org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackedBatchRefUpdate.java - sop/jgit - Git at Google

 /*
  * Copyright (C) 2017, Google Inc.
  * and other copyright owners as documented in the project's IP log.
  *
  * This program and the accompanying materials are made available
  * under the terms of the Eclipse Distribution License v1.0 which
  * accompanies this distribution, is reproduced below, and is
  * available at http://www.eclipse.org/org/documents/edl-v10.php
  *
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or
  * without modification, are permitted provided that the following
  * conditions are met:
  *
  * - Redistributions of source code must retain the above copyright
  *   notice, this list of conditions and the following disclaimer.
  *
  * - Redistributions in binary form must reproduce the above
  *   copyright notice, this list of conditions and the following
  *   disclaimer in the documentation and/or other materials provided
  *   with the distribution.
  *
  * - Neither the name of the Eclipse Foundation, Inc. nor the
  *   names of its contributors may be used to endorse or promote
  *   products derived from this software without specific prior
  *   written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
  * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
  * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

 package org.eclipse.jgit.internal.storage.file;

 import static java.util.stream.Collectors.toList;
 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_NONFASTFORWARD;

 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;

 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.internal.storage.file.RefDirectory.PackedRefList;
 import org.eclipse.jgit.lib.BatchRefUpdate;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectIdRef;
 import org.eclipse.jgit.lib.PersonIdent;
 import org.eclipse.jgit.lib.ProgressMonitor;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.RefDatabase;
 import org.eclipse.jgit.lib.ReflogEntry;
 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.util.RefList;

 /**
  * Implementation of {@link BatchRefUpdate} that uses the {@code packed-refs}
  * file to support atomically updating multiple refs.
  * <p>
  * The algorithm is designed to be compatible with traditional single ref
  * updates operating on single refs only. Regardless of success or failure, the
  * results are atomic: from the perspective of any reader, either all updates in
  * the batch will be visible, or none will. In the case of process failure
  * during any of the following steps, removal of stale lock files is always
  * safe, and will never result in an inconsistent state, although the update may
  * or may not have been applied.
  * <p>
  * The algorithm is:
  * <ol>
  * <li>Pack loose refs involved in the transaction using the normal pack-refs
  * operation. This ensures that creating lock files in the following step
  * succeeds even if a batch contains both a delete of {@code refs/x} (loose) and
  * a create of {@code refs/x/y}.</li>
  * <li>Create locks for all loose refs involved in the transaction, even if they
  * are not currently loose.</li>
  * <li>Pack loose refs again, this time while holding all lock files (see {@link
  * RefDirectory#pack(Map)}), without deleting them afterwards. This covers a
  * potential race where new loose refs were created after the initial packing
  * step. If no new loose refs were created during this race, this step does not
  * modify any files on disk. Keep the merged state in memory.</li>
  * <li>Update the in-memory packed refs with the commands in the batch, possibly
  * failing the whole batch if any old ref values do not match.</li>
  * <li>If the update succeeds, lock {@code packed-refs} and commit by atomically
  * renaming the lock file.</li>
  * <li>Delete loose ref lock files.</li>
  * </ol>
  *
  * Because the packed-refs file format is a sorted list, this algorithm is
  * linear in the total number of refs, regardless of the batch size. This can be
  * a significant slowdown on repositories with large numbers of refs; callers
  * that prefer speed over atomicity should use {@code setAtomic(false)}. As an
  * optimization, an update containing a single ref update does not use the
  * packed-refs protocol.
  */
 class PackedBatchRefUpdate extends BatchRefUpdate {
 	private RefDirectory refdb;

 	PackedBatchRefUpdate(RefDirectory refdb) {
 		super(refdb);
 		this.refdb = refdb;
 	}

 	@Override
 	public void execute(RevWalk walk, ProgressMonitor monitor,
 			List<String> options) throws IOException {
 		if (!isAtomic()) {
 			// Use default one-by-one implementation.
 			super.execute(walk, monitor, options);
 			return;
 		}
 		List<ReceiveCommand> pending =
 				ReceiveCommand.filter(getCommands(), NOT_ATTEMPTED);
 		if (pending.isEmpty()) {
 			return;
 		}
 		if (pending.size() == 1) {
 			// Single-ref updates are always atomic, no need for packed-refs.
 			super.execute(walk, monitor, options);
 			return;
 		}

 		// Required implementation details copied from super.execute.
 		if (!blockUntilTimestamps(MAX_WAIT)) {
 			return;
 		}
 		if (options != null) {
 			setPushOptions(options);
 		}
 		// End required implementation details.

