I noticed that FilterOutputStream is really not implemented for a simple delegation. Look at how its write method is implemented:

public void write(byte b[], int off, int len) throws IOException {
        if ((off | len | (b.length - (len + off)) | (off + len)) < 0)
            throw new IndexOutOfBoundsException();
        for (int i = 0 ; i < len ; i++) {
            write(b[off + i]);
        }
    }


git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@2793 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/CloseProofOutputStream.java

@@ -1,12 +1,14 @@
 package hudson;
 
-import java.io.FilterOutputStream;
+import hudson.util.DelegatingOutputStream;
+
 import java.io.OutputStream;
 
 /**
+ * {@link OutputStream} that blocks {@link #close()} method.
  * @author Kohsuke Kawaguchi
  */
-public class CloseProofOutputStream extends FilterOutputStream {
+public class CloseProofOutputStream extends DelegatingOutputStream {
     public CloseProofOutputStream(OutputStream out) {
         super(out);
     }