Rework SearchingChangeCacheImpl

When filtering refs, we previously had two mechanisms for getting change
information somewhat efficiently. If the instance is running as a
master, we used SearchingChangeCacheImpl to get all change information
from the change index. We parsed this and used if for permission
filtering.

In slaves, we scanned the repo and parsed the changes from disk. There
are two reasons for this: (1) we don't have a change index
available in slaves. (2) There was no cross-machine eviction.

In the master, SearchingChangeCacheImpl was disabled by default, so
without manual configuration, we issued one index call for each
list-refs call.

In addition, we accepted potential staleness of the index. If a change
was moved (= the target branch changed) or marked as private and we
missed the change index update, it would still be available in Git.

This commit reworks the way how we retrieve and store change data for
ref filtering:

We remove SearchingChangeCacheImpl and replace it with ChangeRefCache.
ChangeRefCache has a different caching mechanism. The key contains
project, changeId and the SHA1 of the meta ref. This makes it so that we
can spare any custom eviction logic and have the Guava cache do it's
internal eviction purely based on cache size.

This makes the cache suitable for Gerrit slaves as well.

In addition, we adapt the way how we load change information: If the
change index is available, we bootstrap the cache once per JVM for each
project using the index. All subsequent updates are done incrementally
using the persisted ChangeNotesCache. This drastically cuts down on the
number of index calls we do (one per project per instance vs one per
request) while not sacrificing on the benefits. Due to the (comparably)
low number of change updates per instance and the (comparably) high number
of list ref calls, we will do a very small amount of incremental updates
of the cache for any given list ref call.

For slaves that don't have a change index available, we don't do any
bootstrapping for now and will use the ChangeNotesCache as requests come
in. In case this is too slow, we can easily bootstrap the cache in a
slave using a lifecycle listener. This will be added in the future if
there is a need.

Why do we need this cache at all now that we have a ChangeNotesCache?
To efficiently filter change refs, all (or nearly all) of the
information needs to be in-memory. The ChangeNotesCache is rather large
and on googlesource.com we can't hold all of it in memory.
ChangeRefCache has a small enough footprint that we can. This might
very well be true for other Gerrit instances as well. In case it is not,
administrators can choose to disable ChangeRefCache and have their ref
filtering be backed by ChangeNotesCache.

With this commit, we add integration tests for the new cache and remove
complexity from DefaultRefFilter.

