[development] Choosing the N number for .install's hook_update_N

[Nathaniel Catchpole]

>
> What other software has the very first digit go up and the feature set go
> down?  I'm surprised to hear you have this expectation, and I'm curious how
> wide-spread it is.
>

I think it arises from the way contrib modules are so intricately
linked to major core versions. When PHP issues a new release, we don't
(usually) have to release a new Drupal version to keep up with it.
Since there's no backwards compatibility for contrib, it's a bit more
complicated.

So, when a module is ported, one of at least three to four situations
might occur:

5.x-1.x branch gets ported to 6.x-1.x branch - just a straight port

5.x-1.x branch gets ported to 6.x-1.x branch - new features are added
to the 6.x-1.x branch during this process

5.x-1.x branch gets a straight port to 6.x-1.x, and 5.x-2.x gets a
straight port to 6.x-2.x

5.x-1.x branch is abandoned, users have to upgrade to 5.x-2.x then to
6.x-1.x (if the maintainer has dropped old updates, like core did in
5-6).

So when looking at a 6.x-1.x (or 2.x) version of a module, you have no
way of knowing whether it's a straight port, or a rewrite, or
somewhere in-between.

It's a bit like computer games - when they release a Grand Theft Auto
game on a new platform, they don't call it GTA1 all over again ;).
That said, I'm undecided on what the ideal path ought to be with this.

Nat