/*
    * Copyright (C) 2008-2010, Google Inc.
    * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>
    * 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.lib;
  
  import java.io.IOException;
  import java.text.MessageFormat;
  
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.internal.JGitText;
  import org.eclipse.jgit.revwalk.RevCommit;
  import org.eclipse.jgit.revwalk.RevObject;
  import org.eclipse.jgit.revwalk.RevWalk;
  import org.eclipse.jgit.transport.PushCertificate;
  
  /**
   * Creates, updates or deletes any reference.
   */
  public abstract class RefUpdate {
  	/**
  	 * Status of an update request.
  	 * <p>
  	 * New values may be added to this enum in the future. Callers may assume that
  	 * unknown values are failures, and may generally treat them the same as
  	 * {@link #REJECTED_OTHER_REASON}.
  	 */
  	public static enum Result {
  		/** The ref update/delete has not been attempted by the caller. */
  		NOT_ATTEMPTED,
  
  		/**
  		 * The ref could not be locked for update/delete.
  		 * <p>
  		 * This is generally a transient failure and is usually caused by
  		 * another process trying to access the ref at the same time as this
  		 * process was trying to update it. It is possible a future operation
  		 * will be successful.
  		 */
  		LOCK_FAILURE,
  
  		/**
  		 * Same value already stored.
  		 * <p>
  		 * Both the old value and the new value are identical. No change was
  		 * necessary for an update. For delete the branch is removed.
  		 */
  		NO_CHANGE,
  
  		/**
  		 * The ref was created locally for an update, but ignored for delete.
  		 * <p>
  		 * The ref did not exist when the update started, but it was created
  		 * successfully with the new value.
  		 */
  		NEW,
  
  		/**
  		 * The ref had to be forcefully updated/deleted.
 		 * <p>
 		 * The ref already existed but its old value was not fully merged into
 		 * the new value. The configuration permitted a forced update to take
 		 * place, so ref now contains the new value. History associated with the
 		 * objects not merged may no longer be reachable.
 		 */
 		FORCED,
 
 		/**
 		 * The ref was updated/deleted in a fast-forward way.
 		 * <p>
 		 * The tracking ref already existed and its old value was fully merged
 		 * into the new value. No history was made unreachable.
 		 */
 		FAST_FORWARD,
 
 		/**
 		 * Not a fast-forward and not stored.
 		 * <p>
 		 * The tracking ref already existed but its old value was not fully
 		 * merged into the new value. The configuration did not allow a forced
 		 * update/delete to take place, so ref still contains the old value. No
 		 * previous history was lost.
 		 * <p>
 		 * <em>Note:</em> Despite the general name, this result only refers to the
 		 * non-fast-forward case. For more general errors, see {@link
 		 * #REJECTED_OTHER_REASON}.
 		 */
 		REJECTED,
 
 		/**
 		 * Rejected because trying to delete the current branch.
 		 * <p>
 		 * Has no meaning for update.
 		 */
 		REJECTED_CURRENT_BRANCH,
 
 		/**
 		 * The ref was probably not updated/deleted because of I/O error.
 		 * <p>
 		 * Unexpected I/O error occurred when writing new ref. Such error may
 		 * result in uncertain state, but most probably ref was not updated.
 		 * <p>
 		 * This kind of error doesn't include {@link #LOCK_FAILURE}, which is a
 		 * different case.
 		 */
 		IO_FAILURE,
 
 		/**
 		 * The ref was renamed from another name
 		 * <p>
 		 */
 		RENAMED,
 
 		/**
 		 * One or more objects aren't in the repository.
 		 * <p>
 		 * This is severe indication of either repository corruption on the
 		 * server side, or a bug in the client wherein the client did not supply
 		 * all required objects during the pack transfer.
 		 *
 		 * @since 4.9
 		 */
 		REJECTED_MISSING_OBJECT,
 
 		/**
 		 * Rejected for some other reason not covered by another enum value.
 		 *
 		 * @since 4.9
 		 */
 		REJECTED_OTHER_REASON;
 	}
 
 	/** New value the caller wants this ref to have. */
 	private ObjectId newValue;
 
