[FIXED JENKINS-23277] Better diagnosis for an NPE probably involving cloud slaves.

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

@@ -87,6 +87,7 @@ import javax.annotation.CheckForNull;
 import org.kohsuke.stapler.interceptor.RequirePOST;
 
 import static java.util.logging.Level.WARNING;
+import javax.annotation.Nonnull;
 import jenkins.model.lazy.BuildReference;
 import jenkins.model.lazy.LazyBuildMixIn;
 
@@ -435,9 +436,19 @@ public abstract class AbstractBuild<P extends AbstractProject<P,R>,R extends Abs
 
         /**
          * Returns the current {@link Node} on which we are building.
+         * @throws IllegalStateException if that cannot be determined
          */
-        protected final Node getCurrentNode() {
-            return Executor.currentExecutor().getOwner().getNode();
+        protected final @Nonnull Node getCurrentNode() throws IllegalStateException {
+            Executor exec = Executor.currentExecutor();
+            if (exec == null) {
+                throw new IllegalStateException("not being called from an executor thread");
+            }
+            Computer c = exec.getOwner();
+            Node node = c.getNode();
+            if (node == null) {
+                throw new IllegalStateException("no longer a configured node for " + c.getName());
+            }
+            return node;
         }
 
         public Launcher getLauncher() {

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

@@ -59,6 +59,7 @@ import java.util.logging.Logger;
 import static hudson.model.queue.Executables.*;
 import static java.util.logging.Level.*;
 import javax.annotation.CheckForNull;
+import javax.annotation.Nonnull;
 
 
 /**
@@ -70,7 +71,7 @@ import javax.annotation.CheckForNull;
  */
 @ExportedBean
 public class Executor extends Thread implements ModelObject {
-    protected final Computer owner;
+    protected final @Nonnull Computer owner;
     private final Queue queue;
 
     private long startTime;
@@ -111,7 +112,7 @@ public class Executor extends Thread implements ModelObject {
      */
     private final List<CauseOfInterruption> causes = new Vector<CauseOfInterruption>();
 
-    public Executor(Computer owner, int n) {
+    public Executor(@Nonnull Computer owner, int n) {
         super("Executor #"+n+" for "+owner.getDisplayName());
         this.owner = owner;
         this.queue = Jenkins.getInstance().getQueue();
@@ -534,7 +535,7 @@ public class Executor extends Thread implements ModelObject {
         return e!=null && Tasks.getOwnerTaskOf(getParentOf(e)).hasAbortPermission();
     }
 
-    public Computer getOwner() {
+    public @Nonnull Computer getOwner() {
         return owner;
     }