 		// Check for conflicting names before attempting to acquire locks, since
 		// lockfile creation may fail on file/directory conflicts.
 		if (!checkConflictingNames(pending)) {
 			return;
 		}

 		if (!checkObjectExistence(walk, pending)) {
 			return;
 		}

 		if (!checkNonFastForwards(walk, pending)) {
 			return;
 		}

 		// Pack refs normally, so we can create lock files even in the case where
 		// refs/x is deleted and refs/x/y is created in this batch.
 		refdb.pack(
 				pending.stream().map(ReceiveCommand::getRefName).collect(toList()));

 		Map<String, LockFile> locks = new HashMap<>();
 		try {
 			if (!lockLooseRefs(pending, locks)) {
 				return;
 			}
 			PackedRefList oldPackedList = refdb.pack(locks);
 			RefList<Ref> newRefs = applyUpdates(walk, oldPackedList, pending);
 			if (newRefs == null) {
 				return;
 			}
 			LockFile packedRefsLock = new LockFile(refdb.packedRefsFile);
 			try {
 				packedRefsLock.lock();
 				refdb.commitPackedRefs(packedRefsLock, newRefs, oldPackedList);
 			} finally {
 				packedRefsLock.unlock();
 			}
 		} finally {
 			locks.values().forEach(LockFile::unlock);
 		}

 		refdb.fireRefsChanged();
 		pending.forEach(c -> c.setResult(ReceiveCommand.Result.OK));
 		writeReflog(pending);
 	}

 	private boolean checkConflictingNames(List<ReceiveCommand> commands)
 			throws IOException {
 		Set<String> takenNames = new HashSet<>();
 		Set<String> takenPrefixes = new HashSet<>();
 		Set<String> deletes = new HashSet<>();
 		for (ReceiveCommand cmd : commands) {
 			if (cmd.getType() != ReceiveCommand.Type.DELETE) {
 				takenNames.add(cmd.getRefName());
 				addPrefixesTo(cmd.getRefName(), takenPrefixes);
 			} else {
 				deletes.add(cmd.getRefName());
 			}
 		}
 		Set<String> initialRefs = refdb.getRefs(RefDatabase.ALL).keySet();
 		for (String name : initialRefs) {
 			if (!deletes.contains(name)) {
 				takenNames.add(name);
 				addPrefixesTo(name, takenPrefixes);
 			}
 		}

 		for (ReceiveCommand cmd : commands) {
 			if (cmd.getType() != ReceiveCommand.Type.DELETE &&
 					takenPrefixes.contains(cmd.getRefName())) {
 				// This ref is a prefix of some other ref. This check doesn't apply when
 				// this command is a delete, because if the ref is deleted nobody will
 				// ever be creating a loose ref with that name.
 				lockFailure(cmd, commands);
 				return false;
 			}
 			for (String prefix : getPrefixes(cmd.getRefName())) {
 				if (takenNames.contains(prefix)) {
 					// A prefix of this ref is already a refname. This check does apply
 					// when this command is a delete, because we would need to create the
 					// refname as a directory in order to create a lockfile for the
 					// to-be-deleted ref.
 					lockFailure(cmd, commands);
 					return false;
 				}
 			}
 		}
 		return true;
 	}

