javatests/com/google/gerrit/acceptance/server/notedb/NoteDbOnlyIT.java - gerrit - Git at Google

 // Copyright (C) 2017 The Android Open Source Project
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 // http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 package com.google.gerrit.acceptance.server.notedb;

 import static com.google.common.truth.Truth.assertThat;
 import static com.google.common.truth.Truth8.assertThat;
 import static com.google.gerrit.extensions.client.ListChangesOption.MESSAGES;
 import static java.util.stream.Collectors.toList;

 import com.google.common.collect.Iterables;
 import com.google.common.collect.Streams;
 import com.google.gerrit.acceptance.AbstractDaemonTest;
 import com.google.gerrit.acceptance.PushOneCommit;
 import com.google.gerrit.extensions.api.changes.ReviewInput;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.server.project.NoSuchChangeException;
 import com.google.gerrit.server.update.BatchUpdate;
 import com.google.gerrit.server.update.BatchUpdateListener;
 import com.google.gerrit.server.update.BatchUpdateOp;
 import com.google.gerrit.server.update.ChangeContext;
 import com.google.gerrit.server.update.RepoContext;
 import com.google.gerrit.server.update.RetryHelper;
 import com.google.gerrit.server.util.time.TimeUtil;
 import com.google.gerrit.testing.ConfigSuite;
 import com.google.inject.Inject;
 import java.io.IOException;
 import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.Callable;
 import java.util.concurrent.atomic.AtomicInteger;
 import org.eclipse.jgit.internal.storage.dfs.InMemoryRepository;
 import org.eclipse.jgit.lib.CommitBuilder;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectInserter;
 import org.eclipse.jgit.lib.PersonIdent;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.RefUpdate;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevSort;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.junit.Test;

 public class NoteDbOnlyIT extends AbstractDaemonTest {
   @ConfigSuite.Default
   public static Config defaultConfig() {
     Config cfg = new Config();
     // Avoid spurious timeouts during intentional retries due to overloaded test machines.
     cfg.setString("retry", null, "timeout", Integer.MAX_VALUE + "s");
     return cfg;
   }

   @Inject private RetryHelper retryHelper;

   @Test
   public void updateChangeFailureRollsBackRefUpdate() throws Exception {
     PushOneCommit.Result r = createChange();
     Change.Id id = r.getChange().getId();

     String master = "refs/heads/master";
     String backup = "refs/backup/master";
     ObjectId master1 = getRef(master).get();
     assertThat(getRef(backup)).isEmpty();

     // Toy op that copies the value of refs/heads/master to refs/backup/master.
     BatchUpdateOp backupMasterOp =
         new BatchUpdateOp() {
           ObjectId newId;

           @Override
           public void updateRepo(RepoContext ctx) throws IOException {
             ObjectId oldId = ctx.getRepoView().getRef(backup).orElse(ObjectId.zeroId());
             newId = ctx.getRepoView().getRef(master).get();
             ctx.addRefUpdate(oldId, newId, backup);
           }

           @Override
           public boolean updateChange(ChangeContext ctx) {
             ctx.getUpdate(ctx.getChange().currentPatchSetId())
                 .setChangeMessage("Backed up master branch to " + newId.name());
             return true;
           }
         };

     try (BatchUpdate bu = newBatchUpdate(batchUpdateFactory)) {
       bu.addOp(id, backupMasterOp);
       bu.execute();
     }

     // Ensure backupMasterOp worked.
     assertThat(getRef(backup)).hasValue(master1);
     assertThat(getMessages(id)).contains("Backed up master branch to " + master1.name());

     // Advance master by submitting the change.
     gApi.changes().id(id.get()).current().review(ReviewInput.approve());
     gApi.changes().id(id.get()).current().submit();
     ObjectId master2 = getRef(master).get();
     assertThat(master2).isNotEqualTo(master1);
     int msgCount = getMessages(id).size();

     try (BatchUpdate bu = newBatchUpdate(batchUpdateFactory)) {
       // This time, we attempt to back up master, but we fail during updateChange.
       bu.addOp(id, backupMasterOp);
       String msg = "Change is bad";
       bu.addOp(
           id,
           new BatchUpdateOp() {
             @Override
             public boolean updateChange(ChangeContext ctx) throws ResourceConflictException {
               throw new ResourceConflictException(msg);
             }
           });
       try {
         bu.execute();
         fail("expected ResourceConflictException");
       } catch (ResourceConflictException e) {
         assertThat(e).hasMessageThat().isEqualTo(msg);
       }
     }

