {
 				// A broken symbolic reference, we have to drop it from the
 				// collections the client is about to receive. Should be a
 				// rare occurrence so pay a copy penalty.
 				symbolic.remove(idx);
 				final int toRemove = loose.find(symbolicRef.getName());
 				if (0 <= toRemove)
 					loose = loose.remove(toRemove);
 			}
 		}
 		symbolic.sort();
 
 		return new RefMap(prefix, packed, upcast(loose), symbolic.toRefList());
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public List<Ref> getAdditionalRefs() throws IOException {
 		List<Ref> ret = new LinkedList<>();
 		for (String name : additionalRefsNames) {
 			Ref r = exactRef(name);
 			if (r != null)
 				ret.add(r);
 		}
 		return ret;
 	}
 
 	@SuppressWarnings("unchecked")
 	private RefList<Ref> upcast(RefList<? extends Ref> loose) {
 		return (RefList<Ref>) loose;
 	}
 
 	private class LooseScanner {
 		private final RefList<LooseRef> curLoose;
 
 		private int curIdx;
 
 		final RefList.Builder<Ref> symbolic = new RefList.Builder<>(4);
 
 		RefList.Builder<LooseRef> newLoose;
 
 		LooseScanner(RefList<LooseRef> curLoose) {
 			this.curLoose = curLoose;
 		}
 
 		void scan(String prefix) {
 			if (ALL.equals(prefix)) {
 				scanOne(HEAD);
 				scanTree(R_REFS, refsDir);
 
 				// If any entries remain, they are deleted, drop them.
 				if (newLoose == null && curIdx < curLoose.size())
 					newLoose = curLoose.copy(curIdx);
 
 			} else if (prefix.startsWith(R_REFS) && prefix.endsWith("/")) { //$NON-NLS-1$
 				curIdx = -(curLoose.find(prefix) + 1);
 				File dir = new File(refsDir, prefix.substring(R_REFS.length()));
 				scanTree(prefix, dir);
 
 				// Skip over entries still within the prefix; these have
 				// been removed from the directory.
 				while (curIdx < curLoose.size()) {
 					if (!curLoose.get(curIdx).getName().startsWith(prefix))
 						break;
 					if (newLoose == null)
 						newLoose = curLoose.copy(curIdx);
 					curIdx++;
 				}
 
 				// Keep any entries outside of the prefix space, we
 				// do not know anything about their status.
 				if (newLoose != null) {
 					while (curIdx < curLoose.size())
 						newLoose.add(curLoose.get(curIdx++));
 				}
 			}
 		}
 
 		private boolean scanTree(String prefix, File dir) {
 			final String[] entries = dir.list(LockFile.FILTER);
 			if (entries == null) // not a directory or an I/O error
 				return false;
 			if (0 < entries.length) {
 				for (int i = 0; i < entries.length; ++i) {
 					String e = entries[i];
 					File f = new File(dir, e);
 					if (f.isDirectory())
 						entries[i] += '/';
 				}
 				Arrays.sort(entries);
 				for (String name : entries) {
 					if (name.charAt(name.length() - 1) == '/')
 						scanTree(prefix + name, new File(dir, name));
 					else
 						scanOne(prefix + name);
 				}
 			}
 			return true;
 		}
 
 		private void scanOne(String name) {
 			LooseRef cur;
 
 			if (curIdx < curLoose.size()) {
 				do {
 					cur = curLoose.get(curIdx);
 					int cmp = RefComparator.compareTo(cur, name);
 					if (cmp < 0) {
 						// Reference is not loose anymore, its been deleted.
 						// Skip the name in the new result list.
 						if (newLoose == null)
 							newLoose = curLoose.copy(curIdx);
 						curIdx++;
 						cur = null;
 						continue;
 					}
 
 					if (cmp > 0) // Newly discovered loose reference.
 						cur = null;
 					break;
 				} while (curIdx < curLoose.size());
 			} else
 				cur = null; // Newly discovered loose reference.
 
 			LooseRef n;
 			try {
 				n = scanRef(cur, name);
 			} catch (IOException notValid) {
 				n = null;
 			}
 
 			if (n != null) {
 				if (cur != n && newLoose == null)
 					newLoose = curLoose.copy(curIdx);
 				if (newLoose != null)
 					newLoose.add(n);
 				if (n.isSymbolic())
 					symbolic.add(n);
 			} else if (cur != null) {
 				// Tragically, this file is no longer a loose reference.
 				// Kill our cached entry of it.
 				if (newLoose == null)
 					newLoose = curLoose.copy(curIdx);
 			}
 
 			if (cur != null)
 				curIdx++;
 		}
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public Ref" href="../../../../../../org/eclipse/jgit/lib/Ref.html#Ref">Ref peel(Ref ref) throws IOException {
 		final Ref leaf = ref.getLeaf();
 		if (leaf.isPeeled() || leaf.getObjectId() == null)
 			return ref;
 