 	private boolean checkObjectExistence(RevWalk walk,
 			List<ReceiveCommand> commands) throws IOException {
 		for (ReceiveCommand cmd : commands) {
 			try {
 				if (!cmd.getNewId().equals(ObjectId.zeroId())) {
 					walk.parseAny(cmd.getNewId());
 				}
 			} catch (MissingObjectException e) {
 				// ReceiveCommand#setResult(Result) converts REJECTED to
 				// REJECTED_NONFASTFORWARD, even though that result is also used for a
 				// missing object. Eagerly handle this case so we can set the right
 				// result.
 				reject(cmd, ReceiveCommand.Result.REJECTED_MISSING_OBJECT, commands);
 				return false;
 			}
 		}
 		return true;
 	}

 	private boolean checkNonFastForwards(RevWalk walk,
 			List<ReceiveCommand> commands) throws IOException {
 		if (isAllowNonFastForwards()) {
 			return true;
 		}
 		for (ReceiveCommand cmd : commands) {
 			cmd.updateType(walk);
 			if (cmd.getType() == ReceiveCommand.Type.UPDATE_NONFASTFORWARD) {
 				reject(cmd, REJECTED_NONFASTFORWARD, commands);
 				return false;
 			}
 		}
 		return true;
 	}

 	private boolean lockLooseRefs(List<ReceiveCommand> commands,
 			Map<String, LockFile> locks) throws IOException {
 		for (ReceiveCommand c : commands) {
 			LockFile lock = new LockFile(refdb.fileFor(c.getRefName()));
 			if (!lock.lock()) {
 				lockFailure(c, commands);
 				return false;
 			}
 			locks.put(c.getRefName(), lock);
 		}
 		return true;
 	}

 	private static RefList<Ref> applyUpdates(RevWalk walk, RefList<Ref> refs,
 			List<ReceiveCommand> commands) throws IOException {
 		int nDeletes = 0;
 		List<ReceiveCommand> adds = new ArrayList<>(commands.size());
 		for (ReceiveCommand c : commands) {
 			if (c.getType() == ReceiveCommand.Type.CREATE) {
 				adds.add(c);
 			} else if (c.getType() == ReceiveCommand.Type.DELETE) {
 				nDeletes++;
 			}
 		}
 		int addIdx = 0;

 		// Construct a new RefList by linearly scanning the old list, and merging in
 		// any updates.
 		Map<String, ReceiveCommand> byName = byName(commands);
 		RefList.Builder<Ref> b =
 				new RefList.Builder<>(refs.size() - nDeletes + adds.size());
 		for (Ref ref : refs) {
 			String name = ref.getName();
 			ReceiveCommand cmd = byName.remove(name);
 			if (cmd == null) {
 				b.add(ref);
 				continue;
 			}
 			if (!cmd.getOldId().equals(ref.getObjectId())) {
 				lockFailure(cmd, commands);
 				return null;
 			}

 			// Consume any adds between the last and current ref.
 			while (addIdx < adds.size()) {
 				ReceiveCommand currAdd = adds.get(addIdx);
 				if (currAdd.getRefName().compareTo(name) < 0) {
 					b.add(peeledRef(walk, currAdd));
 					byName.remove(currAdd.getRefName());
 				} else {
 					break;
 				}
 				addIdx++;
 			}

 			if (cmd.getType() != ReceiveCommand.Type.DELETE) {
 				b.add(peeledRef(walk, cmd));
 			}
 		}

 		// All remaining adds are valid, since the refs didn't exist.
 		while (addIdx < adds.size()) {
 			ReceiveCommand cmd = adds.get(addIdx++);
 			byName.remove(cmd.getRefName());
 			b.add(peeledRef(walk, cmd));
 		}

 		// Any remaining updates/deletes do not correspond to any existing refs, so
 		// they are lock failures.
 		if (!byName.isEmpty()) {
 			lockFailure(byName.values().iterator().next(), commands);
 			return null;
 		}

 		return b.toRefList();
 	}

 	private void writeReflog(List<ReceiveCommand> commands) {
 		PersonIdent ident = getRefLogIdent();
 		if (ident == null) {
 			ident = new PersonIdent(refdb.getRepository());
 		}
 		ReflogWriter w = refdb.getLogWriter();
 		for (ReceiveCommand cmd : commands) {
 			// Assume any pending commands have already been executed atomically.
 			if (cmd.getResult() != ReceiveCommand.Result.OK) {
 				continue;
 			}
 			String name = cmd.getRefName();

