Bug#400292: exim4-config: Should remove /etc/mailname if created automatically

[Javier Fernández-Sanguino Peña]

Package: exim4-config
Version: 4.63-10
Priority: wishlist

Exim4-config generates /etc/mailname automatically on postinst, based on the
information provided by the user. It fails, however, to remove this
configuration file if the package is purged from the system.

I have recently had to debug a misconfiguration issue which prevent a mail
system from locally delivering mail. Here's what happened:

- User installed Debian, when the exim4-config package is installed he 
  (by mistake) presses a key in the keyboard that generates a control
  character in the debconf prompt for the local system name 
  (using someother name instead of the default 'localhost.localdomain')

- The debconf prompt fails to notice that the user introduced an invalid
  (non-ASCII) char and happily uses it (this is another, different, bug)

- The user configures the 'things we relay from' with the same local system
  name he (thought) he had introduced.

- Exim4 fails to deliver local mail (logs show that the local system is not
  authorised to use the daemon as a realy)

There is no way to inmediately fix this:

- If the user tries to fix this (reconfiguring the package) he is presented
with a debconf prompt (the first one) in which no matter what characters he
introduces, the prompt does not change and remains blank (this is due to the
control character). 

- If the user tries to puge the package and reinstall then he is presented
  with the same non-functioning debconf prompt. (this is because
  /etc/mailname is preserved after purge and reused in the debconf prompt)

IMHO, exim4-config should remove /etc/mailname if it can ascertain (through
it's checksum) that *he* (and not the admin) created the file. That would
make it possible to fix the above situation by purging the package and
reinstalling.

Regards

Javier

PS: I'm going to fix a separate bug for the control chars issue
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