 		ObjectIdRef newLeaf = doPeel(leaf);
 
 		// Try to remember this peeling in the cache, so we don't have to do
 		// it again in the future, but only if the reference is unchanged.
 		if (leaf.getStorage().isLoose()) {
 			RefList<LooseRef> curList = looseRefs.get();
 			int idx = curList.find(leaf.getName());
 			if (0 <= idx && curList.get(idx) == leaf) {
 				LooseRef asPeeled = ((LooseRef) leaf).peel(newLeaf);
 				RefList<LooseRef> newList = curList.set(idx, asPeeled);
 				looseRefs.compareAndSet(curList, newList);
 			}
 		}
 
 		return recreate(ref, newLeaf);
 	}
 
 	private ObjectIdRef doPeel(Ref leaf) throws MissingObjectException,
 			IOException {
 		try (RevWalkvwalk/RevWalk.html#RevWalk">RevWalk rw = new RevWalk(getRepository())) {
 			RevObject obj = rw.parseAny(leaf.getObjectId());
 			if (obj instanceof RevTag) {
 				return new ObjectIdRef.PeeledTag(leaf.getStorage(), leaf
 						.getName(), leaf.getObjectId(), rw.peel(obj).copy());
 			}
 			return new ObjectIdRef.PeeledNonTag(leaf.getStorage(),
 					leaf.getName(), leaf.getObjectId());
 		}
 	}
 
 	private static Refef="../../../../../../org/eclipse/jgit/lib/Ref.html#Ref">Ref recreate(Ref old, ObjectIdRef leaf) {
 		if (old.isSymbolic()) {
 			Ref dst = recreate(old.getTarget(), leaf);
 			return new SymbolicRef(old.getName(), dst);
 		}
 		return leaf;
 	}
 
 	void storedSymbolicRef(RefDirectoryUpdate u, FileSnapshot snapshot,
 			String target) {
 		putLooseRef(newSymbolicRef(snapshot, u.getRef().getName(), target));
 		fireRefsChanged();
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public RefDirectoryUpdate newUpdate(String name, boolean detach)
 			throws IOException {
 		boolean detachingSymbolicRef = false;
 		final RefList<Ref> packed = getPackedRefs();
 		Ref ref = readRef(name, packed);
 		if (ref != null)
 			ref = resolve(ref, 0, null, null, packed);
 		if (ref == null)
 			ref = new ObjectIdRef.Unpeeled(NEW, name, null);
 		else {
 			detachingSymbolicRef = detach && ref.isSymbolic();
 		}
 		RefDirectoryUpdate refDirUpdate = new RefDirectoryUpdate(this, ref);
 		if (detachingSymbolicRef)
 			refDirUpdate.setDetachingSymbolicRef();
 		return refDirUpdate;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public RefDirectoryRename newRename(String fromName, String toName)
 			throws IOException {
 		RefDirectoryUpdate from = newUpdate(fromName, false);
 		RefDirectoryUpdate to = newUpdate(toName, false);
 		return new RefDirectoryRename(from, to);
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public PackedBatchRefUpdate newBatchUpdate() {
 		return new PackedBatchRefUpdate(this);
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public boolean performsAtomicTransactions() {
 		return true;
 	}
 
 	void stored(RefDirectoryUpdate update, FileSnapshot snapshot) {
 		final ObjectId target = update.getNewObjectId().copy();
 		final Ref leaf = update.getRef().getLeaf();
 		putLooseRef(new LooseUnpeeled(snapshot, leaf.getName(), target));
 	}
 
 	private void putLooseRef(LooseRef ref) {
 		RefList<LooseRef> cList, nList;
 		do {
 			cList = looseRefs.get();
 			nList = cList.put(ref);
 		} while (!looseRefs.compareAndSet(cList, nList));
 		modCnt.incrementAndGet();
 		fireRefsChanged();
 	}
 
 	void delete(RefDirectoryUpdate update) throws IOException {
 		Ref dst = update.getRef();
 		if (!update.isDetachingSymbolicRef()) {
 			dst = dst.getLeaf();
 		}
 		String name = dst.getName();
 
 		// Write the packed-refs file using an atomic update. We might
 		// wind up reading it twice, before and after the lock, to ensure
 		// we don't miss an edit made externally.
 		final PackedRefList packed = getPackedRefs();
 		if (packed.contains(name)) {
 			inProcessPackedRefsLock.lock();
 			try {
 				LockFile lck = lockPackedRefsOrThrow();
 				try {
 					PackedRefList cur = readPackedRefs();
 					int idx = cur.find(name);
 					if (0 <= idx) {
 						commitPackedRefs(lck, cur.remove(idx), packed, true);
 					}
 				} finally {
 					lck.unlock();
 				}
 			} finally {
 				inProcessPackedRefsLock.unlock();
 			}
 		}
 
