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diff --git a/java/com/google/gerrit/server/notedb/NotesMigration.java b/java/com/google/gerrit/server/notedb/NotesMigration.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.notedb;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.gerrit.server.notedb.NoteDbTable.CHANGES;
+
+import com.google.auto.value.AutoValue;
+import com.google.gerrit.server.notedb.NoteDbChangeState.PrimaryStorage;
+import com.google.inject.AbstractModule;
+import java.util.concurrent.atomic.AtomicReference;
+import org.eclipse.jgit.lib.Config;
+
+/**
+ * Current low-level settings of the NoteDb migration for changes.
+ *
+ * <p>This class only describes the migration state of the {@link
+ * com.google.gerrit.reviewdb.client.Change Change} entity group, since it is possible for a given
+ * site to be in different states of the Change NoteDb migration process while staying at the same
+ * ReviewDb schema version. It does <em>not</em> describe the migration state of non-Change tables;
+ * those are automatically migrated using the ReviewDb schema migration process, so the NoteDb
+ * migration state at a given ReviewDb schema cannot vary.
+ *
+ * <p>In many places, core Gerrit code should not directly care about the NoteDb migration state,
+ * and should prefer high-level APIs like {@link com.google.gerrit.server.ApprovalsUtil
+ * ApprovalsUtil} that don't require callers to inspect the migration state. The
+ * <em>implementation</em> of those utilities does care about the state, and should query the {@code
+ * NotesMigration} for the properties of the migration, for example, {@link #changePrimaryStorage()
+ * where new changes should be stored}.
+ *
+ * <p>Core Gerrit code is mostly interested in one facet of the migration at a time (reading or
+ * writing, say), but not all combinations of return values are supported or even make sense.
+ *
+ * <p>This class controls the state of the migration according to options in {@code gerrit.config}.
+ * In general, any changes to these options should only be made by adventurous administrators, who
+ * know what they're doing, on non-production data, for the purposes of testing the NoteDb
+ * implementation. Changing options quite likely requires re-running {@code MigrateToNoteDb}. For
+ * these reasons, the options remain undocumented.
+ *
+ * <p><strong>Note:</strong> Callers should not assume the values returned by {@code
+ * NotesMigration}'s methods will not change in a running server.
+ */
+public abstract class NotesMigration {
+  public static final String SECTION_NOTE_DB = "noteDb";
+  public static final String READ = "read";
+  public static final String WRITE = "write";
+  public static final String DISABLE_REVIEW_DB = "disableReviewDb";
+
+  private static final String PRIMARY_STORAGE = "primaryStorage";
+  private static final String SEQUENCE = "sequence";
+
+  public static class Module extends AbstractModule {
+    @Override
+    public void configure() {
+      bind(MutableNotesMigration.class);
+      bind(NotesMigration.class).to(MutableNotesMigration.class);
+    }
+  }
+
+  @AutoValue
+  abstract static class Snapshot {
+    static Builder builder() {
+      // Default values are defined as what we would read from an empty config.
+      return create(new Config()).toBuilder();
+    }
+
+    static Snapshot create(Config cfg) {
+      return new AutoValue_NotesMigration_Snapshot.Builder()
+          .setWriteChanges(cfg.getBoolean(SECTION_NOTE_DB, CHANGES.key(), WRITE, false))
+          .setReadChanges(cfg.getBoolean(SECTION_NOTE_DB, CHANGES.key(), READ, false))
+          .setReadChangeSequence(cfg.getBoolean(SECTION_NOTE_DB, CHANGES.key(), SEQUENCE, false))
+          .setChangePrimaryStorage(
+              cfg.getEnum(
+                  SECTION_NOTE_DB, CHANGES.key(), PRIMARY_STORAGE, PrimaryStorage.REVIEW_DB))
+          .setDisableChangeReviewDb(
+              cfg.getBoolean(SECTION_NOTE_DB, CHANGES.key(), DISABLE_REVIEW_DB, false))
+          .setFailOnLoadForTest(false) // Only set in tests, can't be set via config.