 	/** Does this specification ask for forced updated (rewind/reset)? */
 	private boolean force;
 
 	/** Identity to record action as within the reflog. */
 	private PersonIdent refLogIdent;
 
 	/** Message the caller wants included in the reflog. */
 	private String refLogMessage;
 
 	/** Should the Result value be appended to {@link #refLogMessage}. */
 	private boolean refLogIncludeResult;
 
 	/**
 	 * Should reflogs be written even if the configured default for this ref is
 	 * not to write it.
 	 */
 	private boolean forceRefLog;
 
 	/** Old value of the ref, obtained after we lock it. */
 	private ObjectId oldValue;
 
 	/** If non-null, the value {@link #oldValue} must have to continue. */
 	private ObjectId expValue;
 
 	/** Result of the update operation. */
 	private Result result = Result.NOT_ATTEMPTED;
 
 	/** Push certificate associated with this update. */
 	private PushCertificate pushCert;
 
 	private final Ref ref;
 
 	/**
 	 * Is this RefUpdate detaching a symbolic ref?
 	 *
 	 * We need this info since this.ref will normally be peeled of in case of
 	 * detaching a symbolic ref (HEAD for example).
 	 *
 	 * Without this flag we cannot decide whether the ref has to be updated or
 	 * not in case when it was a symbolic ref and the newValue == oldValue.
 	 */
 	private boolean detachingSymbolicRef;
 
 	private boolean checkConflicting = true;
 
 	/**
 	 * Construct a new update operation for the reference.
 	 * <p>
 	 * {@code ref.getObjectId()} will be used to seed {@link #getOldObjectId()},
 	 * which callers can use as part of their own update logic.
 	 *
 	 * @param ref
 	 *            the reference that will be updated by this operation.
 	 */
 	protected RefUpdate(Ref ref) {
 		this.ref = ref;
 		oldValue = ref.getObjectId();
 		refLogMessage = ""; //$NON-NLS-1$
 	}
 
 	/**
 	 * Get the reference database this update modifies.
 	 *
 	 * @return the reference database this update modifies.
 	 */
 	protected abstract RefDatabase getRefDatabase();
 
 	/**
 	 * Get the repository storing the database's objects.
 	 *
 	 * @return the repository storing the database's objects.
 	 */
 	protected abstract Repository getRepository();
 
 	/**
 	 * Try to acquire the lock on the reference.
 	 * <p>
 	 * If the locking was successful the implementor must set the current
 	 * identity value by calling {@link #setOldObjectId(ObjectId)}.
 	 *
 	 * @param deref
 	 *            true if the lock should be taken against the leaf level
 	 *            reference; false if it should be taken exactly against the
 	 *            current reference.
 	 * @return true if the lock was acquired and the reference is likely
 	 *         protected from concurrent modification; false if it failed.
 	 * @throws java.io.IOException
 	 *             the lock couldn't be taken due to an unexpected storage
 	 *             failure, and not because of a concurrent update.
 	 */
 	protected abstract boolean tryLock(boolean deref) throws IOException;
 
 	/**
 	 * Releases the lock taken by {@link #tryLock} if it succeeded.
 	 */
 	protected abstract void unlock();
 
 	/**
 	 * Do update
 	 *
 	 * @param desiredResult
 	 *            a {@link org.eclipse.jgit.lib.RefUpdate.Result} object.
 	 * @return {@code result}
 	 * @throws java.io.IOException
 	 */
 	protected abstract Result doUpdate(Result desiredResult) throws IOException;
 
 	/**
 	 * Do delete
 	 *
 	 * @param desiredResult
 	 *            a {@link org.eclipse.jgit.lib.RefUpdate.Result} object.
 	 * @return {@code result}
 	 * @throws java.io.IOException
 	 */
 	protected abstract Result doDelete(Result desiredResult) throws IOException;
 
 	/**
 	 * Do link
 	 *
 	 * @param target
 	 *            a {@link java.lang.String} object.
 	 * @return {@link org.eclipse.jgit.lib.RefUpdate.Result#NEW} on success.
 	 * @throws java.io.IOException
 	 */
 	protected abstract Result doLink(String target) throws IOException;
 
