Bug: The ApplicationInfo in KVStore may be accessed before it's prepared
Status: Resolved | Priority: Minor | Resolution: Fixed
Created: 03/May/20 21:50
Affects Version/s: 3.0.0

While starting some local tests, I occasionally encountered the following exceptions for Web UI.
{noformat}
23:00:29.845 WARN org.eclipse.jetty.server.HttpChannel: /jobs/
 java.util.NoSuchElementException
 at java.util.Collections$EmptyIterator.next(Collections.java:4191)
 at org.apache.spark.util.kvstore.InMemoryStore$InMemoryIterator.next(InMemoryStore.java:467)
 at org.apache.spark.status.AppStatusStore.applicationInfo(AppStatusStore.scala:39)
 at org.apache.spark.ui.jobs.AllJobsPage.render(AllJobsPage.scala:266)
 at org.apache.spark.ui.WebUI.$anonfun$attachPage$1(WebUI.scala:89)
 at org.apache.spark.ui.JettyUtils$$anon$1.doGet(JettyUtils.scala:80)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
 at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:873)
 at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1623)
 at org.apache.spark.ui.HttpSecurityFilter.doFilter(HttpSecurityFilter.scala:95)
 at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
 at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:540)
 at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:255)
 at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1345)
 at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:203)
 at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:480)
 at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:201)
 at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1247)
 at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:144)
 at org.eclipse.jetty.server.handler.gzip.GzipHandler.handle(GzipHandler.java:753)
 at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:220)
 at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:132)
 at org.eclipse.jetty.server.Server.handle(Server.java:505)
 at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:370)
 at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:267)
 at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:305)
 at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:103)
 at org.eclipse.jetty.io.ChannelEndPoint$2.run(ChannelEndPoint.java:117)
 at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:698)
 at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:804)
 at java.lang.Thread.run(Thread.java:748){noformat}
*Reason*
 That is because {{AppStatusStore.applicationInfo()}} accesses an empty view (iterator) returned by {{InMemoryStore}}.

AppStatusStore
{code:java}
def applicationInfo(): v1.ApplicationInfo = {
    store.view(classOf[ApplicationInfoWrapper]).max(1).iterator().next().info
}
{code}
InMemoryStore
{code:java}
public <T> KVStoreView<T> view(Class<T> type){
    InstanceList<T> list = inMemoryLists.get(type);
    return list != null ? list.view() : emptyView();
 }
{code}
During the initialization of {{SparkContext}}, it first starts the Web UI (SparkContext: L475 _ui.foreach(_.bind())) and then setup the {{LiveListenerBus}} thread (SparkContext: L608 {{setupAndStartListenerBus()}}) for dispatching the {{SparkListenerApplicationStart}} event (which will trigger writing the requested {{ApplicationInfo}} to {{InMemoryStore}}).