+          .build();
+    }
+
+    abstract boolean writeChanges();
+
+    abstract boolean readChanges();
+
+    abstract boolean readChangeSequence();
+
+    abstract PrimaryStorage changePrimaryStorage();
+
+    abstract boolean disableChangeReviewDb();
+
+    abstract boolean failOnLoadForTest();
+
+    abstract Builder toBuilder();
+
+    void setConfigValues(Config cfg) {
+      cfg.setBoolean(SECTION_NOTE_DB, CHANGES.key(), WRITE, writeChanges());
+      cfg.setBoolean(SECTION_NOTE_DB, CHANGES.key(), READ, readChanges());
+      cfg.setBoolean(SECTION_NOTE_DB, CHANGES.key(), SEQUENCE, readChangeSequence());
+      cfg.setEnum(SECTION_NOTE_DB, CHANGES.key(), PRIMARY_STORAGE, changePrimaryStorage());
+      cfg.setBoolean(SECTION_NOTE_DB, CHANGES.key(), DISABLE_REVIEW_DB, disableChangeReviewDb());
+    }
+
+    @AutoValue.Builder
+    abstract static class Builder {
+      abstract Builder setWriteChanges(boolean writeChanges);
+
+      abstract Builder setReadChanges(boolean readChanges);
+
+      abstract Builder setReadChangeSequence(boolean readChangeSequence);
+
+      abstract Builder setChangePrimaryStorage(PrimaryStorage changePrimaryStorage);
+
+      abstract Builder setDisableChangeReviewDb(boolean disableChangeReviewDb);
+
+      abstract Builder setFailOnLoadForTest(boolean failOnLoadForTest);
+
+      abstract Snapshot autoBuild();
+
+      Snapshot build() {
+        Snapshot s = autoBuild();
+        checkArgument(
+            !(s.disableChangeReviewDb() && s.changePrimaryStorage() != PrimaryStorage.NOTE_DB),
+            "cannot disable ReviewDb for changes if default change primary storage is ReviewDb");
+        return s;
+      }
+    }
+  }
+
+  protected final AtomicReference<Snapshot> snapshot;
+
+  /**
+   * Read changes from NoteDb.
+   *
+   * <p>Change data is read from NoteDb refs, but ReviewDb is still the source of truth. If the
+   * loader determines NoteDb is out of date, the change data in NoteDb will be transparently
+   * rebuilt. This means that some code paths that look read-only may in fact attempt to write.
+   *
+   * <p>If true and {@code writeChanges() = false}, changes can still be read from NoteDb, but any
+   * attempts to write will generate an error.
+   */
+  public final boolean readChanges() {
+    return snapshot.get().readChanges();
+  }
+
+  /**
+   * Write changes to NoteDb.
+   *
+   * <p>This method is awkwardly named because you should be using either {@link
+   * #commitChangeWrites()} or {@link #failChangeWrites()} instead.
+   *
+   * <p>Updates to change data are written to NoteDb refs, but ReviewDb is still the source of
+   * truth. Change data will not be written unless the NoteDb refs are already up to date, and the
+   * write path will attempt to rebuild the change if not.
+   *
+   * <p>If false, the behavior when attempting to write depends on {@code readChanges()}. If {@code
+   * readChanges() = false}, writes to NoteDb are simply ignored; if {@code true}, any attempts to
+   * write will generate an error.
+   */
+  public final boolean rawWriteChangesSetting() {
+    return snapshot.get().writeChanges();
+  }
+
+  /**
+   * Read sequential change ID numbers from NoteDb.
+   *
+   * <p>If true, change IDs are read from {@code refs/sequences/changes} in All-Projects. If false,
+   * change IDs are read from ReviewDb's native sequences.
+   */
+  public final boolean readChangeSequence() {
+    return snapshot.get().readChangeSequence();
+  }
+
+  /** @return default primary storage for new changes. */
+  public final PrimaryStorage changePrimaryStorage() {
+    return snapshot.get().changePrimaryStorage();
+  }
+
+  /**
+   * Disable ReviewDb access for changes.
+   *
+   * <p>When set, ReviewDb operations involving the Changes table become no-ops. Lookups return no
+   * results; updates do nothing, as does opening, committing, or rolling back a transaction on the
+   * Changes table.
+   */
+  public final boolean disableChangeReviewDb() {
+    return snapshot.get().disableChangeReviewDb();
+  }
+
+  /**
+   * Whether to fail when reading any data from NoteDb.
+   *
+   * <p>Used in conjunction with {@link #readChanges()} for tests.
+   */
+  public boolean failOnLoadForTest() {
+    return snapshot.get().failOnLoadForTest();
+  }
+
+  public final boolean commitChangeWrites() {
+    // It may seem odd that readChanges() without writeChanges() means we should
+    // attempt to commit writes. However, this method is used by callers to know
+    // whether or not they should short-circuit and skip attempting to read or
+    // write NoteDb refs.
+    //
+    // It is possible for commitChangeWrites() to return true and
+    // failChangeWrites() to also return true, causing an error later in the
+    // same codepath. This specific condition is used by the auto-rebuilding
+    // path to rebuild a change and stage the results, but not commit them due
+    // to failChangeWrites().
+    return rawWriteChangesSetting() || readChanges();
+  }
+
+  public final boolean failChangeWrites() {
+    return !rawWriteChangesSetting() && readChanges();
+  }
+
+  public final void setConfigValues(Config cfg) {
+    snapshot.get().setConfigValues(cfg);
+  }
+
+  @Override
+  public final boolean equals(Object o) {
+    return o instanceof NotesMigration
+        && snapshot.get().equals(((NotesMigration) o).snapshot.get());
+  }
+
+  @Override
+  public final int hashCode() {
+    return snapshot.get().hashCode();
+  }
+
+  protected NotesMigration(Snapshot snapshot) {
+    this.snapshot = new AtomicReference<>(snapshot);
+  }
+
+  final Snapshot snapshot() {
+    return snapshot.get();
+  }
+}