 	/**
 	 * Get the name of the ref this update will operate on.
 	 *
 	 * @return name of underlying ref.
 	 */
 	public String getName() {
 		return getRef().getName();
 	}
 
 	/**
 	 * Get the reference this update will create or modify.
 	 *
 	 * @return the reference this update will create or modify.
 	 */
 	public Ref getRef() {
 		return ref;
 	}
 
 	/**
 	 * Get the new value the ref will be (or was) updated to.
 	 *
 	 * @return new value. Null if the caller has not configured it.
 	 */
 	public ObjectId getNewObjectId() {
 		return newValue;
 	}
 
 	/**
 	 * Tells this RefUpdate that it is actually detaching a symbolic ref.
 	 */
 	public void setDetachingSymbolicRef() {
 		detachingSymbolicRef = true;
 	}
 
 	/**
 	 * Return whether this update is actually detaching a symbolic ref.
 	 *
 	 * @return true if detaching a symref.
 	 * @since 4.9
 	 */
 	public boolean isDetachingSymbolicRef() {
 		return detachingSymbolicRef;
 	}
 
 	/**
 	 * Set the new value the ref will update to.
 	 *
 	 * @param id
 	 *            the new value.
 	 */
 	public void setNewObjectId(AnyObjectId id) {
 		newValue = id.copy();
 	}
 
 	/**
 	 * Get the expected value of the ref after the lock is taken, but before
 	 * update occurs.
 	 *
 	 * @return the expected value of the ref after the lock is taken, but before
 	 *         update occurs. Null to avoid the compare and swap test. Use
 	 *         {@link org.eclipse.jgit.lib.ObjectId#zeroId()} to indicate
 	 *         expectation of a non-existant ref.
 	 */
 	public ObjectId getExpectedOldObjectId() {
 		return expValue;
 	}
 
 	/**
 	 * Set the expected value of the ref after the lock is taken, but before
 	 * update occurs.
 	 *
 	 * @param id
 	 *            the expected value of the ref after the lock is taken, but
 	 *            before update occurs. Null to avoid the compare and swap test.
 	 *            Use {@link org.eclipse.jgit.lib.ObjectId#zeroId()} to indicate
 	 *            expectation of a non-existant ref.
 	 */
 	public void setExpectedOldObjectId(AnyObjectId id) {
 		expValue = id != null ? id.toObjectId() : null;
 	}
 
 	/**
 	 * Check if this update wants to forcefully change the ref.
 	 *
 	 * @return true if this update should ignore merge tests.
 	 */
 	public boolean isForceUpdate() {
 		return force;
 	}
 
 	/**
 	 * Set if this update wants to forcefully change the ref.
 	 *
 	 * @param b
 	 *            true if this update should ignore merge tests.
 	 */
 	public void setForceUpdate(boolean b) {
 		force = b;
 	}
 
 	/**
 	 * Get identity of the user making the change in the reflog.
 	 *
 	 * @return identity of the user making the change in the reflog.
 	 */
 	public PersonIdent getRefLogIdent() {
 		return refLogIdent;
 	}
 
 	/**
 	 * Set the identity of the user appearing in the reflog.
 	 * <p>
 	 * The timestamp portion of the identity is ignored. A new identity with the
 	 * current timestamp will be created automatically when the update occurs
 	 * and the log record is written.
 	 *
 	 * @param pi
 	 *            identity of the user. If null the identity will be
 	 *            automatically determined based on the repository
 	 *            configuration.
 	 */
 	public void setRefLogIdent(PersonIdent pi) {
 		refLogIdent = pi;
 	}
 
 	/**
 	 * Get the message to include in the reflog.
 	 *
 	 * @return message the caller wants to include in the reflog; null if the
 	 *         update should not be logged.
 	 */
 	public String getRefLogMessage() {
 		return refLogMessage;
 	}
 
 	/**
 	 * Whether the ref log message should show the result.
 	 *
 	 * @return {@code true} if the ref log message should show the result.
 	 */
 	protected boolean isRefLogIncludingResult() {
 		return refLogIncludeResult;
 	}
 
