Here's how you can see the bug.

1. Boot a clean copy of Knoppix 4.0.2 CD English
2. Type "apt-get remove mozilla-thunderbird-locale-de"

UnionFS is now in an inconsistent state. To see the problem, type this:

% ls -al /ramdisk/var/lib/mozilla-thunderbird/
total 8
drwxr-xr-x 7 root root 220 Dec 15 19:28 .
drwxr-xr-x 7 root root 140 Dec 15 19:28 ..
-rwxr-xr-x 1 root root 0 Dec 15 19:28 .wh.components.ini
-rwxr-xr-x 1 root root 0 Dec 15 19:28 .wh.defaults.ini
drwxr-xr-x 3 root root 140 Dec 15 19:28 chrome
drwxr-xr-x 2 root root 140 Dec 15 19:28 components
-rw-r--r-- 1 root root 24 Dec 15 19:28 components.ini
-rw-r--r-- 1 root root 48 Dec 15 19:28 defaults.ini
drwxr-xr-x 2 root root 100 Dec 15 19:28 extensions
drwxr-xr-x 2 root root 60 Dec 15 19:28 extensions.d
drwxr-xr-x 2 root root 60 Dec 15 19:28 locales.d

As you can see, the directory listing contains both "components.ini" and its whiteout, ".wh.components.ini". After some testing, I have discovered that when both the file and the whiteout are present, the results are unpredictable - sometimes the file is there, sometimes it's not.

The reason I noticed this is that I was trying to remaster knoppix, but the remastering howto suggests this step:

cp -Rp /KNOPPIX/* /mnt/hda1/knx/source/KNOPPIX

But I thought, instead of copying the CD to a read-write partition, why not just use unionfs to merge /KNOPPIX with an empty read-write directory? But of course, it didn't work right, and this UnionFS bug is the reason.

- Josh