Support GEM_HOME, add limited support for user-installed gems

[mislav]

The rehash operation will now discover and create shims for executables in additional locations:

• ~/.gem/ruby/<version>/bin/*
• $GEM_HOME/bin

The rbenv which (and thus rbenv exec) command will also search these locations when looking up a command. This enables shims to dispatch calls to executables added by GEM_HOME=/my/path gem install or gem install --user-install.

Note that this support is limited:

• It will only work with C Ruby, as it's difficult to guess the ~/.gem/<engine>/<version> directory for other Rubies without actually invoking the Ruby interpreter, which is slow;
• It will only work for RBENV_VERSION values in the format X.Y.Z and not "system".

Fixes #1232, fixes #1382
Closes #639
Supersedes https://github.com/mislav/rbenv-user-gems

[hsbt]

👍

[konsolebox]

Any thoughts on just moving the default user directory to another directory instead, like ~/.rbenv/user? Using ~/.gem may conflict with gems installed using system.

[konsolebox]

With the following code I am able to define a different user directory, and in this case the user directory becomes the default directory itself, but having this new feature that discovers ~/.gem will make avoiding use of ~/.gem pointless.

/etc/rbenv.d/install/custom_defaults/operating_system.rb:

# frozen_string_literal: true

module Gem
  module OperatingSystemDefaults
    def user_dir
      default_dir
    end

    def default_path
      path = []
      path << default_dir
      path << vendor_dir if vendor_dir && File.directory?(vendor_dir)
      path
    end
  end

  self.singleton_class.prepend OperatingSystemDefaults
end

/etc/rbenv.d/install/custom_defaults.bash:

function _after_install_hook {
	local defaults_rb_file defaults_dir_path
	[[ ${PREFIX} && -d ${PREFIX} ]] || return
	defaults_rb_file=$(find "${PREFIX}" -path '*/rubygems/defaults.rb')
	[[ ${defaults_rb_file} == */rubygems/defaults.rb ]] || return
	defaults_dir_path=${defaults_rb_file%.rb}
	mkdir -p "${defaults_dir_path}" || return
	cp "${BASH_SOURCE%.*}/operating_system.rb" "${defaults_dir_path}/"
}

after_install _after_install_hook

I hope there's a way to disable the functionality at least if it will be kept.

[mislav]

Any thoughts on just moving the default user directory to another directory instead, like ~/.rbenv/user? Using ~/.gem may conflict with gems installed using system.

How does the conflict happen in a real-world scenario, can you elaborate a bit more please? Also, feel free to open an issue if this change concerns you.