Split the thread pool out of MergeQueue for reuse

There are other types of tasks that we'd like to perform in a delay
fashion beyond just merge retries.

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

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

@@ -22,39 +22,13 @@ import org.slf4j.LoggerFactory;
 
 import java.util.HashMap;
 import java.util.Map;
-import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 
 public class MergeQueue {
   private static final Logger log = LoggerFactory.getLogger(MergeQueue.class);
-  private static ScheduledThreadPoolExecutor pool;
   private static final Map<Branch.NameKey, MergeEntry> active =
       new HashMap<Branch.NameKey, MergeEntry>();
 
-  public static void terminate() {
-    final ScheduledThreadPoolExecutor p = shutdown();
-    if (p != null) {
-      boolean isTerminated;
-      do {
-        try {
-          isTerminated = p.awaitTermination(10, TimeUnit.SECONDS);
-        } catch (InterruptedException ie) {
-          isTerminated = false;
-        }
-      } while (!isTerminated);
-    }
-  }
-
-  private static synchronized ScheduledThreadPoolExecutor shutdown() {
-    final ScheduledThreadPoolExecutor p = pool;
-    if (p != null) {
-      p.shutdown();
-      pool = null;
-      return p;
-    }
-    return null;
-  }
-
   public static void merge(final Branch.NameKey branch) {
     if (start(branch)) {
       try {
@@ -100,12 +74,8 @@ public class MergeQueue {
       // No job has been scheduled to execute this branch, but it needs
       // to run a merge again.
       //
-      if (pool == null) {
-        pool = new ScheduledThreadPoolExecutor(1);
-      }
-
       e.jobScheduled = true;
-      pool.schedule(new Runnable() {
+      WorkQueue.schedule(new Runnable() {
         public void run() {
           unschedule(e);
           try {

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

@@ -0,0 +1,56 @@
+// 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.git;
+
+import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+public class WorkQueue {
+  private static ScheduledThreadPoolExecutor pool;
+
+  public static synchronized void schedule(final Runnable task,
+      final long delay, final TimeUnit unit) {
+    if (pool == null) {
+      pool = new ScheduledThreadPoolExecutor(1);
+      pool.setKeepAliveTime(60, TimeUnit.SECONDS);
+      pool.setMaximumPoolSize(5);
+    }
+    pool.schedule(task, delay, unit);
+  }
+
+  public static void terminate() {
+    final ScheduledThreadPoolExecutor p = shutdown();
+    if (p != null) {
+      boolean isTerminated;
+      do {
+        try {
+          isTerminated = p.awaitTermination(10, TimeUnit.SECONDS);
+        } catch (InterruptedException ie) {
+          isTerminated = false;
+        }
+      } while (!isTerminated);
+    }
+  }
+
+  private static synchronized ScheduledThreadPoolExecutor shutdown() {
+    final ScheduledThreadPoolExecutor p = pool;
+    if (p != null) {
+      p.shutdown();
+      pool = null;
+      return p;
+    }
+    return null;
+  }
+}

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

@@ -16,7 +16,7 @@ package com.google.gerrit.server;
 
 import com.google.gerrit.client.rpc.NotSignedInException;
 import com.google.gerrit.client.rpc.SignInRequired;
-import com.google.gerrit.git.MergeQueue;
+import com.google.gerrit.git.WorkQueue;
 import com.google.gwtjsonrpc.server.JsonServlet;
 import com.google.gwtjsonrpc.server.SignedToken;
 import com.google.gwtjsonrpc.server.XsrfException;
@@ -53,7 +53,7 @@ public abstract class GerritJsonServlet extends JsonServlet<GerritCall> {
 
   @Override
   public void destroy() {
-    MergeQueue.terminate();
+    WorkQueue.terminate();
     super.destroy();
   }