Allow plugins.checkFrequency = 0 to disable scanner

Production systems might not want to scan for plugins, and instead
rely on system administrators to invoke `gerrit plugin reload` or
`gerrit plugin install` to make explicit plugin changes. Disable the
background scanning thread when the check frequency is set to 0 or any
negative value.

Change-Id: Ib0c1b40db51c960650099e5ae84bda6aebbc572e

Documentation/config-gerrit.txt

@@ -1680,6 +1680,22 @@ auto-detected and one thread per CPU is used, per client request.
 By default, 1.
 
 
+[[plugins]]Section plugins
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+[[plugins.checkFrequency]]plugins.checkFrequency::
++
+How often plugins should be examined for new plugins to load, removed
+plugins to be unloaded, or updated plugins to be reloaded.  Values can
+be specified using standard time unit abbreviations ('ms', 'sec',
+'min', etc.).
++
+If set to 0, automatic plugin reloading is disabled.  Administrators
+may force reloading with link:cmd-plugin.html[gerrit plugin reload].
++
+Default is 1 minute.
+
+
 [[receive]]Section receive
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 This section is used to set who can execute the 'receive-pack' and

gerrit-server/src/main/java/com/google/gerrit/server/plugins/PluginLoader.java

@@ -77,11 +77,15 @@ public class PluginLoader implements LifecycleListener {
     broken = Maps.newHashMap();
     cleanupQueue = new ReferenceQueue<ClassLoader>();
     cleanupHandles = Maps.newConcurrentMap();
-    scanner = new PluginScannerThread(
-        this,
-        ConfigUtil.getTimeUnit(cfg,
-            "plugins", null, "checkFrequency",
-            TimeUnit.MINUTES.toMillis(1), TimeUnit.MILLISECONDS));
+
+    long checkFrequency = ConfigUtil.getTimeUnit(cfg,
+        "plugins", null, "checkFrequency",
+        TimeUnit.MINUTES.toMillis(1), TimeUnit.MILLISECONDS);
+    if (checkFrequency > 0) {
+      scanner = new PluginScannerThread(this, checkFrequency);
+    } else {
+      scanner = null;
+    }
   }
 
   public synchronized List<Plugin> getPlugins() {
@@ -182,12 +186,16 @@ public class PluginLoader implements LifecycleListener {
   public synchronized void start() {
     log.info("Loading plugins from " + pluginsDir.getAbsolutePath());
     rescan(false);
-    scanner.start();
+    if (scanner != null) {
+      scanner.start();
+    }
   }
 
   @Override
   public void stop() {
-    scanner.end();
+    if (scanner != null) {
+      scanner.end();
+    }
     synchronized (this) {
       boolean clean = !running.isEmpty();
       for (Plugin p : running.values()) {