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diff --git a/java/com/google/gerrit/server/project/Reachable.java b/java/com/google/gerrit/server/project/Reachable.java
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+++ b/java/com/google/gerrit/server/project/Reachable.java
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+// 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.server.project;
+
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Maps;
+import com.google.common.flogger.FluentLogger;
+import com.google.gerrit.reviewdb.client.Project;
+import com.google.gerrit.server.change.IncludedInResolver;
+import com.google.gerrit.server.permissions.PermissionBackend;
+import com.google.gerrit.server.permissions.PermissionBackend.RefFilterOptions;
+import com.google.gerrit.server.permissions.PermissionBackendException;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import java.io.IOException;
+import java.util.Collection;
+import java.util.Map;
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.RefDatabase;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
+
+/**
+ * Report whether a commit is reachable from a set of commits. This is used for checking if a user
+ * has read permissions on a commit.
+ */
+@Singleton
+public class Reachable {
+  private static final FluentLogger logger = FluentLogger.forEnclosingClass();
+
+  private final PermissionBackend permissionBackend;
+
+  @Inject
+  Reachable(PermissionBackend permissionBackend) {
+    this.permissionBackend = permissionBackend;
+  }
+
+  /** @return true if a commit is reachable from a given set of refs. */
+  public boolean fromRefs(
+      Project.NameKey project, Repository repo, RevCommit commit, Map<String, Ref> refs) {
+    try (RevWalk rw = new RevWalk(repo)) {
+      Map<String, Ref> filtered =
+          permissionBackend
+              .currentUser()
+              .project(project)
+              .filter(refs, repo, RefFilterOptions.builder().setFilterTagsSeparately(true).build());
+      return IncludedInResolver.includedInAny(repo, rw, commit, filtered.values());
+    } catch (IOException | PermissionBackendException e) {
+      logger.atSevere().withCause(e).log(
+          "Cannot verify permissions to commit object %s in repository %s", commit.name(), project);
+      return false;
+    }
+  }
+
+  /** @return true if a commit is reachable from a repo's branches and tags. */
+  boolean fromHeadsOrTags(Project.NameKey project, Repository repo, RevCommit commit) {
+    try {
+      RefDatabase refdb = repo.getRefDatabase();
+      Collection<Ref> heads = refdb.getRefsByPrefix(Constants.R_HEADS);
+      Collection<Ref> tags = refdb.getRefsByPrefix(Constants.R_TAGS);
+      Map<String, Ref> refs = Maps.newHashMapWithExpectedSize(heads.size() + tags.size());
+      for (Ref r : Iterables.concat(heads, tags)) {
+        refs.put(r.getName(), r);
+      }
+      return fromRefs(project, repo, commit, refs);
+    } catch (IOException e) {
+      logger.atSevere().withCause(e).log(
+          "Cannot verify permissions to commit object %s in repository %s", commit.name(), project);
+      return false;
+    }
+  }
+}