Refactor the ChangeMessage UUID generation to a common utility

We also now only hit the database for a new change message sequence
number once each application startup, or once every 2^31 messages.
Since the application startup is going to be the more frequent case
we should only hit the database once.

Signed-off-by: Shawn O. Pearce <sop@google.com>

appjar/src/main/java/com/google/gerrit/git/MergeOp.java

@@ -20,6 +20,7 @@ import com.google.gerrit.client.reviewdb.ChangeMessage;
 import com.google.gerrit.client.reviewdb.PatchSet;
 import com.google.gerrit.client.reviewdb.ReviewDb;
 import com.google.gerrit.client.rpc.Common;
+import com.google.gerrit.server.ChangeUtil;
 import com.google.gerrit.server.GerritServer;
 import com.google.gwtorm.client.OrmException;
 import com.google.gwtorm.client.Transaction;
@@ -39,8 +40,6 @@ import org.spearce.jgit.merge.Merger;
 import org.spearce.jgit.revwalk.RevCommit;
 import org.spearce.jgit.revwalk.RevSort;
 import org.spearce.jgit.revwalk.RevWalk;
-import org.spearce.jgit.util.Base64;
-import org.spearce.jgit.util.NB;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -442,9 +441,12 @@ public class MergeOp {
   }
 
   private ChangeMessage message(final Change c, final String body) {
-    final byte[] raw = new byte[4];
-    NB.encodeInt32(raw, 0, schema.nextChangeMessageId());
-    final String uuid = Base64.encodeBytes(raw);
+    final String uuid;
+    try {
+      uuid = ChangeUtil.messageUUID(schema);
+    } catch (OrmException e) {
+      return null;
+    }
     final ChangeMessage m =
         new ChangeMessage(new ChangeMessage.Key(c.getId(), uuid), null);
     m.setMessage(body);
@@ -486,7 +488,9 @@ public class MergeOp {
       try {
         final Transaction txn = schema.beginTransaction();
         schema.changes().update(Collections.singleton(c), txn);
-        schema.changeMessages().insert(Collections.singleton(msg), txn);
+        if (msg != null) {
+          schema.changeMessages().insert(Collections.singleton(msg), txn);
+        }
         txn.commit();
         break;
       } catch (OrmException e) {

appjar/src/main/java/com/google/gerrit/server/ChangeUtil.java

@@ -0,0 +1,50 @@
+// Copyright 2009 Google Inc.
+//
+// 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;
+
+import com.google.gerrit.client.reviewdb.ReviewDb;
+import com.google.gwtorm.client.OrmException;
+
+import org.spearce.jgit.util.Base64;
+import org.spearce.jgit.util.NB;
+
+public class ChangeUtil {
+  private static int uuidPrefix;
+  private static int uuidSeq;
+
+  /**
+   * Generate a new unique identifier for change message entities.
+   * 
+   * @param db the database connection, used to increment the change message
+   *        allocation sequence.
+   * @return the new unique identifier.
+   * @throws OrmException the database couldn't be incremented.
+   */
+  public static String messageUUID(final ReviewDb db) throws OrmException {
+    final byte[] raw = new byte[8];
+    fill(raw, db);
+    return Base64.encodeBytes(raw);
+  }
+
+  private static synchronized void fill(byte[] raw, ReviewDb db)
+      throws OrmException {
+    if (uuidSeq == 0) {
+      uuidPrefix = db.nextChangeMessageId();
+      uuidSeq = Integer.MAX_VALUE;
+    }
+    NB.encodeInt32(raw, 0, uuidPrefix);
+    NB.encodeInt32(raw, 4, uuidSeq--);
+  }
+}