RATIS-2721. Notify the StateMachine when a server shutdown starts

[amaliujia]

What changes were proposed in this pull request?

Notify the StateMachine when a server shutdown starts

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-2721

Please replace this section with the link to the Apache JIRA)

How was this patch tested?

Unit test

[szetszwo]

@amaliujia , thanks for working on this!

Please deprecate and override the existing notifyServerShutdown method:
(revision: renamed the new enum to EventStatus)

diff --git a/ratis-server-api/src/main/java/org/apache/ratis/statemachine/StateMachine.java b/ratis-server-api/src/main/java/org/apache/ratis/statemachine/StateMachine.java
index bd009b0e9..967debb04 100644
--- a/ratis-server-api/src/main/java/org/apache/ratis/statemachine/StateMachine.java
+++ b/ratis-server-api/src/main/java/org/apache/ratis/statemachine/StateMachine.java
@@ -58,6 +58,11 @@ import java.util.function.Function;
 public interface StateMachine extends Closeable {
   Logger LOG = LoggerFactory.getLogger(StateMachine.class);
 
+  enum EventStatus {
+    INITIATED,
+    COMPLETED
+  }
+
   /** A registry to support different state machines in multi-raft environment. */
   interface Registry extends Function<RaftGroupId, StateMachine> {
   }
@@ -229,20 +234,33 @@ public interface StateMachine extends Closeable {
     default void notifySnapshotInstalled(InstallSnapshotResult result, long snapshotIndex,  RaftPeer peer) {}
 
     /**
-     * Notify the {@link StateMachine} that the server for this division has been shut down.
-     * @deprecated please use/override {@link #notifyServerShutdown(RoleInfoProto, boolean)} instead
+     * The same as notifyServerShutdown(roleInfo, false, EventStatus.COMPLETED).
+     *
+     * @deprecated please use/override {@link #notifyServerShutdown(RoleInfoProto, boolean, EventStatus)} instead
      */
     @Deprecated
     default void notifyServerShutdown(RoleInfoProto roleInfo) {
-      notifyServerShutdown(roleInfo, false);
+      notifyServerShutdown(roleInfo, false, EventStatus.COMPLETED);
+    }
+
+    /**
+     * The same as notifyServerShutdown(roleInfo, allServer, EventStatus.COMPLETED).
+     *
+     * @deprecated please use/override {@link #notifyServerShutdown(RoleInfoProto, boolean, EventStatus)} instead
+     */
+    @Deprecated
+    default void notifyServerShutdown(RoleInfoProto roleInfo, boolean allServer) {
+      notifyServerShutdown(roleInfo, allServer, EventStatus.COMPLETED);
     }
 
     /**
-     * Notify the {@link StateMachine} that either the server for this division or all the servers have been shut down.
+     * Notify the {@link StateMachine} server shutdown.
+     *
      * @param roleInfo roleInfo this server
      * @param allServer whether all raft servers will be shutdown at this time
+     * @param eventStatus {@link EventStatus#INITIATED} or {@link EventStatus#COMPLETED}
      */
-    default void notifyServerShutdown(RoleInfoProto roleInfo, boolean allServer) {}
+    default void notifyServerShutdown(RoleInfoProto roleInfo, boolean allServer, EventStatus eventStatus) {}
   }
 
   /**