Add more comments to RpcEndpoint.onStop and fix onDrop

Author: zsxwing

core/src/main/scala/org/apache/spark/rpc/RpcEndpoint.scala

@@ -116,7 +116,8 @@ private[spark] trait RpcEndpoint {
   }
 
   /**
-   * Invoked when [[RpcEndpoint]] is stopping.
+   * Invoked when [[RpcEndpoint]] is stopping. `self` will be `null` in this method and you cannot
+   * use it to send or ask messages.
    */
   def onStop(): Unit = {
     // By default, do nothing.

core/src/main/scala/org/apache/spark/rpc/netty/Inbox.scala

@@ -178,7 +178,7 @@ private[netty] class Inbox(
         }
       }
     if (dropped) {
-      onDrop()
+      onDrop(message)
     }
   }
 
@@ -197,7 +197,7 @@ private[netty] class Inbox(
   }
 
   // Visible for testing.
-  protected def onDrop(message: Any): Unit = {
+  protected def onDrop(message: InboxMessage): Unit = {
     logWarning(s"Drop ${message} because $endpointRef is stopped")
   }
 

core/src/test/scala/org/apache/spark/rpc/netty/InboxSuite.scala

@@ -72,7 +72,7 @@ class InboxSuite extends SparkFunSuite {
 
     val numDroppedMessages = new AtomicInteger(0)
     val inbox = new Inbox(endpointRef, endpoint) {
-      override def onDrop(message: Any): Unit = {
+      override def onDrop(message: InboxMessage): Unit = {
         numDroppedMessages.incrementAndGet()
       }
     }