     // If updateChange hadn't failed, backup would have been updated to master2.
     assertThat(getRef(backup)).hasValue(master1);
     assertThat(getMessages(id)).hasSize(msgCount);
   }

   @Test
   public void retryOnLockFailureWithAtomicUpdates() throws Exception {
     PushOneCommit.Result r = createChange();
     Change.Id id = r.getChange().getId();
     String master = "refs/heads/master";
     ObjectId initial;
     try (Repository repo = repoManager.openRepository(project)) {
       ensureAtomicTransactions(repo);
       initial = repo.exactRef(master).getObjectId();
     }

     AtomicInteger updateRepoCalledCount = new AtomicInteger();
     AtomicInteger updateChangeCalledCount = new AtomicInteger();
     AtomicInteger afterUpdateReposCalledCount = new AtomicInteger();

     String result =
         retryHelper.execute(
             batchUpdateFactory -> {
               try (BatchUpdate bu = newBatchUpdate(batchUpdateFactory)) {
                 bu.addOp(
                     id,
                     new UpdateRefAndAddMessageOp(updateRepoCalledCount, updateChangeCalledCount));
                 bu.execute(new ConcurrentWritingListener(afterUpdateReposCalledCount));
               }
               return "Done";
             });

     assertThat(result).isEqualTo("Done");
     assertThat(updateRepoCalledCount.get()).isEqualTo(2);
     assertThat(afterUpdateReposCalledCount.get()).isEqualTo(2);
     assertThat(updateChangeCalledCount.get()).isEqualTo(2);

     List<String> messages = getMessages(id);
     assertThat(Iterables.getLast(messages)).isEqualTo(UpdateRefAndAddMessageOp.CHANGE_MESSAGE);
     assertThat(Collections.frequency(messages, UpdateRefAndAddMessageOp.CHANGE_MESSAGE))
         .isEqualTo(1);

     try (Repository repo = repoManager.openRepository(project)) {
       // Op lost the race, so the other writer's commit happened first. Then op retried and wrote
       // its commit with the other writer's commit as parent.
       assertThat(commitMessages(repo, initial, repo.exactRef(master).getObjectId()))
           .containsExactly(
               ConcurrentWritingListener.MSG_PREFIX + "1", UpdateRefAndAddMessageOp.COMMIT_MESSAGE)
           .inOrder();
     }
   }

   @Test
   public void missingChange() throws Exception {
     Change.Id changeId = new Change.Id(1234567);
     assertNoSuchChangeException(() -> notesFactory.create(project, changeId));
     assertNoSuchChangeException(() -> notesFactory.createChecked(project, changeId));
   }

   private void assertNoSuchChangeException(Callable<?> callable) throws Exception {
     try {
       callable.call();
       fail("expected NoSuchChangeException");
     } catch (NoSuchChangeException e) {
       // Expected.
     }
   }

   private class ConcurrentWritingListener implements BatchUpdateListener {
     static final String MSG_PREFIX = "Other writer ";

     private final AtomicInteger calledCount;

     private ConcurrentWritingListener(AtomicInteger calledCount) {
       this.calledCount = calledCount;
     }

