SHS-NG M2: Store FsHistoryProvider listing data in LevelDB.

The application listing is still generated from event logs, but is now stored
in LevelDB. No data (except for the internal LevelDB pages) is kept in memory.
The actual app UIs are, as of now, still untouched.

The provider stores things internally using the public REST API types; I believe
this is better going forward since it will make it easier to get rid of the
internal history server API which is mostly redundant at this point.

I also added a finalizer to LevelDBIterator, to make sure that resources are
eventually released. This helps when code iterates but does not exhaust the
iterator, thus not triggering the auto-close code.

HistoryServerSuite was modified to not re-start the history server unnecessarily;
this makes the json validation tests run more quickly.

common/kvstore/src/main/java/org/apache/spark/kvstore/LevelDBIterator.java

@@ -164,6 +164,18 @@ public synchronized void close() throws IOException {
     }
   }
 
+  /**
+   * Because it's tricky to expose closeable iterators through many internal APIs, especially
+   * when Scala wrappers are used, this makes sure that, hopefully, the JNI resources held by
+   * the iterator will eventually be released.
+   */
+  @Override
+  protected void finalize() throws Throwable {
+    if (db.db() != null) {
+      close();
+    }
+  }
+
   private T loadNext() {
     try {
       while (true) {

core/pom.xml

@@ -67,6 +67,11 @@
       <artifactId>spark-launcher_${scala.binary.version}</artifactId>
       <version>${project.version}</version>
     </dependency>
+    <dependency>
+      <groupId>org.apache.spark</groupId>
+      <artifactId>spark-kvstore_${scala.binary.version}</artifactId>
+      <version>${project.version}</version>
+    </dependency>
     <dependency>
       <groupId>org.apache.spark</groupId>
       <artifactId>spark-network-common_${scala.binary.version}</artifactId>

core/src/main/resources/org/apache/spark/ui/static/historypage.js

@@ -188,7 +188,7 @@ $(document).ready(function() {
         }
 
         $(selector).DataTable(conf);
-        $('#hisotry-summary [data-toggle="tooltip"]').tooltip();
+        $('#history-summary [data-toggle="tooltip"]').tooltip();
       });
     });
 });

core/src/main/scala/org/apache/spark/deploy/history/ApplicationHistoryProvider.scala

@@ -68,11 +68,19 @@ private[history] abstract class HistoryUpdateProbe {
  * @param ui Spark UI
  * @param updateProbe probe to call to check on the update state of this application attempt
  */
-private[history] case class LoadedAppUI(
+private[spark] case class LoadedAppUI(
     ui: SparkUI,
     updateProbe: () => Boolean)
 
-private[history] abstract class ApplicationHistoryProvider {
+private[spark] abstract class ApplicationHistoryProvider {
+
+  /**
+   * The number of applications available for listing. Separate method in case it's cheaper
+   * to get a count than to calculate the whole listing.
+   *
+   * @return The number of available applications.
+   */
+  def getAppCount(): Int = getListing().size
 
   /**
    * Returns the count of application event logs that the provider is currently still processing.