Description of problem:
Ruby Gems defaults to using a version-specific directory for the gem home.  This allows multiple versions of Ruby to "just work" without any configuration.

On RHEL6:
vagrant@vagrant-smerrill:DEVELOPMENT:~> gem env home
/usr/lib/ruby/gems/1.8

On OSX 10.11:
~ smerrill $ gem env home
/Library/Ruby/Gems/2.0.0

On Ubuntu:
skippy@three:~$ gem env home
/var/lib/gems/1.9.1

But using SCL Ruby versions, the `gem` command appears to default to ~/.gem/ruby/.  This means that using multiple SCL Ruby versions on a RHEL box will result in gems being co-located in the same directory.  For any Ruby-only gems, this likely doesn't matter, but for any gems with native extensions it means that they may conflict with one another.


First, enable an SCL Ruby and install a gem:
vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable ruby200 -- gem env home
/home/vagrant/.gem/ruby
vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable ruby200 -- gem install example
Fetching: example-1.0.2.gem (100%)
Successfully installed example-1.0.2
Parsing documentation for example-1.0.2
Installing ri documentation for example-1.0.2
Done installing documentation for example after 0 seconds
1 gem installed

Now enable a different SCL Ruby and examine whether that gem is available:
vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable rh-ruby22 -- gem list example -d

*** LOCAL GEMS ***

example (1.0.2)
    Author: Curtis Schofield
    Homepage: http://github.com/robotarmy/example
    License: GPLv3
    Installed at: /home/vagrant/.gem/ruby

    This is Example


Note the "installed at" points to ~/.gem/ruby.



Now build a gem that builds a native extension. This means the gem needs the appropriate SCL ruby-devel package, and will build against that:
vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable ruby200 -- gem install rmagick
Fetching: rmagick-2.15.4.gem (100%)
Building native extensions.  This could take a while...
Successfully installed rmagick-2.15.4
Parsing documentation for rmagick-2.15.4
Installing ri documentation for rmagick-2.15.4
Done installing documentation for rmagick after 8 seconds
1 gem installed
vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable ruby200 -- gem list rmagick -d

*** LOCAL GEMS ***

rmagick (2.15.4)
    Authors: Tim Hunter, Omer Bar-or, Benjamin Thomas, Moncef Maiza
    Homepage: https://github.com/rmagick/rmagick
    License: MIT
    Installed at: /home/vagrant/.gem/ruby

    Ruby binding to ImageMagick


Confirm that the native extensions were built:
vagrant@vagrant-smerrill:DEVELOPMENT:~> ls /home/vagrant/.gem/ruby/gems/rmagick-2.15.4/ext/RMagick/
extconf.h   mkmf.log     rmagick.h  rmdraw.o  rmfill.c   rmilist.o  rminfo.c  rmmain.o     rmpixel.c   rmstruct.o
extconf.rb  RMagick2.so  rmagick.o  rmenum.c  rmfill.o   rmimage.c  rminfo.o  rmmontage.c  rmpixel.o   rmutil.c
Makefile    rmagick.c    rmdraw.c   rmenum.o  rmilist.c  rmimage.o  rmmain.c  rmmontage.o  rmstruct.c  rmutil.o

Confirm that the rmagick gem works with SCL Ruby 2.0:
vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable ruby200 -- irb
irb(main):001:0> require 'rmagick'
=> true



Now enable SCL Ruby 2.2 and try to use the RMagick gem:
vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable rh-ruby22 -- gem list rmagick -d

*** LOCAL GEMS ***

rmagick (2.15.4)
    Authors: Tim Hunter, Omer Bar-or, Benjamin Thomas, Moncef Maiza
    Homepage: https://github.com/rmagick/rmagick
    License: MIT
    Installed at: /home/vagrant/.gem/ruby

    Ruby binding to ImageMagick


vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable rh-ruby22 -- irb
irb(main):001:0> require 'rmagick'
LoadError: libruby.so.2.0: cannot open shared object file: No such file or directory - /home/vagrant/.gem/ruby/gems/rmagick-2.15.4/lib/RMagick2.so
	from /opt/rh/rh-ruby22/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:128:in `require'
	from /opt/rh/rh-ruby22/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:128:in `rescue in require'
	from /opt/rh/rh-ruby22/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:39:in `require'
	from /home/vagrant/.gem/ruby/gems/rmagick-2.15.4/lib/rmagick_internal.rb:12:in `<top (required)>'
	from /opt/rh/rh-ruby22/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:54:in `require'
	from /opt/rh/rh-ruby22/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:54:in `require'
	from /home/vagrant/.gem/ruby/gems/rmagick-2.15.4/lib/rmagick.rb:1:in `<top (required)>'
	from /opt/rh/rh-ruby22/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:128:in `require'
	from /opt/rh/rh-ruby22/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:128:in `rescue in require'
	from /opt/rh/rh-ruby22/root/usr/share/rubygems/rubygems/core_ext/kernel_require.rb:39:in `require'
	from (irb):1
	from /opt/rh/rh-ruby22/root/usr/bin/irb:11:in `<main>'





Version-Release number of selected component (if applicable):
RHEL6 and RHEL7
SCL ruby200-rubygems
SCL rh-ruby22-rubygems


How reproducible:
Always.


Steps to Reproduce:
1. Install an SCL Ruby and rubygems packages
2. Execute `scl enable <ruby> -- gem env home
3. Examine the output


Actual results:

vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable ruby200 -- gem env home
/home/vagrant/.gem/ruby
vagrant@vagrant-smerrill:DEVELOPMENT:~> scl enable rh-ruby22 -- gem env home
/home/vagrant/.gem/ruby


Expected results:

I would expect SCL Ruby `gem` commands to default to some directory that contains the name of the Ruby version.

For example, SCL Ruby 200 would use ~/.ruby/gems/2.0/ and SCL Ruby 2.2 would use ~/.ruby/gems/2.2/

Additional info:
This may be a problem with Ruby 2+ in base RHEL, trickling into SCL builds of Ruby.  On a CentOS 7.1 system with no SCL Ruby installed:

[vagrant@c1-smerrill ~]$ gem env home
/home/vagrant/.gem/ruby