     @Override
     public void afterUpdateRepos() throws Exception {
       // Reopen repo and update ref, to simulate a concurrent write in another
       // thread. Only do this the first time the listener is called.
       if (calledCount.getAndIncrement() > 0) {
         return;
       }
       try (Repository repo = repoManager.openRepository(project);
           RevWalk rw = new RevWalk(repo);
           ObjectInserter ins = repo.newObjectInserter()) {
         String master = "refs/heads/master";
         ObjectId oldId = repo.exactRef(master).getObjectId();
         ObjectId newId = newCommit(rw, ins, oldId, MSG_PREFIX + calledCount.get());
         ins.flush();
         RefUpdate ru = repo.updateRef(master);
         ru.setExpectedOldObjectId(oldId);
         ru.setNewObjectId(newId);
         assertThat(ru.update(rw)).isEqualTo(RefUpdate.Result.FAST_FORWARD);
       }
     }
   }

   private class UpdateRefAndAddMessageOp implements BatchUpdateOp {
     static final String COMMIT_MESSAGE = "A commit";
     static final String CHANGE_MESSAGE = "A change message";

     private final AtomicInteger updateRepoCalledCount;
     private final AtomicInteger updateChangeCalledCount;

     private UpdateRefAndAddMessageOp(
         AtomicInteger updateRepoCalledCount, AtomicInteger updateChangeCalledCount) {
       this.updateRepoCalledCount = updateRepoCalledCount;
       this.updateChangeCalledCount = updateChangeCalledCount;
     }

     @Override
     public void updateRepo(RepoContext ctx) throws Exception {
       String master = "refs/heads/master";
       ObjectId oldId = ctx.getRepoView().getRef(master).get();
       ObjectId newId = newCommit(ctx.getRevWalk(), ctx.getInserter(), oldId, COMMIT_MESSAGE);
       ctx.addRefUpdate(oldId, newId, master);
       updateRepoCalledCount.incrementAndGet();
     }

     @Override
     public boolean updateChange(ChangeContext ctx) throws Exception {
       ctx.getUpdate(ctx.getChange().currentPatchSetId()).setChangeMessage(CHANGE_MESSAGE);
       updateChangeCalledCount.incrementAndGet();
       return true;
     }
   }

   private ObjectId newCommit(RevWalk rw, ObjectInserter ins, ObjectId parent, String msg)
       throws IOException {
     PersonIdent ident = serverIdent.get();
     CommitBuilder cb = new CommitBuilder();
     cb.setParentId(parent);
     cb.setTreeId(rw.parseCommit(parent).getTree());
     cb.setMessage(msg);
     cb.setAuthor(ident);
     cb.setCommitter(ident);
     return ins.insert(Constants.OBJ_COMMIT, cb.build());
   }

   private BatchUpdate newBatchUpdate(BatchUpdate.Factory buf) {
     return buf.create(db, project, identifiedUserFactory.create(user.getId()), TimeUtil.nowTs());
   }

   private Optional<ObjectId> getRef(String name) throws Exception {
     try (Repository repo = repoManager.openRepository(project)) {
       return Optional.ofNullable(repo.exactRef(name)).map(Ref::getObjectId);
     }
   }

   private List<String> getMessages(Change.Id id) throws Exception {
     return gApi.changes()
         .id(id.get())
         .get(MESSAGES)
         .messages
         .stream()
         .map(m -> m.message)
         .collect(toList());
   }

   private static List<String> commitMessages(
       Repository repo, ObjectId fromExclusive, ObjectId toInclusive) throws Exception {
     try (RevWalk rw = new RevWalk(repo)) {
       rw.markStart(rw.parseCommit(toInclusive));
       rw.markUninteresting(rw.parseCommit(fromExclusive));
       rw.sort(RevSort.REVERSE);
       rw.setRetainBody(true);
       return Streams.stream(rw).map(RevCommit::getShortMessage).collect(toList());
     }
   }

   private void ensureAtomicTransactions(Repository repo) throws Exception {
     if (repo instanceof InMemoryRepository) {
       ((InMemoryRepository) repo).setPerformsAtomicTransactions(true);
     } else {
       assertThat(repo.getRefDatabase().performsAtomicTransactions())
           .named("performsAtomicTransactions on %s", repo)
           .isTrue();
     }
   }
 }
	// Copyright (C) 2017 The Android Open Source Project
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	package com.google.gerrit.acceptance.server.notedb;

	import static com.google.common.truth.Truth.assertThat;
	import static com.google.common.truth.Truth8.assertThat;
	import static com.google.gerrit.extensions.client.ListChangesOption.MESSAGES;
	import static java.util.stream.Collectors.toList;

