Start migration to notedb implementation of PatchSetApprovals

Encapsulate the current state of the migration in a NotesMigration
class. For each database table (or broader group of functionality)
that we migrate to notedb, configure a boolean indicating whether that
data should be read from notes, defaulting to false.

Unlike reads, NotesMigration contains just a single boolean
indicating that data should be written. We don't attempt to write
just some types of data; as the migration continues we will just
rewrite history.

Since most existing reads of the PatchSetApprovals table have been
migrated to ApprovalsUtil, most implementation changes happen there.
There are a few other implementations scattered around, and some that
will require a bit more work (e.g. stamping normalized approvals at
submit time).

Change-Id: I5676267d4de607c385e8c9917a89333863b9c9e7

gerrit-server/src/main/java/com/google/gerrit/server/notedb/NotesMigration.java

@@ -0,0 +1,62 @@
+// Copyright (C) 2013 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 com.google.common.annotations.VisibleForTesting;
+import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+
+import org.eclipse.jgit.lib.Config;
+
+/**
+ * Holds the current state of the notes DB migration.
+ * <p>
+ * During a transitional period, different subsets of the former gwtorm DB
+ * functionality may be enabled on the site, possibly only for reading or
+ * writing.
+ */
+@Singleton
+public class NotesMigration {
+  @VisibleForTesting
+  static NotesMigration allEnabled() {
+    Config cfg = new Config();
+    cfg.setBoolean("notedb", null, "write", true);
+    //cfg.setBoolean("notedb", "patchSetApprovals", "read", true);
+    return new NotesMigration(cfg);
+  }
+
+  private final boolean write;
+  private final boolean readPatchSetApprovals;
+
+  @Inject
+  NotesMigration(@GerritServerConfig Config cfg) {
+    write = cfg.getBoolean("notedb", null, "write", false);
+    readPatchSetApprovals =
+        cfg.getBoolean("notedb", "patchSetApprovals", "read", false);
+    checkArgument(!readPatchSetApprovals,
+        "notedb.readPatchSetApprovals not yet supported");
+  }
+
+  public boolean write() {
+    return write;
+  }
+
+  public boolean readPatchSetApprovals() {
+    return readPatchSetApprovals;
+  }
+}