 			if (cmd.getType() == ReceiveCommand.Type.DELETE) {
 				try {
 					RefDirectory.delete(w.logFor(name), RefDirectory.levelsIn(name));
 				} catch (IOException e) {
 					// Ignore failures, see below.
 				}
 				continue;
 			}

 			if (isRefLogDisabled(cmd)) {
 				continue;
 			}

 			String msg = getRefLogMessage(cmd);
 			if (isRefLogIncludingResult(cmd)) {
 				String strResult = toResultString(cmd);
 				if (strResult != null) {
 					msg = msg.isEmpty()
 							? strResult : msg + ": " + strResult; //$NON-NLS-1$
 				}
 			}
 			try {
 				w.log(name, cmd.getOldId(), cmd.getNewId(), ident, msg);
 			} catch (IOException e) {
 				// Ignore failures, but continue attempting to write more reflogs.
 				//
 				// In this storage format, it is impossible to atomically write the
 				// reflog with the ref updates, so we have to choose between:
 				// a. Propagating this exception and claiming failure, even though the
 				//    actual ref updates succeeded.
 				// b. Ignoring failures writing the reflog, so we claim success if and
 				//    only if the ref updates succeeded.
 				// We choose (b) in order to surprise callers the least.
 				//
 				// Possible future improvements:
 				// * Log a warning to a logger.
 				// * Retry a fixed number of times in case the error was transient.
 			}
 		}
 	}

 	private String toResultString(ReceiveCommand cmd) {
 		switch (cmd.getType()) {
 		case CREATE:
 			return ReflogEntry.PREFIX_CREATED;
 		case UPDATE:
 			// Match the behavior of a single RefUpdate. In that case, setting the
 			// force bit completely bypasses the potentially expensive isMergedInto
 			// check, by design, so the reflog message may be inaccurate.
 			//
 			// Similarly, this class bypasses the isMergedInto checks when the force
 			// bit is set, meaning we can't actually distinguish between UPDATE and
 			// UPDATE_NONFASTFORWARD when isAllowNonFastForwards() returns true.
 			return isAllowNonFastForwards()
 					? ReflogEntry.PREFIX_FORCED_UPDATE : ReflogEntry.PREFIX_FAST_FORWARD;
 		case UPDATE_NONFASTFORWARD:
 			return ReflogEntry.PREFIX_FORCED_UPDATE;
 		default:
 			return null;
 		}
 	}

 	private static Map<String, ReceiveCommand> byName(
 			List<ReceiveCommand> commands) {
 		Map<String, ReceiveCommand> ret = new LinkedHashMap<>();
 		for (ReceiveCommand cmd : commands) {
 			ret.put(cmd.getRefName(), cmd);
 		}
 		return ret;
 	}

 	private static Ref peeledRef(RevWalk walk, ReceiveCommand cmd)
 			throws IOException {
 		ObjectId newId = cmd.getNewId().copy();
 		RevObject obj = walk.parseAny(newId);
 		if (obj instanceof RevTag) {
 			return new ObjectIdRef.PeeledTag(
 					Ref.Storage.PACKED, cmd.getRefName(), newId, walk.peel(obj).copy());
 		}
 		return new ObjectIdRef.PeeledNonTag(
 				Ref.Storage.PACKED, cmd.getRefName(), newId);
 	}

 	private static void lockFailure(ReceiveCommand cmd,
 			List<ReceiveCommand> commands) {
 		reject(cmd, LOCK_FAILURE, commands);
 	}