	import com.google.common.collect.Iterables;
	import com.google.common.collect.Streams;
	import com.google.gerrit.acceptance.AbstractDaemonTest;
	import com.google.gerrit.acceptance.PushOneCommit;
	import com.google.gerrit.extensions.api.changes.ReviewInput;
	import com.google.gerrit.extensions.restapi.ResourceConflictException;
	import com.google.gerrit.reviewdb.client.Change;
	import com.google.gerrit.server.project.NoSuchChangeException;
	import com.google.gerrit.server.update.BatchUpdate;
	import com.google.gerrit.server.update.BatchUpdateListener;
	import com.google.gerrit.server.update.BatchUpdateOp;
	import com.google.gerrit.server.update.ChangeContext;
	import com.google.gerrit.server.update.RepoContext;
	import com.google.gerrit.server.update.RetryHelper;
	import com.google.gerrit.server.util.time.TimeUtil;
	import com.google.gerrit.testing.ConfigSuite;
	import com.google.inject.Inject;
	import java.io.IOException;
	import java.util.Collections;
	import java.util.List;
	import java.util.Optional;
	import java.util.concurrent.Callable;
	import java.util.concurrent.atomic.AtomicInteger;
	import org.eclipse.jgit.internal.storage.dfs.InMemoryRepository;
	import org.eclipse.jgit.lib.CommitBuilder;
	import org.eclipse.jgit.lib.Config;
	import org.eclipse.jgit.lib.Constants;
	import org.eclipse.jgit.lib.ObjectId;
	import org.eclipse.jgit.lib.ObjectInserter;
	import org.eclipse.jgit.lib.PersonIdent;
	import org.eclipse.jgit.lib.Ref;
	import org.eclipse.jgit.lib.RefUpdate;
	import org.eclipse.jgit.lib.Repository;
	import org.eclipse.jgit.revwalk.RevCommit;
	import org.eclipse.jgit.revwalk.RevSort;
	import org.eclipse.jgit.revwalk.RevWalk;
	import org.junit.Test;

	public class NoteDbOnlyIT extends AbstractDaemonTest {
	@ConfigSuite.Default
	public static Config defaultConfig() {
	Config cfg = new Config();
	// Avoid spurious timeouts during intentional retries due to overloaded test machines.
	cfg.setString("retry", null, "timeout", Integer.MAX_VALUE + "s");
	return cfg;
	}

	@Inject private RetryHelper retryHelper;

	@Test
	public void updateChangeFailureRollsBackRefUpdate() throws Exception {
	PushOneCommit.Result r = createChange();
	Change.Id id = r.getChange().getId();

	String master = "refs/heads/master";
	String backup = "refs/backup/master";
	ObjectId master1 = getRef(master).get();
	assertThat(getRef(backup)).isEmpty();

	// Toy op that copies the value of refs/heads/master to refs/backup/master.
	BatchUpdateOp backupMasterOp =
	new BatchUpdateOp() {
	ObjectId newId;

	@Override
	public void updateRepo(RepoContext ctx) throws IOException {
	ObjectId oldId = ctx.getRepoView().getRef(backup).orElse(ObjectId.zeroId());
	newId = ctx.getRepoView().getRef(master).get();
	ctx.addRefUpdate(oldId, newId, backup);
	}

	@Override
	public boolean updateChange(ChangeContext ctx) {
	ctx.getUpdate(ctx.getChange().currentPatchSetId())
	.setChangeMessage("Backed up master branch to " + newId.name());
	return true;
	}
	};

	try (BatchUpdate bu = newBatchUpdate(batchUpdateFactory)) {
	bu.addOp(id, backupMasterOp);
	bu.execute();
	}

	// Ensure backupMasterOp worked.
	assertThat(getRef(backup)).hasValue(master1);
	assertThat(getMessages(id)).contains("Backed up master branch to " + master1.name());

