[FIXED JENKINS-15578]

The slave launch thread is another background thread, and it should run
like a super-user

changelog.html

@@ -55,6 +55,9 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class=bug>
+    Slave launch thread should have the background activitiy credential.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-15578">issue 15578</a>)
   <li class=bug>
     “Build Now” link did not work for multijobs.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-16974">issue 16974</a>)

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

@@ -72,6 +72,7 @@ import javax.crypto.spec.SecretKeySpec;
 import javax.servlet.RequestDispatcher;
 import jenkins.model.Jenkins;
 import jenkins.slaves.JnlpSlaveAgentProtocol;
+import org.acegisecurity.Authentication;
 import org.kohsuke.stapler.StaplerRequest;
 import org.kohsuke.stapler.StaplerResponse;
 import org.kohsuke.stapler.QueryParameter;
@@ -217,6 +218,9 @@ public class SlaveComputer extends Computer {
             public Object call() throws Exception {
                 // do this on another thread so that the lengthy launch operation
                 // (which is typical) won't block UI thread.
+
+                ACL.impersonate(ACL.SYSTEM);    // background activity should run like a super user
+
                 try {
                     log.rewind();
                     try {