[rubygems/rubygems] Fix require issue with file extension priority

If `require "a"` is run when two folders have been specified in the -I
option including a "a.rb" file and a "a.so" file respectively, the ruby
spec says that the ".rb" file should always be preferred. However, the
logic we added in https://github.com/rubygems/rubygems/commit/6b81076d9
to make the -I option always beat default gems does not respect this
spec, creating a difference from the original ruby-core's require.

[the ruby spec says]: d80a6e2b22/core/kernel/shared/require.rb (L234-L246)

https://github.com/rubygems/rubygems/commit/b3944384f4

lib/rubygems/core_ext/kernel_require.rb

@@ -43,18 +43,18 @@ module Kernel
     # https://github.com/rubygems/rubygems/pull/1868
     resolved_path = begin
       rp = nil
-      $LOAD_PATH[0...Gem.load_path_insert_index || -1].each do |lp|
-        safe_lp = lp.dup.tap(&Gem::UNTAINT)
-        begin
-          if File.symlink? safe_lp # for backward compatibility
-            next
+      Gem.suffixes.each do |s|
+        $LOAD_PATH[0...Gem.load_path_insert_index || -1].each do |lp|
+          safe_lp = lp.dup.tap(&Gem::UNTAINT)
+          begin
+            if File.symlink? safe_lp # for backward compatibility
+              next
+            end
+          rescue SecurityError
+            RUBYGEMS_ACTIVATION_MONITOR.exit
+            raise
           end
-        rescue SecurityError
-          RUBYGEMS_ACTIVATION_MONITOR.exit
-          raise
-        end
 
-        Gem.suffixes.each do |s|
           full_path = File.expand_path(File.join(safe_lp, "#{path}#{s}"))
           if File.file?(full_path)
             rp = full_path
@@ -67,12 +67,8 @@ module Kernel
     end
 
     if resolved_path
-      begin
-        RUBYGEMS_ACTIVATION_MONITOR.exit
-        return gem_original_require(resolved_path)
-      rescue LoadError
-        RUBYGEMS_ACTIVATION_MONITOR.enter
-      end
+      RUBYGEMS_ACTIVATION_MONITOR.exit
+      return gem_original_require(resolved_path)
     end
 
     if spec = Gem.find_unresolved_default_spec(path)

test/rubygems/test_require.rb

@@ -120,6 +120,24 @@ class TestGemRequire < Gem::TestCase
     Object.send :remove_const, :HELLO if Object.const_defined? :HELLO
   end
 
+  def test_dash_i_respects_default_library_extension_priority
+    skip "extensions don't quite work on jruby" if Gem.java_platform?
+
+    dash_i_ext_arg = util_install_extension_file('a')
+    dash_i_lib_arg = util_install_ruby_file('a')
+
+    lp = $LOAD_PATH.dup
+
+    begin
+      $LOAD_PATH.unshift dash_i_lib_arg
+      $LOAD_PATH.unshift dash_i_ext_arg
+      assert_require 'a'
+      assert_match(/a\.rb$/, $LOADED_FEATURES.last)
+    ensure
+      $LOAD_PATH.replace lp
+    end
+  end
+
   def test_concurrent_require
     Object.const_set :FILE_ENTERED_LATCH, Latch.new(2)
     Object.const_set :FILE_EXIT_LATCH, Latch.new(1)
@@ -541,4 +559,48 @@ class TestGemRequire < Gem::TestCase
     $VERBOSE = old_verbose
   end
 
+  def util_install_extension_file(name)
+    spec = quick_gem name
+    util_build_gem spec
+
+    spec.extensions << "extconf.rb"
+    write_file File.join(@tempdir, "extconf.rb") do |io|
+      io.write <<-RUBY
+        require "mkmf"
+        create_makefile("#{name}")
+      RUBY
+    end
+
+    write_file File.join(@tempdir, "#{name}.c") do |io|
+      io.write <<-C
+        #include <ruby.h>
+        void Init_#{name}() { }
+      C
+    end
+
+    spec.files += ["extconf.rb", "#{name}.c"]
+
+    so = File.join(spec.gem_dir, "#{name}.#{RbConfig::CONFIG["DLEXT"]}")
+    refute_path_exists so
+
+    path = Gem::Package.build spec
+    installer = Gem::Installer.at path
+    installer.install
+    assert_path_exists so
+
+    spec.gem_dir
+  end
+
+  def util_install_ruby_file(name)
+    dir_lib = Dir.mktmpdir("test_require_lib", @tempdir)
+    dash_i_lib_arg = File.join dir_lib
+
+    a_rb = File.join dash_i_lib_arg, "#{name}.rb"
+
+    FileUtils.mkdir_p File.dirname a_rb
+    File.open(a_rb, 'w') { |f| f.write "# #{name}.rb" }
+
+    dash_i_lib_arg
+  end
+
 end