Added a new extension point to listen to slave connection/disconnection activity.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@11339 71c3de6d-444a-0410-be80-ed276b4c234a

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

@@ -16,6 +16,7 @@ import static hudson.Util.fixEmpty;
 import hudson.XmlFile;
 import hudson.ProxyConfiguration;
 import hudson.slaves.RetentionStrategy;
+import hudson.slaves.ComputerListener;
 import hudson.model.Descriptor.FormException;
 import hudson.model.listeners.ItemListener;
 import hudson.model.listeners.JobListener;
@@ -271,6 +272,11 @@ public final class Hudson extends View implements ItemGroup<TopLevelItem>, Node,
      */
     private transient final CopyOnWriteList<SCMListener> scmListeners = new CopyOnWriteList<SCMListener>();
 
+    /**
+     * List of registered {@link ComputerListener}s.
+     */
+    private transient final CopyOnWriteList<ComputerListener> computerListeners = new CopyOnWriteList<ComputerListener>();
+
     /**
      * TCP slave agent port.
      * 0 for random, -1 to disable.
@@ -542,6 +548,13 @@ public final class Hudson extends View implements ItemGroup<TopLevelItem>, Node,
         return scmListeners;
     }
 
+    /**
+     * Gets all the installed {@link ComputerListener}s.
+     */
+    public CopyOnWriteList<ComputerListener> getComputerListeners() {
+        return computerListeners;
+    }
+
     /**
      * Gets the plugin object from its short name.
      *

core/src/main/java/hudson/slaves/ComputerListener.java

+package hudson.slaves;
+
+import hudson.model.Computer;
+import hudson.model.Hudson;
+import hudson.ExtensionPoint;
+
+/**
+ * Receives notifications about status changes of {@link Computer}s.
+ *
+ * @author Kohsuke Kawaguchi
+ * @since 1.246
+ */
+public abstract class ComputerListener implements ExtensionPoint {
+    /**
+     * Called right after a {@link Computer} comes online.
+     */
+    public void onOnline(Computer c) {}
+
+    /**
+     * Called right after a {@link Computer} went offline.
+     */
+    public void onOffline(Computer c) {}
+
+    /**
+     * Registers this {@link ComputerListener} so that it will start receiving events.
+     */
+    public final void register() {
+        Hudson.getInstance().getComputerListeners().add(this);
+    }
+
+    /**
+     * Unregisters this {@link ComputerListener} so that it will never receive further events.
+     *
+     * <p>
+     * Unless {@link ComputerListener} is unregistered, it will never be a subject of GC.
+     */
+    public final boolean unregister() {
+        return Hudson.getInstance().getComputerListeners().remove(this);
+    }
+}

core/src/main/java/hudson/slaves/SlaveComputer.java

@@ -229,6 +229,8 @@ public final class SlaveComputer extends Computer {
             numRetryAttempt = 0;
             this.channel = channel;
         }
+        for (ComputerListener cl : Hudson.getInstance().getComputerListeners())
+            cl.onOnline(this);
         Hudson.getInstance().getQueue().scheduleMaintenance();
     }
 
@@ -336,6 +338,8 @@ public final class SlaveComputer extends Computer {
                 logger.log(Level.SEVERE, "Failed to terminate channel to " + getDisplayName(), e);
             }
         }
+        for (ComputerListener cl : Hudson.getInstance().getComputerListeners())
+            cl.onOffline(this);
     }
 
     @Override