Bug#930118: libconfig-model-dpkg-perl: adds debian/compat despite debhelper-compat

[Dominique Dumont]

On Wed, 12 Jun 2019 19:54:51 +0200 gregor herrmann <gregoa at debian.org> wrote:
> One might argue that forcing this change-of-style is a bit intrusive;
> but on the other hand it's supported by debhelper in buster and
> stretch-backports and is now the recommended way … So yes, I think a
> migration would be a good idea.

ok, it turns out that this migration is quite complicated. 

Here's what I plan to implement. I hope this 
takes into account most common mistakes. This list below details the checks that will be
run on debhelper or debherlper-compat dependencies. (this doc looks better with a monospace font).

Please tell me if I missed something (or if it's too much)..

Both check can be applied on a dependency if the dependency is changed
from debhelper to debhelper-compat:

** debhelper dependency check

Depending on ~debian/compat~ content and ~debhelper~ depencency value,
the check and fix steps will behave this way:

| compat | debhelper version      | check                    | fix                                                   |
|--------+------------------------+--------------------------+-------------------------------------------------------|
| undef  | undef                  | error                    | error                                                 |
| undef  | defined                | warn missing compat      | set compat to dh version                              |
| < 9    | consistent with compat | warn about old compat    | update compat and dh version                          |
| < 9    | inconsistent           | warn about old compat    | update compat and dh version                          |
| >=9    | ignored                | warn no debhelper-compat | change dependency to debhelper_compat = $compat_value |

Note that all check are tried one by one. More than one fix can be
applied. For instance, during a fix, a ~compat~ set to 8 is changed to
12, then the ~debhelper~ dependency is changed to ~debhelper-compat =
12~ and ~compat~ is deleted by the rules shown below.

** debhelper-compat dependency check

Depending on ~debian/compat~ content and ~debhelper-compat~ depencency
value, the check and fix steps will behave this way:

| compat    | debhelper-compat version | check             | fix               |
|-----------+--------------------------+-------------------+-------------------|
| defined   | defined                  | warn extra compat | delete compat     |
| undefined | < current dh compat - 1  | warn too old      | update dependency |


** global check

need to check in Buid-Depends that debhelper and debhelper-compat do not coexist