When NodeMonitor is reconfigured or when the instance goes online, recompute the values.

Based on the following IRC conversations:

 (04:02:29 PM) cliffano: my build slave is offline, "Disk space is too low. Only 0.331GB left on" , but the box has 5.5Gb free.. i've tried disabling the disk space checking, restarted both master and slave, that error message is still there
 (04:03:03 PM) rtyler: I've seen that a couple times, I think I just disconnected and reconnected the slave
 (04:03:47 PM) cliffano: no dice for me.. reconnected several times.. even tried deleting more stuffs to free up more space, still exactly the same error message
 (04:04:04 PM) rtyler: you could try reinstalling windows
 (04:04:06 PM) ***rtyler ducks
 (04:04:22 PM) cliffano: lol yea
 (04:04:49 PM) kohsuke: cliffano: you can force the recomputation of those metrics
 (04:04:56 PM) kohsuke: otherwise it's only once an hour
 (04:05:17 PM) rtyler: wait, you can force it?
 (04:05:37 PM) kohsuke: http://ci.jenkins-ci.org/computer/ "refresh status"
 (04:06:26 PM) rtyler: ah
 (04:07:25 PM) cliffano: kohsuke: thanks, that worked..
 (04:07:43 PM) kohsuke: although I expect that if you've been reconnecting that should be sufficient
 (04:08:08 PM) kohsuke: And I probably should also do that automatically when you reconfigure the column, threashold, etc
 (04:10:39 PM) cliffano: kohsuke: reconnecting didn't help somehow... i tried disconnecting from the UI, tried killing the process on the slave machine, restarted etc
 (04:10:56 PM) kohsuke: it's probably caching the value
 (04:12:16 PM) cliffano: kohsuke: does it cache in the filesystem? i restarted both master and slaves
 (04:13:09 PM) kohsuke: it's stored in Computer
 (04:13:11 PM) kohsuke: so I guess that explains
 (04:13:34 PM) kohsuke: let me clear this when a slave connects
 (04:16:23 PM) cliffano: kohsuke: thanks :)

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

@@ -318,6 +318,12 @@ public final class ComputerSet extends AbstractModelObject implements Describabl
                     if(i!=null)
                         monitors.add(i);
                 }
+
+            // recompute the data
+            for (NodeMonitor nm : monitors) {
+                nm.triggerUpdate();
+            }
+
             rsp.sendRedirect2(".");
         } finally {
             bc.commit();

core/src/main/java/hudson/node_monitors/NodeMonitor.java

@@ -83,6 +83,9 @@ public abstract class NodeMonitor implements ExtensionPoint, Describable<NodeMon
         return (AbstractNodeMonitorDescriptor<?>) Jenkins.getInstance().getDescriptorOrDie(getClass());
     }
 
+    /**
+     * Obtains the monitoring result currently available, or null if no data is available.
+     */
     public Object data(Computer c) {
         return getDescriptor().get(c);
     }

core/src/main/java/hudson/node_monitors/NodeMonitorUpdater.java

+package hudson.node_monitors;
+
+import hudson.Extension;
+import hudson.model.Computer;
+import hudson.model.TaskListener;
+import hudson.slaves.ComputerListener;
+import hudson.util.DaemonThreadFactory;
+import jenkins.model.Jenkins;
+
+import java.io.IOException;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * When a slave is connected, redo the node monitoring.
+ *
+ * @author Kohsuke Kawaguchi
+ */
+@Extension
+public class NodeMonitorUpdater extends ComputerListener {
+    // TODO: shutdown hook to kill off this timer
+    private final ScheduledExecutorService timer = new ScheduledThreadPoolExecutor(1,new DaemonThreadFactory());
+
+    private volatile long timestamp;
+
+    /**
+     * Triggers the update with 5 seconds quiet period, to avoid triggering data check too often
+     * when multiple slaves become online at about the same time.
+     */
+    @Override
+    public void onOnline(Computer c, TaskListener listener) throws IOException, InterruptedException {
+        timestamp = System.currentTimeMillis();
+        timer.schedule(new Runnable() {
+            public void run() {
+                if (System.currentTimeMillis()-timestamp<4000)
+                    return;
+
+                for (NodeMonitor nm : Jenkins.getInstance().getComputer().getMonitors()) {
+                    nm.triggerUpdate();
+                }
+            }
+        }, 5, TimeUnit.SECONDS);
+    }
+}