	// Advance master by submitting the change.
	gApi.changes().id(id.get()).current().review(ReviewInput.approve());
	gApi.changes().id(id.get()).current().submit();
	ObjectId master2 = getRef(master).get();
	assertThat(master2).isNotEqualTo(master1);
	int msgCount = getMessages(id).size();

	try (BatchUpdate bu = newBatchUpdate(batchUpdateFactory)) {
	// This time, we attempt to back up master, but we fail during updateChange.
	bu.addOp(id, backupMasterOp);
	String msg = "Change is bad";
	bu.addOp(
	id,
	new BatchUpdateOp() {
	@Override
	public boolean updateChange(ChangeContext ctx) throws ResourceConflictException {
	throw new ResourceConflictException(msg);
	}
	});
	try {
	bu.execute();
	fail("expected ResourceConflictException");
	} catch (ResourceConflictException e) {
	assertThat(e).hasMessageThat().isEqualTo(msg);
	}
	}

	// If updateChange hadn't failed, backup would have been updated to master2.
	assertThat(getRef(backup)).hasValue(master1);
	assertThat(getMessages(id)).hasSize(msgCount);
	}

	@Test
	public void retryOnLockFailureWithAtomicUpdates() throws Exception {
	PushOneCommit.Result r = createChange();
	Change.Id id = r.getChange().getId();
	String master = "refs/heads/master";
	ObjectId initial;
	try (Repository repo = repoManager.openRepository(project)) {
	ensureAtomicTransactions(repo);
	initial = repo.exactRef(master).getObjectId();
	}

	AtomicInteger updateRepoCalledCount = new AtomicInteger();
	AtomicInteger updateChangeCalledCount = new AtomicInteger();
	AtomicInteger afterUpdateReposCalledCount = new AtomicInteger();

	String result =
	retryHelper.execute(
	batchUpdateFactory -> {
	try (BatchUpdate bu = newBatchUpdate(batchUpdateFactory)) {
	bu.addOp(
	id,
	new UpdateRefAndAddMessageOp(updateRepoCalledCount, updateChangeCalledCount));
	bu.execute(new ConcurrentWritingListener(afterUpdateReposCalledCount));
	}
	return "Done";
	});

	assertThat(result).isEqualTo("Done");
	assertThat(updateRepoCalledCount.get()).isEqualTo(2);
	assertThat(afterUpdateReposCalledCount.get()).isEqualTo(2);
	assertThat(updateChangeCalledCount.get()).isEqualTo(2);

	List<String> messages = getMessages(id);
	assertThat(Iterables.getLast(messages)).isEqualTo(UpdateRefAndAddMessageOp.CHANGE_MESSAGE);
	assertThat(Collections.frequency(messages, UpdateRefAndAddMessageOp.CHANGE_MESSAGE))
	.isEqualTo(1);

	try (Repository repo = repoManager.openRepository(project)) {
	// Op lost the race, so the other writer's commit happened first. Then op retried and wrote
	// its commit with the other writer's commit as parent.
	assertThat(commitMessages(repo, initial, repo.exactRef(master).getObjectId()))
	.containsExactly(
	ConcurrentWritingListener.MSG_PREFIX + "1", UpdateRefAndAddMessageOp.COMMIT_MESSAGE)
	.inOrder();
	}
	}

	@Test
	public void missingChange() throws Exception {
	Change.Id changeId = new Change.Id(1234567);
	assertNoSuchChangeException(() -> notesFactory.create(project, changeId));
	assertNoSuchChangeException(() -> notesFactory.createChecked(project, changeId));
	}

	private void assertNoSuchChangeException(Callable<?> callable) throws Exception {
	try {
	callable.call();
	fail("expected NoSuchChangeException");
	} catch (NoSuchChangeException e) {
	// Expected.
	}
	}

	private class ConcurrentWritingListener implements BatchUpdateListener {
	static final String MSG_PREFIX = "Other writer ";

	private final AtomicInteger calledCount;

	private ConcurrentWritingListener(AtomicInteger calledCount) {
	this.calledCount = calledCount;
	}