 	private static void reject(ReceiveCommand cmd, ReceiveCommand.Result result,
 			List<ReceiveCommand> commands) {
 		cmd.setResult(result);
 		for (ReceiveCommand c2 : commands) {
 			if (c2.getResult() == ReceiveCommand.Result.OK) {
 				// Undo OK status so ReceiveCommand#abort aborts it. Assumes this method
 				// is always called before committing any updates to disk.
 				c2.setResult(ReceiveCommand.Result.NOT_ATTEMPTED);
 			}
 		}
 		ReceiveCommand.abort(commands);
 	}
 }
	/*
	* Copyright (C) 2017, Google Inc.
	* and other copyright owners as documented in the project's IP log.
	*
	* This program and the accompanying materials are made available
	* under the terms of the Eclipse Distribution License v1.0 which
	* accompanies this distribution, is reproduced below, and is
	* available at http://www.eclipse.org/org/documents/edl-v10.php
	*
	* All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or
	* without modification, are permitted provided that the following
	* conditions are met:
	*
	* - Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	*
	* - Redistributions in binary form must reproduce the above
	* copyright notice, this list of conditions and the following
	* disclaimer in the documentation and/or other materials provided
	* with the distribution.
	*
	* - Neither the name of the Eclipse Foundation, Inc. nor the
	* names of its contributors may be used to endorse or promote
	* products derived from this software without specific prior
	* written permission.
	*
	* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
	* CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
	* INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
	* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
	* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
	* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
	* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
	* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
	* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
	* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
	* ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

	package org.eclipse.jgit.internal.storage.file;

	import static java.util.stream.Collectors.toList;
	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_NONFASTFORWARD;

	import java.io.IOException;
	import java.util.ArrayList;
	import java.util.HashMap;
	import java.util.HashSet;
	import java.util.LinkedHashMap;
	import java.util.List;
	import java.util.Map;
	import java.util.Set;

	import org.eclipse.jgit.errors.MissingObjectException;
	import org.eclipse.jgit.internal.storage.file.RefDirectory.PackedRefList;
	import org.eclipse.jgit.lib.BatchRefUpdate;
	import org.eclipse.jgit.lib.ObjectId;
	import org.eclipse.jgit.lib.ObjectIdRef;
	import org.eclipse.jgit.lib.PersonIdent;
	import org.eclipse.jgit.lib.ProgressMonitor;
	import org.eclipse.jgit.lib.Ref;
	import org.eclipse.jgit.lib.RefDatabase;
	import org.eclipse.jgit.lib.ReflogEntry;
	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.util.RefList;

	/**
	* Implementation of {@link BatchRefUpdate} that uses the {@code packed-refs}
	* file to support atomically updating multiple refs.
	* <p>
	* The algorithm is designed to be compatible with traditional single ref
	* updates operating on single refs only. Regardless of success or failure, the
	* results are atomic: from the perspective of any reader, either all updates in
	* the batch will be visible, or none will. In the case of process failure
	* during any of the following steps, removal of stale lock files is always
	* safe, and will never result in an inconsistent state, although the update may
	* or may not have been applied.
	* <p>
	* The algorithm is:
	* <ol>
	* <li>Pack loose refs involved in the transaction using the normal pack-refs
	* operation. This ensures that creating lock files in the following step
	* succeeds even if a batch contains both a delete of {@code refs/x} (loose) and
	* a create of {@code refs/x/y}.</li>
	* <li>Create locks for all loose refs involved in the transaction, even if they
	* are not currently loose.</li>
	* <li>Pack loose refs again, this time while holding all lock files (see {@link
	* RefDirectory#pack(Map)}), without deleting them afterwards. This covers a
	* potential race where new loose refs were created after the initial packing
	* step. If no new loose refs were created during this race, this step does not
	* modify any files on disk. Keep the merged state in memory.</li>
	* <li>Update the in-memory packed refs with the commands in the batch, possibly
	* failing the whole batch if any old ref values do not match.</li>
	* <li>If the update succeeds, lock {@code packed-refs} and commit by atomically
	* renaming the lock file.</li>
	* <li>Delete loose ref lock files.</li>
	* </ol>
	*
	* Because the packed-refs file format is a sorted list, this algorithm is
	* linear in the total number of refs, regardless of the batch size. This can be
	* a significant slowdown on repositories with large numbers of refs; callers
	* that prefer speed over atomicity should use {@code setAtomic(false)}. As an
	* optimization, an update containing a single ref update does not use the
	* packed-refs protocol.
	*/
	class PackedBatchRefUpdate extends BatchRefUpdate {
	private RefDirectory refdb;

