Make XDG_DATA_DIRS obey the basedir spec

[cmrd-senya]

The "XDG Base Directory Specification" defines that when XDG_DATA_DIRS is unset the value of /usr/local/share/:/usr/share/ should be used:

If $XDG_DATA_DIRS is either not set or empty, a value equal to /usr/local/share/:/usr/share/ should be used.

::RbConfig::CONFIG['datadir'] call in some environments (e.g. RVM installed Ruby) returns something different. In my case it was "/home/user/.rvm/rubies/ruby-2.6.5/share". This violates the specification. Also due to this violation one specific software project didn't work correctly for me out-of-the box.

I think that most of the GNU/Linux installation actually define XDG_DATA_DIRS explicitly so this code doesn't use the fallback in most of the cases and that's why this went unnoticed. But in the cases when XDG_DATA_DIRS this could lead to bugs.

[peterhoeg]

You probably want to have a look at https://github.com/bkuhlmann/xdg which is the active fork.

[cmrd-senya]

@peterhoeg yes, I've already figured that out, thanks!
Should this repository be archived with a notice about https://github.com/bkuhlmann/xdg being the new place?

[peterhoeg]

Ideally yes, but @trans hasn't been very active here in a long time.

[cmrd-senya]

I've opened an issue here: #9

I think it will make it more visible and obvious to the occasional visitors where to look for the up-to-date version.