 		RefList<LooseRef> curLoose, newLoose;
 		do {
 			curLoose = looseRefs.get();
 			int idx = curLoose.find(name);
 			if (idx < 0)
 				break;
 			newLoose = curLoose.remove(idx);
 		} while (!looseRefs.compareAndSet(curLoose, newLoose));
 
 		int levels = levelsIn(name) - 2;
 		delete(logFor(name), levels);
 		if (dst.getStorage().isLoose()) {
 			update.unlock();
 			delete(fileFor(name), levels);
 		}
 
 		modCnt.incrementAndGet();
 		fireRefsChanged();
 	}
 
 	/**
 	 * Adds a set of refs to the set of packed-refs. Only non-symbolic refs are
 	 * added. If a ref with the given name already existed in packed-refs it is
 	 * updated with the new value. Each loose ref which was added to the
 	 * packed-ref file is deleted. If a given ref can't be locked it will not be
 	 * added to the pack file.
 	 *
 	 * @param refs
 	 *            the refs to be added. Must be fully qualified.
 	 * @throws java.io.IOException
 	 */
 	public void pack(List<String> refs) throws IOException {
 		pack(refs, Collections.emptyMap());
 	}
 
 	PackedRefList pack(Map<String, LockFile> heldLocks) throws IOException {
 		return pack(heldLocks.keySet(), heldLocks);
 	}
 
 	private PackedRefList pack(Collection<String> refs,
 			Map<String, LockFile> heldLocks) throws IOException {
 		for (LockFile ol : heldLocks.values()) {
 			ol.requireLock();
 		}
 		if (refs.isEmpty()) {
 			return null;
 		}
 		FS fs = parent.getFS();
 
 		// Lock the packed refs file and read the content
 		inProcessPackedRefsLock.lock();
 		try {
 			LockFile lck = lockPackedRefsOrThrow();
 			try {
 				final PackedRefList packed = getPackedRefs();
 				RefList<Ref> cur = readPackedRefs();
 
 				// Iterate over all refs to be packed
 				boolean dirty = false;
 				for (String refName : refs) {
 					Ref oldRef = readRef(refName, cur);
 					if (oldRef == null) {
 						continue; // A non-existent ref is already correctly packed.
 					}
 					if (oldRef.isSymbolic()) {
 						continue; // can't pack symbolic refs
 					}
 					// Add/Update it to packed-refs
 					Ref newRef = peeledPackedRef(oldRef);
 					if (newRef == oldRef) {
 						// No-op; peeledPackedRef returns the input ref only if it's already
 						// packed, and readRef returns a packed ref only if there is no
 						// loose ref.
 						continue;
 					}
 
 					dirty = true;
 					int idx = cur.find(refName);
 					if (idx >= 0) {
 						cur = cur.set(idx, newRef);
 					} else {
 						cur = cur.add(idx, newRef);
 					}
 				}
 				if (!dirty) {
 					// All requested refs were already packed accurately
 					return packed;
 				}
 
 				// The new content for packed-refs is collected. Persist it.
 				PackedRefList result = commitPackedRefs(lck, cur, packed,
 						false);
 
 				// Now delete the loose refs which are now packed
 				for (String refName : refs) {
 					// Lock the loose ref
 					File refFile = fileFor(refName);
 					if (!fs.exists(refFile)) {
 						continue;
 					}
 
 					LockFile rLck = heldLocks.get(refName);
 					boolean shouldUnlock;
 					if (rLck == null) {
 						rLck = new LockFile(refFile);
 						if (!rLck.lock()) {
 							continue;
 						}
 						shouldUnlock = true;
 					} else {
 						shouldUnlock = false;
 					}
 
 					try {
 						LooseRef currentLooseRef = scanRef(null, refName);
 						if (currentLooseRef == null || currentLooseRef.isSymbolic()) {
 							continue;
 						}
 						Ref packedRef = cur.get(refName);
 						ObjectId clr_oid = currentLooseRef.getObjectId();
 						if (clr_oid != null
 								&& clr_oid.equals(packedRef.getObjectId())) {
 							RefList<LooseRef> curLoose, newLoose;
 							do {
 								curLoose = looseRefs.get();
 								int idx = curLoose.find(refName);
 								if (idx < 0) {
 									break;
 								}
 								newLoose = curLoose.remove(idx);
 							} while (!looseRefs.compareAndSet(curLoose, newLoose));
 							int levels = levelsIn(refName) - 2;
 							delete(refFile, levels, rLck);
 						}
 					} finally {
 						if (shouldUnlock) {
 							rLck.unlock();
 						}
 					}
 				}
 				// Don't fire refsChanged. The refs have not change, only their
 				// storage.
 				return result;
 			} finally {
 				lck.unlock();
 			}
 		} finally {
 			inProcessPackedRefsLock.unlock();
 		}
 	}
 