	@Override
	public void afterUpdateRepos() throws Exception {
	// Reopen repo and update ref, to simulate a concurrent write in another
	// thread. Only do this the first time the listener is called.
	if (calledCount.getAndIncrement() > 0) {
	return;
	}
	try (Repository repo = repoManager.openRepository(project);
	RevWalk rw = new RevWalk(repo);
	ObjectInserter ins = repo.newObjectInserter()) {
	String master = "refs/heads/master";
	ObjectId oldId = repo.exactRef(master).getObjectId();
	ObjectId newId = newCommit(rw, ins, oldId, MSG_PREFIX + calledCount.get());
	ins.flush();
	RefUpdate ru = repo.updateRef(master);
	ru.setExpectedOldObjectId(oldId);
	ru.setNewObjectId(newId);
	assertThat(ru.update(rw)).isEqualTo(RefUpdate.Result.FAST_FORWARD);
	}
	}
	}

	private class UpdateRefAndAddMessageOp implements BatchUpdateOp {
	static final String COMMIT_MESSAGE = "A commit";
	static final String CHANGE_MESSAGE = "A change message";

	private final AtomicInteger updateRepoCalledCount;
	private final AtomicInteger updateChangeCalledCount;

	private UpdateRefAndAddMessageOp(
	AtomicInteger updateRepoCalledCount, AtomicInteger updateChangeCalledCount) {
	this.updateRepoCalledCount = updateRepoCalledCount;
	this.updateChangeCalledCount = updateChangeCalledCount;
	}

	@Override
	public void updateRepo(RepoContext ctx) throws Exception {
	String master = "refs/heads/master";
	ObjectId oldId = ctx.getRepoView().getRef(master).get();
	ObjectId newId = newCommit(ctx.getRevWalk(), ctx.getInserter(), oldId, COMMIT_MESSAGE);
	ctx.addRefUpdate(oldId, newId, master);
	updateRepoCalledCount.incrementAndGet();
	}

	@Override
	public boolean updateChange(ChangeContext ctx) throws Exception {
	ctx.getUpdate(ctx.getChange().currentPatchSetId()).setChangeMessage(CHANGE_MESSAGE);
	updateChangeCalledCount.incrementAndGet();
	return true;
	}
	}

	private ObjectId newCommit(RevWalk rw, ObjectInserter ins, ObjectId parent, String msg)
	throws IOException {
	PersonIdent ident = serverIdent.get();
	CommitBuilder cb = new CommitBuilder();
	cb.setParentId(parent);
	cb.setTreeId(rw.parseCommit(parent).getTree());
	cb.setMessage(msg);
	cb.setAuthor(ident);
	cb.setCommitter(ident);
	return ins.insert(Constants.OBJ_COMMIT, cb.build());
	}

	private BatchUpdate newBatchUpdate(BatchUpdate.Factory buf) {
	return buf.create(db, project, identifiedUserFactory.create(user.getId()), TimeUtil.nowTs());
	}

	private Optional<ObjectId> getRef(String name) throws Exception {
	try (Repository repo = repoManager.openRepository(project)) {
	return Optional.ofNullable(repo.exactRef(name)).map(Ref::getObjectId);
	}
	}

	private List<String> getMessages(Change.Id id) throws Exception {
	return gApi.changes()
	.id(id.get())
	.get(MESSAGES)
	.messages
	.stream()
	.map(m -> m.message)
	.collect(toList());
	}

	private static List<String> commitMessages(
	Repository repo, ObjectId fromExclusive, ObjectId toInclusive) throws Exception {
	try (RevWalk rw = new RevWalk(repo)) {
	rw.markStart(rw.parseCommit(toInclusive));
	rw.markUninteresting(rw.parseCommit(fromExclusive));
	rw.sort(RevSort.REVERSE);
	rw.setRetainBody(true);
	return Streams.stream(rw).map(RevCommit::getShortMessage).collect(toList());
	}
	}

	private void ensureAtomicTransactions(Repository repo) throws Exception {
	if (repo instanceof InMemoryRepository) {
	((InMemoryRepository) repo).setPerformsAtomicTransactions(true);
	} else {
	assertThat(repo.getRefDatabase().performsAtomicTransactions())
	.named("performsAtomicTransactions on %s", repo)
	.isTrue();
	}
	}
	}