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diff --git a/java/com/google/gerrit/server/account/externalids/ExternalIdReader.java b/java/com/google/gerrit/server/account/externalids/ExternalIdReader.java
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+++ b/java/com/google/gerrit/server/account/externalids/ExternalIdReader.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.account.externalids;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.collect.ImmutableSet;
+import com.google.gerrit.common.Nullable;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.Description.Units;
+import com.google.gerrit.metrics.MetricMaker;
+import com.google.gerrit.metrics.Timer0;
+import com.google.gerrit.reviewdb.client.RefNames;
+import com.google.gerrit.server.config.AllUsersName;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import java.io.IOException;
+import java.util.Optional;
+import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.notes.NoteMap;
+import org.eclipse.jgit.revwalk.RevWalk;
+
+/**
+ * Class to read external IDs from NoteDb.
+ *
+ * <p>In NoteDb external IDs are stored in the All-Users repository in a Git Notes branch called
+ * refs/meta/external-ids where the sha1 of the external ID is used as note name. Each note content
+ * is a git config file that contains an external ID. It has exactly one externalId subsection with
+ * an accountId and optionally email and password:
+ *
+ * <pre>
+ * [externalId "username:jdoe"]
+ *   accountId = 1003407
+ *   email = jdoe@example.com
+ *   password = bcrypt:4:LCbmSBDivK/hhGVQMfkDpA==:XcWn0pKYSVU/UJgOvhidkEtmqCp6oKB7
+ * </pre>
+ */
+@Singleton
+public class ExternalIdReader {
+  public static ObjectId readRevision(Repository repo) throws IOException {
+    Ref ref = repo.exactRef(RefNames.REFS_EXTERNAL_IDS);
+    return ref != null ? ref.getObjectId() : ObjectId.zeroId();
+  }
+
+  public static NoteMap readNoteMap(RevWalk rw, ObjectId rev) throws IOException {
+    if (!rev.equals(ObjectId.zeroId())) {
+      return NoteMap.read(rw.getObjectReader(), rw.parseCommit(rev));
+    }
+    return NoteMap.newEmptyMap();
+  }
+
+  private final GitRepositoryManager repoManager;
+  private final AllUsersName allUsersName;
+  private boolean failOnLoad = false;
+  private final Timer0 readAllLatency;
+
+  @Inject
+  ExternalIdReader(
+      GitRepositoryManager repoManager, AllUsersName allUsersName, MetricMaker metricMaker) {
+    this.repoManager = repoManager;
+    this.allUsersName = allUsersName;
+    this.readAllLatency =
+        metricMaker.newTimer(
+            "notedb/read_all_external_ids_latency",
+            new Description("Latency for reading all external IDs from NoteDb.")
+                .setCumulative()
+                .setUnit(Units.MILLISECONDS));
+  }
+
+  @VisibleForTesting
+  public void setFailOnLoad(boolean failOnLoad) {
+    this.failOnLoad = failOnLoad;
+  }
+
+  ObjectId readRevision() throws IOException {
+    try (Repository repo = repoManager.openRepository(allUsersName)) {
+      return readRevision(repo);
+    }
+  }
+
+  /** Reads and returns all external IDs. */
+  ImmutableSet<ExternalId> all() throws IOException, ConfigInvalidException {
+    checkReadEnabled();
+
+    try (Timer0.Context ctx = readAllLatency.start();
+        Repository repo = repoManager.openRepository(allUsersName)) {
+      return ExternalIdNotes.loadReadOnly(allUsersName, repo).all();
+    }
+  }
+
+  /**
+   * Reads and returns all external IDs from the specified revision of the {@code
+   * refs/meta/external-ids} branch.
+   *
+   * @param rev the revision from which the external IDs should be read, if {@code null} the
+   *     external IDs are read from the current tip, if {@link ObjectId#zeroId()} it's assumed that
+   *     the {@code refs/meta/external-ids} branch doesn't exist and the loaded external IDs will be
+   *     empty
+   * @return all external IDs that were read from the specified revision
+   */
+  ImmutableSet<ExternalId> all(@Nullable ObjectId rev) throws IOException, ConfigInvalidException {
+    checkReadEnabled();
+
+    try (Timer0.Context ctx = readAllLatency.start();
+        Repository repo = repoManager.openRepository(allUsersName)) {
+      return ExternalIdNotes.loadReadOnly(allUsersName, repo, rev).all();
+    }
+  }
+
+  /** Reads and returns the specified external ID. */
+  Optional<ExternalId> get(ExternalId.Key key) throws IOException, ConfigInvalidException {
+    checkReadEnabled();
+
+    try (Repository repo = repoManager.openRepository(allUsersName)) {
+      return ExternalIdNotes.loadReadOnly(allUsersName, repo).get(key);
+    }
+  }
+
+  /** Reads and returns the specified external ID from the given revision. */
+  Optional<ExternalId> get(ExternalId.Key key, ObjectId rev)
+      throws IOException, ConfigInvalidException {
+    checkReadEnabled();
+
+    try (Repository repo = repoManager.openRepository(allUsersName)) {
+      return ExternalIdNotes.loadReadOnly(allUsersName, repo, rev).get(key);
+    }
+  }
+
+  private void checkReadEnabled() throws IOException {
+    if (failOnLoad) {
+      throw new IOException("Reading from external IDs is disabled");
+    }
+  }
+}