[SPARK-4595][Core] Fix MetricsServlet not work issue

`MetricsServlet` handler should be added to the web UI after initialized by `MetricsSystem`, otherwise servlet handler cannot be attached.

Author: Saisai Shao <saisai.shao@intel.com>
Author: Josh Rosen <joshrosen@databricks.com>
Author: jerryshao <saisai.shao@intel.com>

Closes apache#3444 from jerryshao/SPARK-4595 and squashes the following commits:

434d17e [Saisai Shao] Merge pull request #10 from JoshRosen/metrics-system-cleanup
87a2292 [Josh Rosen] Guard against misuse of MetricsSystem methods.
f779fe0 [jerryshao] Fix MetricsServlet not work issue

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -344,6 +344,8 @@ class SparkContext(config: SparkConf) extends Logging {
   // The metrics system for Driver need to be set spark.app.id to app ID.
   // So it should start after we get app ID from the task scheduler and set spark.app.id.
   metricsSystem.start()
+  // Attach the driver metrics servlet handler to the web ui after the metrics system is started.
+  metricsSystem.getServletHandlers.foreach(handler => ui.foreach(_.attachHandler(handler)))
 
   // Optionally log Spark events
   private[spark] val eventLogger: Option[EventLoggingListener] = {

core/src/main/scala/org/apache/spark/deploy/master/Master.scala

@@ -129,6 +129,10 @@ private[spark] class Master(
     masterMetricsSystem.registerSource(masterSource)
     masterMetricsSystem.start()
     applicationMetricsSystem.start()
+    // Attach the master and app metrics servlet handler to the web ui after the metrics systems are
+    // started.
+    masterMetricsSystem.getServletHandlers.foreach(webUi.attachHandler)
+    applicationMetricsSystem.getServletHandlers.foreach(webUi.attachHandler)
 
     val (persistenceEngine_, leaderElectionAgent_) = RECOVERY_MODE match {
       case "ZOOKEEPER" =>

core/src/main/scala/org/apache/spark/deploy/master/ui/MasterWebUI.scala

@@ -41,8 +41,6 @@ class MasterWebUI(val master: Master, requestedPort: Int)
     attachPage(new HistoryNotFoundPage(this))
     attachPage(new MasterPage(this))
     attachHandler(createStaticHandler(MasterWebUI.STATIC_RESOURCE_DIR, "/static"))
-    master.masterMetricsSystem.getServletHandlers.foreach(attachHandler)
-    master.applicationMetricsSystem.getServletHandlers.foreach(attachHandler)
   }
 
   /** Attach a reconstructed UI to this Master UI. Only valid after bind(). */

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -163,6 +163,8 @@ private[spark] class Worker(
 
     metricsSystem.registerSource(workerSource)
     metricsSystem.start()
+    // Attach the worker metrics servlet handler to the web ui after the metrics system is started.
+    metricsSystem.getServletHandlers.foreach(webUi.attachHandler)
   }
 
   def changeMaster(url: String, uiUrl: String) {

core/src/main/scala/org/apache/spark/deploy/worker/ui/WorkerWebUI.scala

@@ -50,7 +50,6 @@ class WorkerWebUI(
     attachHandler(createStaticHandler(WorkerWebUI.STATIC_RESOURCE_BASE, "/static"))
     attachHandler(createServletHandler("/log",
       (request: HttpServletRequest) => logPage.renderLog(request), worker.securityMgr))
-    worker.metricsSystem.getServletHandlers.foreach(attachHandler)
   }
 }
 

core/src/main/scala/org/apache/spark/metrics/MetricsSystem.scala

@@ -76,22 +76,36 @@ private[spark] class MetricsSystem private (
   private val sources = new mutable.ArrayBuffer[Source]
   private val registry = new MetricRegistry()
 
+  private var running: Boolean = false
+
   // Treat MetricsServlet as a special sink as it should be exposed to add handlers to web ui
   private var metricsServlet: Option[MetricsServlet] = None
 
-  /** Get any UI handlers used by this metrics system. */
-  def getServletHandlers = metricsServlet.map(_.getHandlers).getOrElse(Array())
+  /**
+   * Get any UI handlers used by this metrics system; can only be called after start().
+   */
+  def getServletHandlers = {
+    require(running, "Can only call getServletHandlers on a running MetricsSystem")
+    metricsServlet.map(_.getHandlers).getOrElse(Array())
+  }
 
   metricsConfig.initialize()
 
   def start() {
+    require(!running, "Attempting to start a MetricsSystem that is already running")
+    running = true
     registerSources()
     registerSinks()
     sinks.foreach(_.start)
   }
 
   def stop() {
-    sinks.foreach(_.stop)
+    if (running) {
+      sinks.foreach(_.stop)
+    } else {
+      logWarning("Stopping a MetricsSystem that is not running")
+    }
+    running = false
   }
 
   def report() {
@@ -107,7 +121,7 @@ private[spark] class MetricsSystem private (
    * @return An unique metric name for each combination of
    *         application, executor/driver and metric source.
    */
-  def buildRegistryName(source: Source): String = {
+  private[spark] def buildRegistryName(source: Source): String = {
     val appId = conf.getOption("spark.app.id")
     val executorId = conf.getOption("spark.executor.id")
     val defaultName = MetricRegistry.name(source.sourceName)
@@ -144,7 +158,7 @@ private[spark] class MetricsSystem private (
     })
   }
 
-  def registerSources() {
+  private def registerSources() {
     val instConfig = metricsConfig.getInstance(instance)
     val sourceConfigs = metricsConfig.subProperties(instConfig, MetricsSystem.SOURCE_REGEX)
 
@@ -160,7 +174,7 @@ private[spark] class MetricsSystem private (
     }
   }
 
-  def registerSinks() {
+  private def registerSinks() {
     val instConfig = metricsConfig.getInstance(instance)
     val sinkConfigs = metricsConfig.subProperties(instConfig, MetricsSystem.SINK_REGEX)
 

core/src/main/scala/org/apache/spark/ui/SparkUI.scala

@@ -57,8 +57,6 @@ private[spark] class SparkUI private (
     attachHandler(createRedirectHandler("/", "/jobs", basePath = basePath))
     attachHandler(
       createRedirectHandler("/stages/stage/kill", "/stages", stagesTab.handleKillRequest))
-    // If the UI is live, then serve
-    sc.foreach { _.env.metricsSystem.getServletHandlers.foreach(attachHandler) }
   }
   initialize()