 	@Nullable
 	LockFile lockPackedRefs() throws IOException {
 		LockFile lck = new LockFile(packedRefsFile);
 		for (int ms : getRetrySleepMs()) {
 			sleep(ms);
 			if (lck.lock()) {
 				return lck;
 			}
 		}
 		return null;
 	}
 
 	private LockFile lockPackedRefsOrThrow() throws IOException {
 		LockFile lck = lockPackedRefs();
 		if (lck == null) {
 			throw new LockFailedException(packedRefsFile);
 		}
 		return lck;
 	}
 
 	/**
 	 * Make sure a ref is peeled and has the Storage PACKED. If the given ref
 	 * has this attributes simply return it. Otherwise create a new peeled
 	 * {@link ObjectIdRef} where Storage is set to PACKED.
 	 *
 	 * @param f
 	 * @return a ref for Storage PACKED having the same name, id, peeledId as f
 	 * @throws MissingObjectException
 	 * @throws IOException
 	 */
 	private Ref../../../../../org/eclipse/jgit/lib/Ref.html#Ref">Ref peeledPackedRef(Ref f)
 			throws MissingObjectException, IOException {
 		if (f.getStorage().isPacked() && f.isPeeled()) {
 			return f;
 		}
 		if (!f.isPeeled()) {
 			f = peel(f);
 		}
 		ObjectId peeledObjectId = f.getPeeledObjectId();
 		if (peeledObjectId != null) {
 			return new ObjectIdRef.PeeledTag(PACKED, f.getName(),
 					f.getObjectId(), peeledObjectId);
 		}
 		return new ObjectIdRef.PeeledNonTag(PACKED, f.getName(),
 				f.getObjectId());
 	}
 
 	void log(boolean force, RefUpdate update, String msg, boolean deref)
 			throws IOException {
 		newLogWriter(force).log(update, msg, deref);
 	}
 
 	private Ref/Ref.html#Ref">Ref resolve(final Ref ref, int depth, String prefix,
 			RefList<LooseRef> loose, RefList<Ref> packed) throws IOException {
 		if (ref.isSymbolic()) {
 			Ref dst = ref.getTarget();
 
 			if (MAX_SYMBOLIC_REF_DEPTH <= depth)
 				return null; // claim it doesn't exist
 
 			// If the cached value can be assumed to be current due to a
 			// recent scan of the loose directory, use it.
 			if (loose != null && dst.getName().startsWith(prefix)) {
 				int idx;
 				if (0 <= (idx = loose.find(dst.getName())))
 					dst = loose.get(idx);
 				else if (0 <= (idx = packed.find(dst.getName())))
 					dst = packed.get(idx);
 				else
 					return ref;
 			} else {
 				dst = readRef(dst.getName(), packed);
 				if (dst == null)
 					return ref;
 			}
 
 			dst = resolve(dst, depth + 1, prefix, loose, packed);
 			if (dst == null)
 				return null;
 			return new SymbolicRef(ref.getName(), dst);
 		}
 		return ref;
 	}
 
 	PackedRefList getPackedRefs() throws IOException {
 		boolean trustFolderStat = getRepository().getConfig().getBoolean(
 				ConfigConstants.CONFIG_CORE_SECTION,
 				ConfigConstants.CONFIG_KEY_TRUSTFOLDERSTAT, true);
 
 		final PackedRefList curList = packedRefs.get();
 		if (trustFolderStat && !curList.snapshot.isModified(packedRefsFile)) {
 			return curList;
 		}
 
 		final PackedRefList newList = readPackedRefs();
 		if (packedRefs.compareAndSet(curList, newList)
 				&& !curList.id.equals(newList.id)) {
 			modCnt.incrementAndGet();
 		}
 		return newList;
 	}
 
