title: db_input in postinst doesn't work
body: <p>I have a problem with postinst script, I have to ask root password for MySQL to create some user for my application. I know, it's bad practice to use db_input in postinst however I really need it.
  Why this postinst doesn't work when I install my app form repository but works properly when I install it as .deb package?</p>
  
  <p>postinst:</p>
  
  <pre><code>#!/bin/bash -e
  
  . /usr/share/debconf/confmodule
  
  db_input high my_app/mysql_root_password || true
  db_go
  </code></pre>
  
  <p>error:</p>
  
  <pre><code>dpkg: error processing my-app (--configure):
   subprocess installed post-installation script returned error exit status 30
  configured to not write apport reports
                                        Errors were encountered while processing:
   my-app
  E: Sub-process /usr/bin/dpkg returned an error code (1)
  </code></pre>