 	/**
 	 * Set the message to include in the reflog.
 	 * <p>
 	 * Repository implementations may limit which reflogs are written by default,
 	 * based on the project configuration. If a repo is not configured to write
 	 * logs for this ref by default, setting the message alone may have no effect.
 	 * To indicate that the repo should write logs for this update in spite of
 	 * configured defaults, use {@link #setForceRefLog(boolean)}.
 	 *
 	 * @param msg
 	 *            the message to describe this change. It may be null if
 	 *            appendStatus is null in order not to append to the reflog
 	 * @param appendStatus
 	 *            true if the status of the ref change (fast-forward or
 	 *            forced-update) should be appended to the user supplied
 	 *            message.
 	 */
 	public void setRefLogMessage(String msg, boolean appendStatus) {
 		if (msg == null && !appendStatus)
 			disableRefLog();
 		else if (msg == null && appendStatus) {
 			refLogMessage = ""; //$NON-NLS-1$
 			refLogIncludeResult = true;
 		} else {
 			refLogMessage = msg;
 			refLogIncludeResult = appendStatus;
 		}
 	}
 
 	/**
 	 * Don't record this update in the ref's associated reflog.
 	 */
 	public void disableRefLog() {
 		refLogMessage = null;
 		refLogIncludeResult = false;
 	}
 
 	/**
 	 * Force writing a reflog for the updated ref.
 	 *
 	 * @param force whether to force.
 	 * @since 4.9
 	 */
 	public void setForceRefLog(boolean force) {
 		forceRefLog = force;
 	}
 
 	/**
 	 * Check whether the reflog should be written regardless of repo defaults.
 	 *
 	 * @return whether force writing is enabled.
 	 * @since 4.9
 	 */
 	protected boolean isForceRefLog() {
 		return forceRefLog;
 	}
 
 	/**
 	 * The old value of the ref, prior to the update being attempted.
 	 * <p>
 	 * This value may differ before and after the update method. Initially it is
 	 * populated with the value of the ref before the lock is taken, but the old
 	 * value may change if someone else modified the ref between the time we
 	 * last read it and when the ref was locked for update.
 	 *
 	 * @return the value of the ref prior to the update being attempted; null if
 	 *         the updated has not been attempted yet.
 	 */
 	public ObjectId getOldObjectId() {
 		return oldValue;
 	}
 
 	/**
 	 * Set the old value of the ref.
 	 *
 	 * @param old
 	 *            the old value.
 	 */
 	protected void setOldObjectId(ObjectId old) {
 		oldValue = old;
 	}
 
 	/**
 	 * Set a push certificate associated with this update.
 	 * <p>
 	 * This usually includes a command to update this ref, but is not required to.
 	 *
 	 * @param cert
 	 *            push certificate, may be null.
 	 * @since 4.1
 	 */
 	public void setPushCertificate(PushCertificate cert) {
 		pushCert = cert;
 	}
 
 	/**
 	 * Set the push certificate associated with this update.
 	 * <p>
 	 * This usually includes a command to update this ref, but is not required to.
 	 *
 	 * @return push certificate, may be null.
 	 * @since 4.1
 	 */
 	protected PushCertificate getPushCertificate() {
 		return pushCert;
 	}
 
 	/**
 	 * Get the status of this update.
 	 * <p>
 	 * The same value that was previously returned from an update method.
 	 *
 	 * @return the status of the update.
 	 */
 	public Result getResult() {
 		return result;
 	}
 
 	private void requireCanDoUpdate() {
 		if (newValue == null)
 			throw new IllegalStateException(JGitText.get().aNewObjectIdIsRequired);
 	}
 
 	/**
 	 * Force the ref to take the new value.
 	 * <p>
 	 * This is just a convenient helper for setting the force flag, and as such
 	 * the merge test is performed.
 	 *
 	 * @return the result status of the update.
 	 * @throws java.io.IOException
 	 *             an unexpected IO error occurred while writing changes.
 	 */
 	public Result forceUpdate() throws IOException {
 		force = true;
 		return update();
 	}
 