	PackedBatchRefUpdate(RefDirectory refdb) {
	super(refdb);
	this.refdb = refdb;
	}

	@Override
	public void execute(RevWalk walk, ProgressMonitor monitor,
	List<String> options) throws IOException {
	if (!isAtomic()) {
	// Use default one-by-one implementation.
	super.execute(walk, monitor, options);
	return;
	}
	List<ReceiveCommand> pending =
	ReceiveCommand.filter(getCommands(), NOT_ATTEMPTED);
	if (pending.isEmpty()) {
	return;
	}
	if (pending.size() == 1) {
	// Single-ref updates are always atomic, no need for packed-refs.
	super.execute(walk, monitor, options);
	return;
	}

	// Required implementation details copied from super.execute.
	if (!blockUntilTimestamps(MAX_WAIT)) {
	return;
	}
	if (options != null) {
	setPushOptions(options);
	}
	// End required implementation details.

	// Check for conflicting names before attempting to acquire locks, since
	// lockfile creation may fail on file/directory conflicts.
	if (!checkConflictingNames(pending)) {
	return;
	}

	if (!checkObjectExistence(walk, pending)) {
	return;
	}

	if (!checkNonFastForwards(walk, pending)) {
	return;
	}

	// Pack refs normally, so we can create lock files even in the case where
	// refs/x is deleted and refs/x/y is created in this batch.
	refdb.pack(
	pending.stream().map(ReceiveCommand::getRefName).collect(toList()));

	Map<String, LockFile> locks = new HashMap<>();
	try {
	if (!lockLooseRefs(pending, locks)) {
	return;
	}
	PackedRefList oldPackedList = refdb.pack(locks);
	RefList<Ref> newRefs = applyUpdates(walk, oldPackedList, pending);
	if (newRefs == null) {
	return;
	}
	LockFile packedRefsLock = new LockFile(refdb.packedRefsFile);
	try {
	packedRefsLock.lock();
	refdb.commitPackedRefs(packedRefsLock, newRefs, oldPackedList);
	} finally {
	packedRefsLock.unlock();
	}
	} finally {
	locks.values().forEach(LockFile::unlock);
	}

	refdb.fireRefsChanged();
	pending.forEach(c -> c.setResult(ReceiveCommand.Result.OK));
	writeReflog(pending);
	}

	private boolean checkConflictingNames(List<ReceiveCommand> commands)
	throws IOException {
	Set<String> takenNames = new HashSet<>();
	Set<String> takenPrefixes = new HashSet<>();
	Set<String> deletes = new HashSet<>();
	for (ReceiveCommand cmd : commands) {
	if (cmd.getType() != ReceiveCommand.Type.DELETE) {
	takenNames.add(cmd.getRefName());
	addPrefixesTo(cmd.getRefName(), takenPrefixes);
	} else {
	deletes.add(cmd.getRefName());
	}
	}
	Set<String> initialRefs = refdb.getRefs(RefDatabase.ALL).keySet();
	for (String name : initialRefs) {
	if (!deletes.contains(name)) {
	takenNames.add(name);
	addPrefixesTo(name, takenPrefixes);
	}
	}

	for (ReceiveCommand cmd : commands) {
	if (cmd.getType() != ReceiveCommand.Type.DELETE &&
	takenPrefixes.contains(cmd.getRefName())) {
	// This ref is a prefix of some other ref. This check doesn't apply when
	// this command is a delete, because if the ref is deleted nobody will
	// ever be creating a loose ref with that name.
	lockFailure(cmd, commands);
	return false;
	}
	for (String prefix : getPrefixes(cmd.getRefName())) {
	if (takenNames.contains(prefix)) {
	// A prefix of this ref is already a refname. This check does apply
	// when this command is a delete, because we would need to create the
	// refname as a directory in order to create a lockfile for the
	// to-be-deleted ref.
	lockFailure(cmd, commands);
	return false;
	}
	}
	}
	return true;
	}

