added another callback

core/src/main/java/hudson/model/AbstractCIBase.java

@@ -140,6 +140,7 @@ public abstract class AbstractCIBase extends Node implements ItemGroup<TopLevelI
         for (Map.Entry<Node, Computer> e : computers.entrySet()) {
             if (e.getValue() == computer) {
                 computers.remove(e.getKey());
+                computer.onRemoved();
                 return;
             }
         }

core/src/main/java/hudson/model/Computer.java

@@ -663,11 +663,26 @@ public /*transient*/ abstract class Computer extends Actionable implements Acces
 
     /**
      * Called by {@link Jenkins#updateComputerList()} to notify {@link Computer} that it will be discarded.
+     *
+     * @see #onRemoved()
      */
     protected void kill() {
         setNumExecutors(0);
     }
 
+    /**
+     * Called by {@link Jenkins} when this computer is removed.
+     *
+     * This happens when list of nodes are updated (for example by {@link Jenkins#setNodes(List)} and
+     * the computer becomes redundant. Such {@link Computer}s get {@linkplain #kill() killed}, then
+     * after all its executors are finished, this method is called.
+     *
+     * @see #kill()
+     * @since 1.510
+     */
+    protected void onRemoved(){
+    }
+
     private synchronized void setNumExecutors(int n) {
         if(numExecutors==n) return; // no-op
 
@@ -1076,8 +1091,8 @@ public /*transient*/ abstract class Computer extends Actionable implements Acces
         offlineMessage = Util.fixEmptyAndTrim(offlineMessage);
         setTemporarilyOffline(true,
                 OfflineCause.create(hudson.slaves.Messages._SlaveComputer_DisconnectedBy(
-                    Jenkins.getAuthentication().getName(),
-                    offlineMessage!=null ? " : " + offlineMessage : "")));
+                        Jenkins.getAuthentication().getName(),
+                        offlineMessage != null ? " : " + offlineMessage : "")));
         return HttpResponses.redirectToDot();
     }