[SPARK-23608][CORE][WEBUI] Add synchronization in SHS between attachSparkUI and detachSparkUI functions to avoid concurrent modification issue to Jetty Handlers

Jetty handlers are dynamically attached/detached while SHS is running. But the attach and detach operations might be taking place at the same time due to the async in load/clear in Guava Cache.

## What changes were proposed in this pull request?
Add synchronization between attachSparkUI and detachSparkUI in SHS.

## How was this patch tested?
With this patch, the jetty handlers missing issue never happens again in our production cluster SHS.

Author: Ye Zhou <[email protected]>

Closes apache#20744 from zhouyejoe/SPARK-23608.

Author: zhouyejoe

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

@@ -150,14 +150,18 @@ class HistoryServer(
       ui: SparkUI,
       completed: Boolean) {
     assert(serverInfo.isDefined, "HistoryServer must be bound before attaching SparkUIs")
-    ui.getHandlers.foreach(attachHandler)
-    addFilters(ui.getHandlers, conf)
+    handlers.synchronized {
+      ui.getHandlers.foreach(attachHandler)
+      addFilters(ui.getHandlers, conf)
+    }
   }
 
   /** Detach a reconstructed UI from this server. Only valid after bind(). */
   override def detachSparkUI(appId: String, attemptId: Option[String], ui: SparkUI): Unit = {
     assert(serverInfo.isDefined, "HistoryServer must be bound before detaching SparkUIs")
-    ui.getHandlers.foreach(detachHandler)
+    handlers.synchronized {
+      ui.getHandlers.foreach(detachHandler)
+    }
     provider.onUIDetached(appId, attemptId, ui)
   }