[rubygems/rubygems] Fixed false positive SymlinkError in symbolic link directory

https://github.com/rubygems/rubygems/commit/58173ff2ea

lib/rubygems/package.rb

@@ -413,6 +413,8 @@ EOM
   # extracted.
 
   def extract_tar_gz(io, destination_dir, pattern = "*") # :nodoc:
+    real_destination_dir = File.realpath(destination_dir)
+
     directories = []
     symlinks = []
 
@@ -428,7 +430,7 @@ EOM
           real_destination = link_target.start_with?("/") ? link_target : File.expand_path(link_target, File.dirname(destination))
 
           raise Gem::Package::SymlinkError.new(full_name, real_destination, destination_dir) unless
-            normalize_path(real_destination).start_with? normalize_path(destination_dir + "/")
+            normalize_path(real_destination).start_with? normalize_path(real_destination_dir + "/")
 
           symlinks << [full_name, link_target, destination, real_destination]
         end

test/rubygems/test_gem_package.rb

@@ -574,6 +574,32 @@ class TestGemPackage < Gem::Package::TarTestCase
                  File.read(extracted)
   end
 
+  def test_extract_symlink_into_symlink_dir
+    package = Gem::Package.new @gem
+    tgz_io = util_tar_gz do |tar|
+      tar.mkdir       "lib", 0o755
+      tar.add_symlink "lib/link", "./inside.rb", 0o644
+      tar.add_file    "lib/inside.rb", 0o644 do |io|
+        io.write "hi"
+      end
+    end
+
+    destination_subdir = File.join @destination, "subdir"
+    FileUtils.mkdir_p destination_subdir
+
+    destination_linkdir = File.join @destination, "linkdir"
+    File.symlink(destination_subdir, destination_linkdir)
+
+    package.extract_tar_gz tgz_io, destination_linkdir
+
+    extracted = File.join destination_subdir, "lib/link"
+    assert_path_exist extracted
+    assert_equal "./inside.rb",
+                 File.readlink(extracted)
+    assert_equal "hi",
+                 File.read(extracted)
+  end
+
   def test_extract_tar_gz_symlink_broken_relative_path
     package = Gem::Package.new @gem
     package.verify