 	private PackedRefList readPackedRefs() throws IOException {
 		int maxStaleRetries = 5;
 		int retries = 0;
 		while (true) {
 			final FileSnapshot snapshot = FileSnapshot.save(packedRefsFile);
 			final MessageDigest digest = Constants.newMessageDigest();
 			try (BufferedReader br = new BufferedReader(new InputStreamReader(
 					new DigestInputStream(new FileInputStream(packedRefsFile),
 							digest),
 					UTF_8))) {
 				try {
 					return new PackedRefList(parsePackedRefs(br), snapshot,
 							ObjectId.fromRaw(digest.digest()));
 				} catch (IOException e) {
 					if (FileUtils.isStaleFileHandleInCausalChain(e)
 							&& retries < maxStaleRetries) {
 						if (LOG.isDebugEnabled()) {
 							LOG.debug(MessageFormat.format(
 									JGitText.get().packedRefsHandleIsStale,
 									Integer.valueOf(retries)), e);
 						}
 						retries++;
 						continue;
 					}
 					throw e;
 				}
 			} catch (FileNotFoundException noPackedRefs) {
 				if (packedRefsFile.exists()) {
 					throw noPackedRefs;
 				}
 				// Ignore it and leave the new list empty.
 				return NO_PACKED_REFS;
 			}
 		}
 	}
 
 	private RefList<Ref> parsePackedRefs(BufferedReader br)
 			throws IOException {
 		RefList.Builder<Ref> all = new RefList.Builder<>();
 		Ref last = null;
 		boolean peeled = false;
 		boolean needSort = false;
 
 		String p;
 		while ((p = br.readLine()) != null) {
 			if (p.charAt(0) == '#') {
 				if (p.startsWith(PACKED_REFS_HEADER)) {
 					p = p.substring(PACKED_REFS_HEADER.length());
 					peeled = p.contains(PACKED_REFS_PEELED);
 				}
 				continue;
 			}
 
 			if (p.charAt(0) == '^') {
 				if (last == null)
 					throw new IOException(JGitText.get().peeledLineBeforeRef);
 
 				ObjectId id = ObjectId.fromString(p.substring(1));
 				last = new ObjectIdRef.PeeledTag(PACKED, last.getName(), last
 						.getObjectId(), id);
 				all.set(all.size() - 1, last);
 				continue;
 			}
 
 			int sp = p.indexOf(' ');
 			if (sp < 0) {
 				throw new IOException(MessageFormat.format(
 						JGitText.get().packedRefsCorruptionDetected,
 						packedRefsFile.getAbsolutePath()));
 			}
 			ObjectId id = ObjectId.fromString(p.substring(0, sp));
 			String name = copy(p, sp + 1, p.length());
 			ObjectIdRef cur;
 			if (peeled)
 				cur = new ObjectIdRef.PeeledNonTag(PACKED, name, id);
 			else
 				cur = new ObjectIdRef.Unpeeled(PACKED, name, id);
 			if (last != null && RefComparator.compareTo(last, cur) > 0)
 				needSort = true;
 			all.add(cur);
 			last = cur;
 		}
 
 		if (needSort)
 			all.sort();
 		return all.toRefList();
 	}
 
 	private static String copy(String src, int off, int end) {
 		// Don't use substring since it could leave a reference to the much
 		// larger existing string. Force construction of a full new object.
 		return new StringBuilder(end - off).append(src, off, end).toString();
 	}
 
 	PackedRefList commitPackedRefs(final LockFile lck, final RefList<Ref> refs,
 			final PackedRefList oldPackedList, boolean changed)
 			throws IOException {
 		// Can't just return packedRefs.get() from this method; it might have been
 		// updated again after writePackedRefs() returns.
 		AtomicReference<PackedRefList> result = new AtomicReference<>();
 		new RefWriter(refs) {
 			@Override
 			protected void writeFile(String name, byte[] content)
 					throws IOException {
 				lck.setFSync(true);
 				lck.setNeedSnapshot(true);
 				try {
 					lck.write(content);
 				} catch (IOException ioe) {
 					throw new ObjectWritingException(MessageFormat.format(JGitText.get().unableToWrite, name), ioe);
 				}
 				try {
 					lck.waitForStatChange();
 				} catch (InterruptedException e) {
 					lck.unlock();
 					throw new ObjectWritingException(
 							MessageFormat.format(
 									JGitText.get().interruptedWriting, name),
 							e);
 				}
 				if (!lck.commit())
 					throw new ObjectWritingException(MessageFormat.format(JGitText.get().unableToWrite, name));
 
 				byte[] digest = Constants.newMessageDigest().digest(content);
 				PackedRefList newPackedList = new PackedRefList(
 						refs, lck.getCommitSnapshot(), ObjectId.fromRaw(digest));
 
