[gentoo-dev] restricting phases where enew{user,group} is allowed

[Mike Frysinger]

currently we blacklist certain phases (which is largely based on EAPI=0 and 
blocking src_*) for enew{user,group}.  moving forward, ferringb suggested we 
invert this into a whitelist of allowed phases.

afaict, the blacklisting + dev documentation has done a good job of 
restricting calls to three places: pkg_{setup,preinst,postinst}.  so inverting 
the logic should largely be safe.  on the off chance it isn't, i think letting 
the ebuild `die` and getting it fixed up via bug reports is acceptable (i 
grepped through the tree a bit and looked sane).

moving beyond that, i'd like to also ban pkg_postinst usage.  the trouble with 
using this phase is that `die` isn't fatal because the package has already 
been merged to $ROOT, so there's no going back.  and while logically i can see 
that people might prefer pkg_postinst vs pkg_preinst (no point in creating a 
user/group if the pkg isn't actually yet merged), for all real world usage, 
there's no need to delay it, and it makes the ebuilds a bit more robust as 
errors get caught before things get merged to $ROOT.  i believe the 
documentation has always recommended pkg_setup and pkg_preinst anyways.

moving even beyond that, i'd like to update the documentation to push people 
to prefer pkg_preinst.  if your package doesn't require the user/group to be 
available at compile/install time, then there's no need to use pkg_setup.  
this improves the use case of attempting to install a package from source, 
things failing for whatever reason, and then the pkg is never actually 
installed, but the user/group is left behind.

any feedback before i implement ?
-mike