Weird errors from update-alternatives

[Daniel Burrows <Daniel_Burrows@brown.edu>]

  My system upgrade today (from yesterday's potato to today's potato) produced
the following odd output:

Setting up tk8.2 (8.2.0-3) ...
Checking available versions of wish, updating links in /etc/alternatives ...
(You may modify the symlinks there yourself if desired - see `man ln'.)
Leaving wish (/usr/bin/wish) pointing to /usr/bin/wish8.2.
Updating wish.1 (/usr/share/man/man1/wish.1.gz) to point to /usr/share/man/man1/wish8.2.1.gz.
Removing wish8.0.1 (/usr/man/man1/wish8.0.1.gz), not appropriate with /usr/bin/wish8.2.
warning: /usr/bin/wish8.0 is supposed to be a slave symlink to
 /etc/alternatives/wish8.0, or nonexistent; however, readlink failed: Invalid argument
Removing wish8.0 (/usr/bin/wish8.0), not appropriate with /usr/bin/wish8.2.

  In particular, note the last three lines.  /usr/bin/wish8.0 is in the package
tk8.0, and I got set to file a Grave bug against something for clobbering
a package's files arbitrarily (I wasn't sure whether dpkg or tk8.0 was at
fault).  However, I decided first to examine /usr/bin/wish8.0 and see what was
there now.  To my surprise, it was a binary which appears to belong to the
tk8.0 package!  So there are two things going on here:

  -> The readlink failure.  I suspect that this is occuring because the file
    is not a symlink.
    
  -> The removal message.  First, shouldn't update-alternatives do something
    more graceful than unconditionally clobbering whatever existed there before?
    Second, why didn't it delete the file?

  Anyone know why this is, and why I haven't seen this behavior before? (I
noticed that dpkg was upgraded over two version numbers, but the only
change to update-alternatives doesn't sound like it would be likely to be
affected by this..)

  Daniel

-- 
  The Disc, being flat, has no real horizon.  Any adventurous sailors who get
funny ideas from staring at eggs and oranges too long and set out for the
antipodes soon discovert that the reason that distant ships sometimes look
like they're disappearing over the edge of the world is that they *are*
disappearing over the edge of the world.
  -- Terry Pratchett, _The Light Fantastic_