 				// This thread holds the file lock, so no other thread or process should
 				// be able to modify the packed-refs file on disk. If the list changed,
 				// it means something is very wrong, so throw an exception.
 				//
 				// However, we can't use a naive compareAndSet to check whether the
 				// update was successful, because another thread might _read_ the
 				// packed refs file that was written out by this thread while holding
 				// the lock, and update the packedRefs reference to point to that. So
 				// compare the actual contents instead.
 				PackedRefList afterUpdate = packedRefs.updateAndGet(
 						p -> p.id.equals(oldPackedList.id) ? newPackedList : p);
 				if (!afterUpdate.id.equals(newPackedList.id)) {
 					throw new ObjectWritingException(
 							MessageFormat.format(JGitText.get().unableToWrite, name));
 				}
 				if (changed) {
 					modCnt.incrementAndGet();
 				}
 				result.set(newPackedList);
 			}
 		}.writePackedRefs();
 		return result.get();
 	}
 
 	private Ref readRef(String name, RefList<Ref> packed) throws IOException {
 		final RefList<LooseRef> curList = looseRefs.get();
 		final int idx = curList.find(name);
 		if (0 <= idx) {
 			final LooseRef o = curList.get(idx);
 			final LooseRef n = scanRef(o, name);
 			if (n == null) {
 				if (looseRefs.compareAndSet(curList, curList.remove(idx)))
 					modCnt.incrementAndGet();
 				return packed.get(name);
 			}
 
 			if (o == n)
 				return n;
 			if (looseRefs.compareAndSet(curList, curList.set(idx, n)))
 				modCnt.incrementAndGet();
 			return n;
 		}
 
 		final LooseRef n = scanRef(null, name);
 		if (n == null)
 			return packed.get(name);
 
 		// check whether the found new ref is the an additional ref. These refs
 		// should not go into looseRefs
 		for (String additionalRefsName : additionalRefsNames) {
 			if (name.equals(additionalRefsName)) {
 				return n;
 			}
 		}
 
 		if (looseRefs.compareAndSet(curList, curList.add(idx, n)))
 			modCnt.incrementAndGet();
 		return n;
 	}
 
 	LooseRef scanRef(LooseRef ref, String name) throws IOException {
 		final File path = fileFor(name);
 		FileSnapshot currentSnapshot = null;
 
 		if (ref != null) {
 			currentSnapshot = ref.getSnapShot();
 			if (!currentSnapshot.isModified(path))
 				return ref;
 			name = ref.getName();
 		}
 
 		final int limit = 4096;
 		final byte[] buf;
 		FileSnapshot otherSnapshot = FileSnapshot.save(path);
 		try {
 			buf = IO.readSome(path, limit);
 		} catch (FileNotFoundException noFile) {
 			if (path.exists() && path.isFile()) {
 				throw noFile;
 			}
 			return null; // doesn't exist or no file; not a reference.
 		}
 
 		int n = buf.length;
 		if (n == 0)
 			return null; // empty file; not a reference.
 
 		if (isSymRef(buf, n)) {
 			if (n == limit)
 				return null; // possibly truncated ref
 
 			// trim trailing whitespace
 			while (0 < n && Character.isWhitespace(buf[n - 1]))
 				n--;
 			if (n < 6) {
 				String content = RawParseUtils.decode(buf, 0, n);
 				throw new IOException(MessageFormat.format(JGitText.get().notARef, name, content));
 			}
 			final String target = RawParseUtils.decode(buf, 5, n);
 			if (ref != null && ref.isSymbolic()
 					&& ref.getTarget().getName().equals(target)) {
 				assert(currentSnapshot != null);
 				currentSnapshot.setClean(otherSnapshot);
 				return ref;
 			}
 			return newSymbolicRef(otherSnapshot, name, target);
 		}
 
 		if (n < OBJECT_ID_STRING_LENGTH)
 			return null; // impossibly short object identifier; not a reference.
 
 		final ObjectId id;
 		try {
 			id = ObjectId.fromString(buf, 0);
 			if (ref != null && !ref.isSymbolic()
 					&& id.equals(ref.getTarget().getObjectId())) {
 				assert(currentSnapshot != null);
 				currentSnapshot.setClean(otherSnapshot);
 				return ref;
 			}
 
 		} catch (IllegalArgumentException notRef) {
 			while (0 < n && Character.isWhitespace(buf[n - 1]))
 				n--;
 			String content = RawParseUtils.decode(buf, 0, n);
 
 			throw new IOException(MessageFormat.format(JGitText.get().notARef,
 					name, content), notRef);
 		}
 		return new LooseUnpeeled(otherSnapshot, name, id);
 	}
 
 	private static boolean isSymRef(byte[] buf, int n) {
 		if (n < 6)
 			return false;
 		return /**/buf[0] == 'r' //
 				&& buf[1] == 'e' //
 				&& buf[2] == 'f' //
 				&& buf[3] == ':' //
 				&& buf[4] == ' ';
 	}
 