	private boolean checkObjectExistence(RevWalk walk,
	List<ReceiveCommand> commands) throws IOException {
	for (ReceiveCommand cmd : commands) {
	try {
	if (!cmd.getNewId().equals(ObjectId.zeroId())) {
	walk.parseAny(cmd.getNewId());
	}
	} catch (MissingObjectException e) {
	// ReceiveCommand#setResult(Result) converts REJECTED to
	// REJECTED_NONFASTFORWARD, even though that result is also used for a
	// missing object. Eagerly handle this case so we can set the right
	// result.
	reject(cmd, ReceiveCommand.Result.REJECTED_MISSING_OBJECT, commands);
	return false;
	}
	}
	return true;
	}

	private boolean checkNonFastForwards(RevWalk walk,
	List<ReceiveCommand> commands) throws IOException {
	if (isAllowNonFastForwards()) {
	return true;
	}
	for (ReceiveCommand cmd : commands) {
	cmd.updateType(walk);
	if (cmd.getType() == ReceiveCommand.Type.UPDATE_NONFASTFORWARD) {
	reject(cmd, REJECTED_NONFASTFORWARD, commands);
	return false;
	}
	}
	return true;
	}

	private boolean lockLooseRefs(List<ReceiveCommand> commands,
	Map<String, LockFile> locks) throws IOException {
	for (ReceiveCommand c : commands) {
	LockFile lock = new LockFile(refdb.fileFor(c.getRefName()));
	if (!lock.lock()) {
	lockFailure(c, commands);
	return false;
	}
	locks.put(c.getRefName(), lock);
	}
	return true;
	}

	private static RefList<Ref> applyUpdates(RevWalk walk, RefList<Ref> refs,
	List<ReceiveCommand> commands) throws IOException {
	int nDeletes = 0;
	List<ReceiveCommand> adds = new ArrayList<>(commands.size());
	for (ReceiveCommand c : commands) {
	if (c.getType() == ReceiveCommand.Type.CREATE) {
	adds.add(c);
	} else if (c.getType() == ReceiveCommand.Type.DELETE) {
	nDeletes++;
	}
	}
	int addIdx = 0;

	// Construct a new RefList by linearly scanning the old list, and merging in
	// any updates.
	Map<String, ReceiveCommand> byName = byName(commands);
	RefList.Builder<Ref> b =
	new RefList.Builder<>(refs.size() - nDeletes + adds.size());
	for (Ref ref : refs) {
	String name = ref.getName();
	ReceiveCommand cmd = byName.remove(name);
	if (cmd == null) {
	b.add(ref);
	continue;
	}
	if (!cmd.getOldId().equals(ref.getObjectId())) {
	lockFailure(cmd, commands);
	return null;
	}

	// Consume any adds between the last and current ref.
	while (addIdx < adds.size()) {
	ReceiveCommand currAdd = adds.get(addIdx);
	if (currAdd.getRefName().compareTo(name) < 0) {
	b.add(peeledRef(walk, currAdd));
	byName.remove(currAdd.getRefName());
	} else {
	break;
	}
	addIdx++;
	}

	if (cmd.getType() != ReceiveCommand.Type.DELETE) {
	b.add(peeledRef(walk, cmd));
	}
	}

	// All remaining adds are valid, since the refs didn't exist.
	while (addIdx < adds.size()) {
	ReceiveCommand cmd = adds.get(addIdx++);
	byName.remove(cmd.getRefName());
	b.add(peeledRef(walk, cmd));
	}

