[rubygems/rubygems] Make --build-root disable auto-user-install.

https://github.com/rubygems/rubygems/commit/6a06b0763f

lib/rubygems/installer.rb

@@ -675,14 +675,16 @@ class Gem::Installer
 
     @build_args = options[:build_args]
 
-    @gem_home = @install_dir
-    @gem_home ||= if options[:user_install]
-      Gem.user_dir
-    elsif !ENV.key?("GEM_HOME") && (File.exist?(Gem.dir) && !File.writable?(Gem.dir))
-      say "Defaulting to user installation because default installation directory (#{Gem.dir}) is not writable."
-      Gem.user_dir
-    else
-      Gem.dir
+    @gem_home = @install_dir || Gem.dir
+
+    # `--build-root` overrides `--user-install` and auto-user-install
+    if @build_root.nil? && @install_dir.nil?
+      if options[:user_install]
+        @gem_home = Gem.user_dir
+      elsif !ENV.key?("GEM_HOME") && (File.exist?(Gem.dir) && !File.writable?(Gem.dir))
+        say "Defaulting to user installation because default installation directory (#{Gem.dir}) is not writable."
+        @gem_home = Gem.user_dir
+      end
     end
 
     # If the user has asked for the gem to be installed in a directory that is

test/rubygems/test_gem_installer.rb

@@ -1285,6 +1285,26 @@ end
     assert_equal @spec, installer.install
   end
 
+  def test_install_build_root_when_gem_home_not_writable_does_not_fallback_to_user_install_inside_build_root
+    build_root = File.join(@tempdir, "build_root")
+
+    orig_gem_home = ENV.delete("GEM_HOME")
+
+    @gem = setup_base_gem
+
+    FileUtils.chmod "-w", @gemhome
+
+    installer = Gem::Installer.at @gem, :build_root => build_root
+
+    assert_equal @spec, installer.install
+
+    build_root_path = File.join(build_root, @gemhome.gsub(/^[a-zA-Z]:/, ""))
+    assert File.exist?(build_root_path), "gem not written to build_root"
+  ensure
+    FileUtils.chmod "+w", @gemhome
+    ENV["GEM_HOME"] = orig_gem_home
+  end
+
   def test_install_missing_dirs
     installer = setup_base_installer