 	/**
 	 * Gracefully update the ref to the new value.
 	 * <p>
 	 * Merge test will be performed according to {@link #isForceUpdate()}.
 	 * <p>
 	 * This is the same as:
 	 *
 	 * <pre>
 	 * return update(new RevWalk(getRepository()));
 	 * </pre>
 	 *
 	 * @return the result status of the update.
 	 * @throws java.io.IOException
 	 *             an unexpected IO error occurred while writing changes.
 	 */
 	public Result update() throws IOException {
 		try (RevWalk rw = new RevWalk(getRepository())) {
 			return update(rw);
 		}
 	}
 
 	/**
 	 * Gracefully update the ref to the new value.
 	 * <p>
 	 * Merge test will be performed according to {@link #isForceUpdate()}.
 	 *
 	 * @param walk
 	 *            a RevWalk instance this update command can borrow to perform
 	 *            the merge test. The walk will be reset to perform the test.
 	 * @return the result status of the update.
 	 * @throws java.io.IOException
 	 *             an unexpected IO error occurred while writing changes.
 	 */
 	public Result update(RevWalk walk) throws IOException {
 		requireCanDoUpdate();
 		try {
 			return result = updateImpl(walk, new Store() {
 				@Override
 				Result execute(Result status) throws IOException {
 					if (status == Result.NO_CHANGE)
 						return status;
 					return doUpdate(status);
 				}
 			});
 		} catch (IOException x) {
 			result = Result.IO_FAILURE;
 			throw x;
 		}
 	}
 
 	/**
 	 * Delete the ref.
 	 * <p>
 	 * This is the same as:
 	 *
 	 * <pre>
 	 * return delete(new RevWalk(getRepository()));
 	 * </pre>
 	 *
 	 * @return the result status of the delete.
 	 * @throws java.io.IOException
 	 */
 	public Result delete() throws IOException {
 		try (RevWalk rw = new RevWalk(getRepository())) {
 			return delete(rw);
 		}
 	}
 
 	/**
 	 * Delete the ref.
 	 *
 	 * @param walk
 	 *            a RevWalk instance this delete command can borrow to perform
 	 *            the merge test. The walk will be reset to perform the test.
 	 * @return the result status of the delete.
 	 * @throws java.io.IOException
 	 */
 	public Result delete(RevWalk walk) throws IOException {
 		final String myName = detachingSymbolicRef
 				? getRef().getName()
 				: getRef().getLeaf().getName();
 		if (myName.startsWith(Constants.R_HEADS) && !getRepository().isBare()) {
 			// Don't allow the currently checked out branch to be deleted.
 			Ref head = getRefDatabase().getRef(Constants.HEAD);
 			while (head != null && head.isSymbolic()) {
 				head = head.getTarget();
 				if (myName.equals(head.getName()))
 					return result = Result.REJECTED_CURRENT_BRANCH;
 			}
 		}
 
 		try {
 			return result = updateImpl(walk, new Store() {
 				@Override
 				Result execute(Result status) throws IOException {
 					return doDelete(status);
 				}
 			});
 		} catch (IOException x) {
 			result = Result.IO_FAILURE;
 			throw x;
 		}
 	}
 
 	/**
 	 * Replace this reference with a symbolic reference to another reference.
 	 * <p>
 	 * This exact reference (not its traversed leaf) is replaced with a symbolic
 	 * reference to the requested name.
 	 *
 	 * @param target
 	 *            name of the new target for this reference. The new target name
 	 *            must be absolute, so it must begin with {@code refs/}.
 	 * @return {@link org.eclipse.jgit.lib.RefUpdate.Result#NEW} or
 	 *         {@link org.eclipse.jgit.lib.RefUpdate.Result#FORCED} on success.
 	 * @throws java.io.IOException
 	 */
 	public Result link(String target) throws IOException {
 		if (!target.startsWith(Constants.R_REFS))
 			throw new IllegalArgumentException(MessageFormat.format(JGitText.get().illegalArgumentNotA, Constants.R_REFS));
 		if (checkConflicting && getRefDatabase().isNameConflicting(getName()))
 			return Result.LOCK_FAILURE;
 		try {
 			if (!tryLock(false))
 				return Result.LOCK_FAILURE;
 