	// Any remaining updates/deletes do not correspond to any existing refs, so
	// they are lock failures.
	if (!byName.isEmpty()) {
	lockFailure(byName.values().iterator().next(), commands);
	return null;
	}

	return b.toRefList();
	}

	private void writeReflog(List<ReceiveCommand> commands) {
	PersonIdent ident = getRefLogIdent();
	if (ident == null) {
	ident = new PersonIdent(refdb.getRepository());
	}
	ReflogWriter w = refdb.getLogWriter();
	for (ReceiveCommand cmd : commands) {
	// Assume any pending commands have already been executed atomically.
	if (cmd.getResult() != ReceiveCommand.Result.OK) {
	continue;
	}
	String name = cmd.getRefName();

	if (cmd.getType() == ReceiveCommand.Type.DELETE) {
	try {
	RefDirectory.delete(w.logFor(name), RefDirectory.levelsIn(name));
	} catch (IOException e) {
	// Ignore failures, see below.
	}
	continue;
	}

	if (isRefLogDisabled(cmd)) {
	continue;
	}

	String msg = getRefLogMessage(cmd);
	if (isRefLogIncludingResult(cmd)) {
	String strResult = toResultString(cmd);
	if (strResult != null) {
	msg = msg.isEmpty()
	? strResult : msg + ": " + strResult; //$NON-NLS-1$
	}
	}
	try {
	w.log(name, cmd.getOldId(), cmd.getNewId(), ident, msg);
	} catch (IOException e) {
	// Ignore failures, but continue attempting to write more reflogs.
	//
	// In this storage format, it is impossible to atomically write the
	// reflog with the ref updates, so we have to choose between:
	// a. Propagating this exception and claiming failure, even though the
	// actual ref updates succeeded.
	// b. Ignoring failures writing the reflog, so we claim success if and
	// only if the ref updates succeeded.
	// We choose (b) in order to surprise callers the least.
	//
	// Possible future improvements:
	// * Log a warning to a logger.
	// * Retry a fixed number of times in case the error was transient.
	}
	}
	}

	private String toResultString(ReceiveCommand cmd) {
	switch (cmd.getType()) {
	case CREATE:
	return ReflogEntry.PREFIX_CREATED;
	case UPDATE:
	// Match the behavior of a single RefUpdate. In that case, setting the
	// force bit completely bypasses the potentially expensive isMergedInto
	// check, by design, so the reflog message may be inaccurate.
	//
	// Similarly, this class bypasses the isMergedInto checks when the force
	// bit is set, meaning we can't actually distinguish between UPDATE and
	// UPDATE_NONFASTFORWARD when isAllowNonFastForwards() returns true.
	return isAllowNonFastForwards()
	? ReflogEntry.PREFIX_FORCED_UPDATE : ReflogEntry.PREFIX_FAST_FORWARD;
	case UPDATE_NONFASTFORWARD:
	return ReflogEntry.PREFIX_FORCED_UPDATE;
	default:
	return null;
	}
	}

	private static Map<String, ReceiveCommand> byName(
	List<ReceiveCommand> commands) {
	Map<String, ReceiveCommand> ret = new LinkedHashMap<>();
	for (ReceiveCommand cmd : commands) {
	ret.put(cmd.getRefName(), cmd);
	}
	return ret;
	}

	private static Ref peeledRef(RevWalk walk, ReceiveCommand cmd)
	throws IOException {
	ObjectId newId = cmd.getNewId().copy();
	RevObject obj = walk.parseAny(newId);
	if (obj instanceof RevTag) {
	return new ObjectIdRef.PeeledTag(
	Ref.Storage.PACKED, cmd.getRefName(), newId, walk.peel(obj).copy());
	}
	return new ObjectIdRef.PeeledNonTag(
	Ref.Storage.PACKED, cmd.getRefName(), newId);
	}

	private static void lockFailure(ReceiveCommand cmd,
	List<ReceiveCommand> commands) {
	reject(cmd, LOCK_FAILURE, commands);
	}

	private static void reject(ReceiveCommand cmd, ReceiveCommand.Result result,
	List<ReceiveCommand> commands) {
	cmd.setResult(result);
	for (ReceiveCommand c2 : commands) {
	if (c2.getResult() == ReceiveCommand.Result.OK) {
	// Undo OK status so ReceiveCommand#abort aborts it. Assumes this method
	// is always called before committing any updates to disk.
	c2.setResult(ReceiveCommand.Result.NOT_ATTEMPTED);
	}
	}
	ReceiveCommand.abort(commands);
	}
	}