 	/**
 	 * Detect if we are in a clone command execution
 	 *
 	 * @return {@code true} if we are currently cloning a repository
 	 * @throws IOException
 	 */
 	boolean isInClone() throws IOException {
 		return hasDanglingHead() && !packedRefsFile.exists() && !hasLooseRef();
 	}
 
 	private boolean hasDanglingHead() throws IOException {
 		Ref head = exactRef(Constants.HEAD);
 		if (head != null) {
 			ObjectId id = head.getObjectId();
 			return id == null || id.equals(ObjectId.zeroId());
 		}
 		return false;
 	}
 
 	private boolean hasLooseRef() throws IOException {
 		try (Stream<Path> stream = Files.walk(refsDir.toPath())) {
 			return stream.anyMatch(Files::isRegularFile);
 		}
 	}
 
 	/** If the parent should fire listeners, fires them. */
 	void fireRefsChanged() {
 		final int last = lastNotifiedModCnt.get();
 		final int curr = modCnt.get();
 		if (last != curr && lastNotifiedModCnt.compareAndSet(last, curr) && last != 0)
 			parent.fireEvent(new RefsChangedEvent());
 	}
 
 	/**
 	 * Create a reference update to write a temporary reference.
 	 *
 	 * @return an update for a new temporary reference.
 	 * @throws IOException
 	 *             a temporary name cannot be allocated.
 	 */
 	RefDirectoryUpdate newTemporaryUpdate() throws IOException {
 		File tmp = File.createTempFile("renamed_", "_ref", refsDir); //$NON-NLS-1$ //$NON-NLS-2$
 		String name = Constants.R_REFS + tmp.getName();
 		Ref ref = new ObjectIdRef.Unpeeled(NEW, name, null);
 		return new RefDirectoryUpdate(this, ref);
 	}
 
 	/**
 	 * Locate the file on disk for a single reference name.
 	 *
 	 * @param name
 	 *            name of the ref, relative to the Git repository top level
 	 *            directory (so typically starts with refs/).
 	 * @return the loose file location.
 	 */
 	File fileFor(String name) {
 		if (name.startsWith(R_REFS)) {
 			name = name.substring(R_REFS.length());
 			return new File(refsDir, name);
 		}
 		return new File(gitDir, name);
 	}
 
 	static int levelsIn(String name) {
 		int count = 0;
 		for (int p = name.indexOf('/'); p >= 0; p = name.indexOf('/', p + 1))
 			count++;
 		return count;
 	}
 
 	static void delete(File file, int depth) throws IOException {
 		delete(file, depth, null);
 	}
 
 	private static void delete(File file, int depth, LockFile rLck)
 			throws IOException {
 		if (!file.delete() && file.isFile()) {
 			throw new IOException(MessageFormat.format(
 					JGitText.get().fileCannotBeDeleted, file));
 		}
 
 		if (rLck != null) {
 			rLck.unlock(); // otherwise cannot delete dir below
 		}
 		File dir = file.getParentFile();
 		for (int i = 0; i < depth; ++i) {
 			try {
 				Files.deleteIfExists(dir.toPath());
 			} catch (DirectoryNotEmptyException e) {
 				// Don't log; normal case when there are other refs with the
 				// same prefix
 				break;
 			} catch (IOException e) {
 				LOG.warn(MessageFormat.format(JGitText.get().unableToRemovePath,
 						dir), e);
 				break;
 			}
 			dir = dir.getParentFile();
 		}
 	}
 
 	/**
 	 * Get times to sleep while retrying a possibly contentious operation.
 	 * <p>
 	 * For retrying an operation that might have high contention, such as locking
 	 * the {@code packed-refs} file, the caller may implement a retry loop using
 	 * the returned values:
 	 *
 	 * <pre>
 	 * for (int toSleepMs : getRetrySleepMs()) {
 	 *   sleep(toSleepMs);
 	 *   if (isSuccessful(doSomething())) {
 	 *     return success;
 	 *   }
 	 * }
 	 * return failure;
 	 * </pre>
 	 *
 	 * The first value in the returned iterable is 0, and the caller should treat
 	 * a fully-consumed iterator as a timeout.
 	 *
 	 * @return iterable of times, in milliseconds, that the caller should sleep
 	 *         before attempting an operation.
 	 */
 	Iterable<Integer> getRetrySleepMs() {
 		return retrySleepMs;
 	}
 
 	void setRetrySleepMs(List<Integer> retrySleepMs) {
 		if (retrySleepMs == null || retrySleepMs.isEmpty()
 				|| retrySleepMs.get(0).intValue() != 0) {
 			throw new IllegalArgumentException();
 		}
 		this.retrySleepMs = retrySleepMs;
 	}
 
