[FIXED JENKINS-9003] NPE when polling after node removed

changelog.html

@@ -69,6 +69,9 @@ Upcoming changes</a>
   <li class=bug>
     Catch FileNotFoundException in Maven builds if Mojos are executed from a classes directory.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-5044">issue 5044</a>)
+  <li class=bug>
+    Fix NPE if node of last build isn't available anymore while polling for SCM changes.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-9003">issue 9003 </a>)
   <li class=rfe>
     Set NODE_NAME for master node to "master"
     (<a href="http://issues.jenkins-ci.org/browse/JENKINS-9671">issue 9671</a>)

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

@@ -1263,7 +1263,7 @@ public abstract class AbstractProject<P extends AbstractProject<P,R>,R extends A
                 // lock the workspace of the last build
                 FilePath ws=lb.getWorkspace();
 
-                if (ws==null || !ws.exists()) {
+                if (workspaceOffline(lb)) {
                     // workspace offline. build now, or nothing will ever be built
                     Label label = getAssignedLabel();
                     if (label != null && label.isSelfLabel()) {
@@ -1326,6 +1326,24 @@ public abstract class AbstractProject<P extends AbstractProject<P,R>,R extends A
             return NO_CHANGES;
         }
     }
+    
+    private boolean workspaceOffline(R build) throws IOException, InterruptedException {
+        FilePath ws = build.getWorkspace();
+        if (ws==null || !ws.exists()) {
+            return true;
+        }
+        
+        Node builtOn = build.getBuiltOn();
+        if (builtOn == null) { // node built-on doesn't exist anymore
+            return true;
+        }
+        
+        if (builtOn.toComputer() == null) { // node still exists, but has 0 executors - o.s.l.t.
+            return true;
+        }
+        
+        return false;
+    }
 
     /**
      * Returns true if this user has made a commit to this project.