 			final Ref old = getRefDatabase().getRef(getName());
 			if (old != null && old.isSymbolic()) {
 				final Ref dst = old.getTarget();
 				if (target.equals(dst.getName()))
 					return result = Result.NO_CHANGE;
 			}
 
 			if (old != null && old.getObjectId() != null)
 				setOldObjectId(old.getObjectId());
 
 			final Ref dst = getRefDatabase().getRef(target);
 			if (dst != null && dst.getObjectId() != null)
 				setNewObjectId(dst.getObjectId());
 
 			return result = doLink(target);
 		} catch (IOException x) {
 			result = Result.IO_FAILURE;
 			throw x;
 		} finally {
 			unlock();
 		}
 	}
 
 	private Result updateImpl(RevWalk walk, Store store)
 			throws IOException {
 		RevObject newObj;
 		RevObject oldObj;
 
 		// don't make expensive conflict check if this is an existing Ref
 		if (oldValue == null && checkConflicting
 				&& getRefDatabase().isNameConflicting(getName())) {
 			return Result.LOCK_FAILURE;
 		}
 		try {
 			// If we're detaching a symbolic reference, we should update the reference
 			// itself. Otherwise, we will update the leaf reference, which should be
 			// an ObjectIdRef.
 			if (!tryLock(!detachingSymbolicRef)) {
 				return Result.LOCK_FAILURE;
 			}
 			if (expValue != null) {
 				final ObjectId o;
 				o = oldValue != null ? oldValue : ObjectId.zeroId();
 				if (!AnyObjectId.equals(expValue, o)) {
 					return Result.LOCK_FAILURE;
 				}
 			}
 			try {
 				newObj = safeParseNew(walk, newValue);
 			} catch (MissingObjectException e) {
 				return Result.REJECTED_MISSING_OBJECT;
 			}
 
 			if (oldValue == null) {
 				return store.execute(Result.NEW);
 			}
 
 			oldObj = safeParseOld(walk, oldValue);
 			if (newObj == oldObj && !detachingSymbolicRef) {
 				return store.execute(Result.NO_CHANGE);
 			}
 
 			if (isForceUpdate()) {
 				return store.execute(Result.FORCED);
 			}
 
 			if (newObj instanceof RevCommit && oldObj instanceof RevCommit) {
 				if (walk.isMergedInto((RevCommit) oldObj, (RevCommit) newObj)) {
 					return store.execute(Result.FAST_FORWARD);
 				}
 			}
 
 			return Result.REJECTED;
 		} finally {
 			unlock();
 		}
 	}
 
 	/**
 	 * Enable/disable the check for conflicting ref names. By default conflicts
 	 * are checked explicitly.
 	 *
 	 * @param check
 	 *            whether to enable the check for conflicting ref names.
 	 * @since 3.0
 	 */
 	public void setCheckConflicting(boolean check) {
 		checkConflicting = check;
 	}
 
 	private static RevObject safeParseNew(RevWalk rw, AnyObjectId newId)
 			throws IOException {
 		if (newId == null || ObjectId.zeroId().equals(newId)) {
 			return null;
 		}
 		return rw.parseAny(newId);
 	}
 
 	private static RevObject safeParseOld(RevWalk rw, AnyObjectId oldId)
 			throws IOException {
 		try {
 			return oldId != null ? rw.parseAny(oldId) : null;
 		} catch (MissingObjectException e) {
 			// We can expect some old objects to be missing, like if we are trying to
 			// force a deletion of a branch and the object it points to has been
 			// pruned from the database due to freak corruption accidents (it happens
 			// with 'git new-work-dir').
 			return null;
 		}
 	}
 
 	/**
 	 * Handle the abstraction of storing a ref update. This is because both
 	 * updating and deleting of a ref have merge testing in common.
 	 */
 	private abstract class Store {
 		abstract Result execute(Result status) throws IOException;
 	}
 }