 	/**
 	 * Sleep with {@link Thread#sleep(long)}, converting {@link
 	 * InterruptedException} to {@link InterruptedIOException}.
 	 *
 	 * @param ms
 	 *            time to sleep, in milliseconds; zero or negative is a no-op.
 	 * @throws InterruptedIOException
 	 *             if sleeping was interrupted.
 	 */
 	static void sleep(long ms) throws InterruptedIOException {
 		if (ms <= 0) {
 			return;
 		}
 		try {
 			Thread.sleep(ms);
 		} catch (InterruptedException e) {
 			InterruptedIOException ie = new InterruptedIOException();
 			ie.initCause(e);
 			throw ie;
 		}
 	}
 
 	static class PackedRefList extends RefList<Ref> {
 
 		private final FileSnapshot snapshot;
 
 		private final ObjectId id;
 
 		private PackedRefList(RefList<Ref> src, FileSnapshot s, ObjectId i) {
 			super(src);
 			snapshot = s;
 			id = i;
 		}
 	}
 
 	private static final PackedRefList NO_PACKED_REFS = new PackedRefList(
 			RefList.emptyList(), FileSnapshot.MISSING_FILE,
 			ObjectId.zeroId());
 
 	private static LooseSymbolicRef newSymbolicRef(FileSnapshot snapshot,
 			String name, String target) {
 		Ref dst = new ObjectIdRef.Unpeeled(NEW, target, null);
 		return new LooseSymbolicRef(snapshot, name, dst);
 	}
 
 	private static interface LooseRef extends Ref {
 		FileSnapshot getSnapShot();
 
 		LooseRef peel(ObjectIdRef newLeaf);
 	}
 
 	private static final class LoosePeeledTag extends ObjectIdRef.PeeledTag
 			implements LooseRef {
 		private final FileSnapshot snapShot;
 
 		LoosePeeledTag(FileSnapshot snapshot, @NonNull String refName,
 				@NonNull ObjectId/../../../../org/eclipse/jgit/lib/ObjectId.html#ObjectId">ObjectId id, @NonNull ObjectId p) {
 			super(LOOSE, refName, id, p);
 			this.snapShot = snapshot;
 		}
 
 		@Override
 		public FileSnapshot getSnapShot() {
 			return snapShot;
 		}
 
 		@Override
 		public LooseRef peel(ObjectIdRef newLeaf) {
 			return this;
 		}
 	}
 
 	private static final class LooseNonTag extends ObjectIdRef.PeeledNonTag
 			implements LooseRef {
 		private final FileSnapshot snapShot;
 
 		LooseNonTag(FileSnapshot snapshot, @NonNull String refName,
 				@NonNull ObjectId id) {
 			super(LOOSE, refName, id);
 			this.snapShot = snapshot;
 		}
 
 		@Override
 		public FileSnapshot getSnapShot() {
 			return snapShot;
 		}
 
 		@Override
 		public LooseRef peel(ObjectIdRef newLeaf) {
 			return this;
 		}
 	}
 
 	private static final class LooseUnpeeled extends ObjectIdRef.Unpeeled
 			implements LooseRef {
 		private FileSnapshot snapShot;
 
 		LooseUnpeeled(FileSnapshot snapShot, @NonNull String refName,
 				@NonNull ObjectId id) {
 			super(LOOSE, refName, id);
 			this.snapShot = snapShot;
 		}
 
 		@Override
 		public FileSnapshot getSnapShot() {
 			return snapShot;
 		}
 
 		@NonNull
 		@Override
 		public ObjectId getObjectId() {
 			ObjectId id = super.getObjectId();
 			assert id != null; // checked in constructor
 			return id;
 		}
 
 		@Override
 		public LooseRef peel(ObjectIdRef newLeaf) {
 			ObjectId peeledObjectId = newLeaf.getPeeledObjectId();
 			ObjectId objectId = getObjectId();
 			if (peeledObjectId != null) {
 				return new LoosePeeledTag(snapShot, getName(),
 						objectId, peeledObjectId);
 			}
 			return new LooseNonTag(snapShot, getName(), objectId);
 		}
 	}
 
 	private static final class LooseSymbolicRef extends SymbolicRef implements
 			LooseRef {
 		private final FileSnapshot snapShot;
 
 		LooseSymbolicRef(FileSnapshot snapshot, @NonNull String refName,
 				@NonNull Ref target) {
 			super(refName, target);
 			this.snapShot = snapshot;
 		}
 
 		@Override
 		public FileSnapshot getSnapShot() {
 			return snapShot;
 		}
 
 		@Override
 		public LooseRef peel(ObjectIdRef newLeaf) {
 			// We should never try to peel the symbolic references.
 			throw new UnsupportedOperationException();
 		}
 	}
 }