Trying to avoid an NPE reported by a 1.537 user with the Multijob plugin (patched 1.10).

java.lang.NullPointerException
	at hudson.model.queue.FutureImpl.cancel(FutureImpl.java:78)
	at com.tikal.jenkins.plugins.multijob.MultiJobBuilder.perform(MultiJobBuilder.java:156)
	at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
	at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:781)
	at hudson.model.Build$BuildExecution.build(Build.java:199)
	at hudson.model.Build$BuildExecution.doRun(Build.java:160)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:562)
	at com.tikal.jenkins.plugins.multijob.MultiJobBuild$MultiJobRunnerImpl.run(MultiJobBuild.java:70)
	at hudson.model.Run.execute(Run.java:1665)
	at hudson.model.Run.run(Run.java:1612)
	at com.tikal.jenkins.plugins.multijob.MultiJobBuild.run(MultiJobBuild.java:63)
	at hudson.model.ResourceController.execute(ResourceController.java:88)
	at hudson.model.Executor.run(Executor.java:230)

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

@@ -58,6 +58,7 @@ import java.util.logging.Logger;
 
 import static hudson.model.queue.Executables.*;
 import static java.util.logging.Level.*;
+import javax.annotation.CheckForNull;
 
 
 /**
@@ -566,7 +567,7 @@ public class Executor extends Thread implements ModelObject {
     /**
      * Returns the executor of the current thread or null if current thread is not an executor.
      */
-    public static Executor currentExecutor() {
+    public static @CheckForNull Executor currentExecutor() {
         Thread t = Thread.currentThread();
         if (t instanceof Executor) return (Executor) t;
         return IMPERSONATION.get();

core/src/main/java/hudson/model/queue/FutureImpl.java

@@ -34,6 +34,7 @@ import java.util.HashSet;
 import java.util.Set;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
+import javax.annotation.Nonnull;
 
 /**
  * Created when {@link hudson.model.Queue.Item} is created so that the caller can track the progress of the task.
@@ -83,7 +84,7 @@ public final class FutureImpl extends AsyncFutureImpl<Executable> implements Que
         }
     }
 
-    synchronized void addExecutor(Executor executor) {
+    synchronized void addExecutor(@Nonnull Executor executor) {
         this.executors.add(executor);
     }
 }

core/src/main/java/hudson/model/queue/WorkUnitContext.java

@@ -91,7 +91,10 @@ public final class WorkUnitContext {
      * to create its {@link WorkUnit}.
      */
     public WorkUnit createWorkUnit(SubTask execUnit) {
-        future.addExecutor(Executor.currentExecutor());
+        Executor executor = Executor.currentExecutor();
+        if (executor != null) { // TODO is it legal for this to be called by a non-executor thread?
+            future.addExecutor(executor);
+        }
         WorkUnit wu = new WorkUnit(this, execUnit);
         workUnits.add(wu);
         return wu;