Bug#400294: exim4-config: Should not allow non-ASCII characters in /etc/mailname (and maybe in some other values)

[Javier Fernández-Sanguino Peña]

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

Exim4-config fails to detect if a user introduced non-ASCII characters
in the debconf prompt that might lead to an impromer /etc/mailname being
automatically generated on postinst.

I have recently had to debug a misconfiguration issue which stemmed on the 
fact that the user introduced a control character (by pressing some of the
"Print Screen" / Scroll Lock keys in the keyboard) when introducing the
local system name which resulted in local mail not being delivered.

Notice that since the value is reused in the exim4-config debconf values and
Debconf does not properly handle control characters there is no way to fix
this issue through either Debconf reconfiguration or package purge:

- If the user tries to fix this by reconfiguring the package he is presented
  with a debconf prompt in which, no matter what characters he/she
  introduces, the prompt does not change and remains blank (because of 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)

I don't think that exim4-config should allow non-ASCII characters in answer
to his configuration questions for the local mailname and other values.

Maybe this should be something requested of Debconf prompts as they
cannot handle non-ASCII information. Maybe a new 'string-ascii' type should
be selected that made Debconf printout an error (and remade the question) if
the user introduced invalid characteres there.  I envision that many Debconf
questions cannot handle non-ASCII information and asking maintainer scripts
to code in that detection would be maybe too much. If you agree, please
reassign this bug to Debconf.

Regards

Javier
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