merged additional changes

core/src/main/java/hudson/ExtensionList.java

@@ -80,7 +80,7 @@ public class ExtensionList<T> extends AbstractList<T> {
     private volatile List<ExtensionComponent<T>> extensions;
 
     /**
-     * Place to store manually registered instances with the per-Jenkins scope.
+     * Place to store manually registered instances with the per-Hudson scope.
      * {@link CopyOnWriteArrayList} is used here to support concurrent iterations and mutation.
      */
     private final CopyOnWriteArrayList<ExtensionComponent<T>> legacyInstances;

core/src/main/java/hudson/slaves/SlaveComputer.java

@@ -197,7 +197,6 @@ public class SlaveComputer extends Computer {
                             cl.preLaunch(SlaveComputer.this, taskListener);
 
                         launcher.launch(SlaveComputer.this, taskListener);
-                        return null;
                     } catch (AbortException e) {
                         taskListener.error(e.getMessage());
                         throw e;
@@ -216,6 +215,10 @@ public class SlaveComputer extends Computer {
                             cl.onLaunchFailure(SlaveComputer.this, taskListener);
                     }
                 }
+
+                if (channel==null)
+                    throw new IOException("Slave failed to connect, even though the launcher didn't report it. See the log output for details.");
+                return null;
             }
         });
     }