[HUDSON-5073] if the build fails because the node went offline, point the user...

[HUDSON-5073] if the build fails because the node went offline, point the user to the slave log where the details are.

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

@@ -29,6 +29,7 @@ import hudson.Functions;
 import hudson.Launcher;
 import hudson.Util;
 import hudson.FilePath;
+import hudson.console.HyperlinkNote;
 import hudson.slaves.WorkspaceList;
 import hudson.slaves.NodeProperty;
 import hudson.slaves.WorkspaceList.Lease;
@@ -416,6 +417,14 @@ public abstract class AbstractBuild<P extends AbstractProject<P,R>,R extends Abs
 
                 Result result = doRun(listener);
 
+                Computer c = node.toComputer();
+                if (c==null || c.isOffline()) {
+                    // See HUDSON-5073. looks like we went offline during the build
+                    // point the user to the slave log
+                    listener.hyperlink("/computer/"+builtOn+"/log","Looks like the node went offline during the build. Check the slave log for the details.");
+                    listener.getLogger().println();
+                }
+
                 // kill run-away processes that are left
                 // use multiple environment variables so that people can escape this massacre by overriding an environment
                 // variable for some processes

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

@@ -76,6 +76,9 @@ public interface TaskListener extends Serializable {
 
     /**
      * Places a {@link HyperlinkNote} on the given text.
+     *
+     * @param url
+     *      If this starts with '/', it's interpreted as a path within the context path.
      */
     void hyperlink(String url, String text) throws IOException;