Add Gem.default_install

Most people don't install gems in the default directory, but in the
user's directory (--user-install), this can be configured by
distributions using /etc/gemrc, but that doesn't work for bundler since
it doesn't read gem configurations.

The default installation directory should be Gem.user_dir, not
Gem.default_dir, but changing this invariably would break backwards
compatibility.

Instead we can add another default directory which is where gems will be
installed to by default.

Gem.default_dir is where the system gems are installed to,
Gem.default_install is where the gems installed by the user are
installed to.

The method Gem.default_install can be overridden by distributions to
Gem.user_dir, therefore making all installations user installations by
default (essentially the same as --user-install).

This way both bundler and gem will install gems in the same directory by
default without users having to manually set GEM_HOME to volatile
locations such as $HOME/.local/share/gem/ruby/3.0.0.

All distributions need to do is turn this new behavior on.

If a user has sudo privileges, he/she can do --no-user-install to install
gems in the system directory (Gem.default_dir), which is the current
default behavior.

The current behavior remains unaffected: gems are still installed to
Gem.default_dir by default.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

lib/rubygems/defaults.rb

@@ -8,6 +8,13 @@ module Gem
   @pre_uninstall_hooks  ||= []
   @pre_install_hooks    ||= []
 
+  ##
+  # Determines the default install directory
+
+  def self.default_install
+    @default_install ||= Gem.default_dir
+  end
+
   ##
   # An Array of the default sources that come with RubyGems
 

lib/rubygems/path_support.rb

@@ -23,7 +23,7 @@ class Gem::PathSupport
   # hashtable, or defaults to ENV, the system environment.
   #
   def initialize(env)
-    @home = env["GEM_HOME"] || Gem.default_dir
+    @home = env["GEM_HOME"] || Gem.default_install
 
     if File::ALT_SEPARATOR
       @home = @home.gsub(File::ALT_SEPARATOR, File::SEPARATOR)

test/rubygems/helper.rb

@@ -378,6 +378,7 @@ def setup
       @orig_default_gem_home = RbConfig::CONFIG["default_gem_home"]
       RbConfig::CONFIG["default_gem_home"] = @gemhome
     else
+      Gem.instance_variable_set(:@default_install, @gemhome)
       Gem.instance_variable_set(:@default_dir, @gemhome)
     end
 
@@ -469,6 +470,7 @@ def teardown
     if Gem.java_platform?
       RbConfig::CONFIG["default_gem_home"] = @orig_default_gem_home
     else
+      Gem.instance_variable_set :@default_install, nil
       Gem.instance_variable_set :@default_dir, nil
     end
 

test/rubygems/test_rubygems.rb

@@ -49,6 +49,22 @@ def self.default_dir
     end
   end
 
+  def test_operating_system_customizing_default_install
+    path = util_install_operating_system_rb <<-RUBY
+      module Gem
+        def self.default_install
+          Gem.user_dir
+        end
+      end
+    RUBY
+
+    assert system(
+      *ruby_with_rubygems_and_fake_operating_system_in_load_path(path),
+      "-e",
+      "raise unless Gem.dir == Gem.user_dir"
+    )
+  end
+
   private
 
   def util_install_operating_system_rb(content)