Bug#133030: debian-policy: debconf policy (specification) implies dpkg will run .config before preinst ALWAYS

[Joey Hess <joeyh@debian.org>]

Henrique de Moraes Holschuh wrote:
> Please document this, it may save someone a grave bug someday, and maybe
> even avoid a lot of headaches.

Does it really need to be documented in policy? debconf-devel(8)
documents it:

       The config script can be run in one of three ways:

       1      If  a  package is pre-configured, with dpkg-precon­
              figure, its config script is run, and is passed the
              parameters "configure", and installed-version.

       2      When  a package's postinst is run, debconf will try
              to run the config script then too, and it  will  be
              passed the same parameters it was passed when it is
              pre-configured. This is necessary because the pack­
              age  might  not  have  been pre-configured, and the
              config script still needs to get a chance  to  run.
              See HACKS for details.

       3      If  a  package is reconfigured, with dpkg-reconfig­
              ure, its config script it run, and  is  passed  the
              parameters "reconfigure" and installed-version.

       Note that since a typical package install or upgrade using
       apt runs steps 1 and 3, the config script  will  typically
       be run twice. It should do nothing the second time (to ask
       questions twice in a row is annoying), and it should defi­
       nitely  be  idempotent.  Luckily, debconf avoids repeating
       questions by default, so this is generally easy to  accom­
       plish.

...

HACKS
       Debconf is currently not fully integrated into dpkg (but I
       want  to  change  this  in  the future), and so some messy
       hacks are currently called for.

       The worst of these involves getting the config  script  to
       run.  The  way that works now is the config script will be
       run when the package is  pre-configured.  Then,  when  the
       postinst  script runs, it starts up debconf again. Debconf
       notices it is being used by the postinst script, and so it
       goes off and runs the config script. This can only work if
       your postinst  loads  up  one  of  the  debconf  libraries
       though,  so postinsts always have to take care to do that.
       We hope to address this later by adding  explicit  support
       to dpkg for debconf.

(This man page will be in the next release of debconf BTW.)

-- 
see shy jo