Make stdout swapping more reliable.

This matches fb65a2c3c8584a9ac4daa49ecbce7a540dc7f5a5 in remoting.

core/src/main/java/jenkins/slaves/StandardOutputSwapper.java

@@ -38,28 +38,39 @@ public class StandardOutputSwapper extends ComputerListener {
     private static final class ChannelSwapper extends MasterToSlaveCallable<Boolean,Exception> {
         public Boolean call() throws Exception {
             if (File.pathSeparatorChar==';')    return false;   // Windows
-            
-            OutputStream o = Channel.current().getUnderlyingOutput();
-            if (o instanceof StandardOutputStream) {
-                StandardOutputStream stdout = (StandardOutputStream)o;
 
-                // duplicate the OS file descriptor and create FileOutputStream around it
-                int out = GNUCLibrary.LIBC.dup(1);
-                if (out<0)      throw new IOException("Failed to dup(1)");
-                Constructor<FileDescriptor> c = FileDescriptor.class.getDeclaredConstructor(int.class);
-                c.setAccessible(true);
-                FileOutputStream fos = new FileOutputStream(c.newInstance(out));
+            Channel c = Channel.current();
 
-                // swap it into channel so that it'll use the new file descriptor
-                stdout.swap(fos);
+            StandardOutputStream sos = (StandardOutputStream) c.getProperty(StandardOutputStream.class);
+            if (sos!=null) {
+                swap(sos);
+                return true;
+            }
 
-                // close fd=1 (stdout) and duplicate fd=2 (stderr) into fd=1 (stdout)
-                GNUCLibrary.LIBC.close(1);
-                GNUCLibrary.LIBC.dup2(2,1);
+            OutputStream o = c.getUnderlyingOutput();
+            if (o instanceof StandardOutputStream) {
+                swap((StandardOutputStream) o);
                 return true;
             }
+
             return false;
         }
+
+        private void swap(StandardOutputStream stdout) throws IOException, NoSuchMethodException, InstantiationException, IllegalAccessException, java.lang.reflect.InvocationTargetException {
+            // duplicate the OS file descriptor and create FileOutputStream around it
+            int out = GNUCLibrary.LIBC.dup(1);
+            if (out<0)      throw new IOException("Failed to dup(1)");
+            Constructor<FileDescriptor> c = FileDescriptor.class.getDeclaredConstructor(int.class);
+            c.setAccessible(true);
+            FileOutputStream fos = new FileOutputStream(c.newInstance(out));
+
+            // swap it into channel so that it'll use the new file descriptor
+            stdout.swap(fos);
+
+            // close fd=1 (stdout) and duplicate fd=2 (stderr) into fd=1 (stdout)
+            GNUCLibrary.LIBC.close(1);
+            GNUCLibrary.LIBC.dup2(2,1);
+        }
     }
 
     private static final Logger LOGGER = Logger.getLogger(StandardOutputSwapper.class.getName());