Change-Id: I5eda9d411e97925e3e8b450fe32693a936164f96

java/com/google/gerrit/server/git/SearchingChangeCacheImpl.java

@@ -1,162 +0,0 @@
-// Copyright (C) 2012 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.server.git;
-
-import com.google.auto.value.AutoValue;
-import com.google.common.cache.CacheLoader;
-import com.google.common.cache.LoadingCache;
-import com.google.common.flogger.FluentLogger;
-import com.google.gerrit.common.Nullable;
-import com.google.gerrit.extensions.events.GitReferenceUpdatedListener;
-import com.google.gerrit.extensions.registration.DynamicSet;
-import com.google.gerrit.reviewdb.client.Change;
-import com.google.gerrit.reviewdb.client.Project;
-import com.google.gerrit.reviewdb.client.RefNames;
-import com.google.gerrit.server.ReviewerSet;
-import com.google.gerrit.server.cache.CacheModule;
-import com.google.gerrit.server.index.change.ChangeField;
-import com.google.gerrit.server.logging.TraceContext;
-import com.google.gerrit.server.logging.TraceContext.TraceTimer;
-import com.google.gerrit.server.query.change.ChangeData;
-import com.google.gerrit.server.query.change.InternalChangeQuery;
-import com.google.gerrit.server.util.ManualRequestContext;
-import com.google.gerrit.server.util.OneOffRequestContext;
-import com.google.inject.Inject;
-import com.google.inject.Provider;
-import com.google.inject.Singleton;
-import com.google.inject.TypeLiteral;
-import com.google.inject.name.Named;
-import com.google.inject.util.Providers;
-import java.util.ArrayList;
-import java.util.Collections;
-import java.util.List;
-import java.util.concurrent.ExecutionException;
-
-@Singleton
-public class SearchingChangeCacheImpl implements GitReferenceUpdatedListener {
-  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
-
-  static final String ID_CACHE = "changes";
-
-  public static class Module extends CacheModule {
-    private final boolean slave;
-
-    public Module() {
-      this(false);
-    }
-
-    public Module(boolean slave) {
-      this.slave = slave;
-    }
-
-    @Override
-    protected void configure() {
-      if (slave) {
-        bind(SearchingChangeCacheImpl.class).toProvider(Providers.of(null));
-      } else {
-        cache(ID_CACHE, Project.NameKey.class, new TypeLiteral<List<CachedChange>>() {})
-            .maximumWeight(0)
-            .loader(Loader.class);
-
-        bind(SearchingChangeCacheImpl.class);
-        DynamicSet.bind(binder(), GitReferenceUpdatedListener.class)
-            .to(SearchingChangeCacheImpl.class);
-      }
-    }
-  }
-
-  @AutoValue
-  abstract static class CachedChange {
-    // Subset of fields in ChangeData, specifically fields needed to serve
-    // VisibleRefFilter without touching the database. More can be added as
-    // necessary.
-    abstract Change change();
-
-    @Nullable
-    abstract ReviewerSet reviewers();
-  }
-
-  private final LoadingCache<Project.NameKey, List<CachedChange>> cache;
-  private final ChangeData.Factory changeDataFactory;
-
-  @Inject
-  SearchingChangeCacheImpl(
-      @Named(ID_CACHE) LoadingCache<Project.NameKey, List<CachedChange>> cache,
-      ChangeData.Factory changeDataFactory) {
-    this.cache = cache;
-    this.changeDataFactory = changeDataFactory;
-  }
-
-  /**
-   * Read changes for the project from the secondary index.
-   *
-   * <p>Returned changes only include the {@code Change} object (with id, branch) and the reviewers.
-   * Additional stored fields are not loaded from the index.
-   *
-   * @param project project to read.
-   * @return list of known changes; empty if no changes.
-   */
-  public List<ChangeData> getChangeData(Project.NameKey project) {
-    try {
-      List<CachedChange> cached = cache.get(project);
-      List<ChangeData> cds = new ArrayList<>(cached.size());
-      for (CachedChange cc : cached) {
-        ChangeData cd = changeDataFactory.create(cc.change());
-        cd.setReviewers(cc.reviewers());
-        cds.add(cd);
-      }
-      return Collections.unmodifiableList(cds);
-    } catch (ExecutionException e) {
-      logger.atWarning().withCause(e).log("Cannot fetch changes for %s", project);
-      return Collections.emptyList();
-    }
-  }
-
-  @Override
-  public void onGitReferenceUpdated(GitReferenceUpdatedListener.Event event) {
-    if (event.getRefName().startsWith(RefNames.REFS_CHANGES)) {
-      cache.invalidate(new Project.NameKey(event.getProjectName()));
-    }
-  }
-
-  static class Loader extends CacheLoader<Project.NameKey, List<CachedChange>> {
-    private final OneOffRequestContext requestContext;
-    private final Provider<InternalChangeQuery> queryProvider;
-
-    @Inject
-    Loader(OneOffRequestContext requestContext, Provider<InternalChangeQuery> queryProvider) {
-      this.requestContext = requestContext;
-      this.queryProvider = queryProvider;
-    }
-
-    @Override
-    public List<CachedChange> load(Project.NameKey key) throws Exception {
-      try (TraceTimer timer = TraceContext.newTimer("Loading changes of project %s", key);
-          ManualRequestContext ctx = requestContext.open()) {
-        List<ChangeData> cds =
-            queryProvider
-                .get()
-                .setRequestedFields(ChangeField.CHANGE, ChangeField.REVIEWER)
-                .byProject(key);
-        List<CachedChange> result = new ArrayList<>(cds.size());
-        for (ChangeData cd : cds) {
-          result.add(
-              new AutoValue_SearchingChangeCacheImpl_CachedChange(cd.change(), cd.getReviewers()));
-        }
-        return Collections.unmodifiableList(result);
-      }
-    }
-  }
-}