[rubygems/rubygems] Clear `YAML` constant if it was undefined previously

https://github.com/rubygems/rubygems/commit/31d0311258

lib/rubygems/specification.rb

@@ -1301,6 +1301,8 @@ class Gem::Specification < Gem::BasicSpecification
   def self._load(str)
     Gem.load_yaml
 
+    yaml_set = false
+
     array = begin
       Marshal.load str
     rescue ArgumentError => e
@@ -1313,7 +1315,10 @@ class Gem::Specification < Gem::BasicSpecification
       message = e.message
       raise unless message.include?("YAML::")
 
-      Object.const_set "YAML", Psych unless Object.const_defined?(:YAML)
+      unless Object.const_defined?(:YAML)
+        Object.const_set "YAML", Psych
+        yaml_set = true
+      end
 
       if message.include?("YAML::Syck::")
         YAML.const_set "Syck", YAML unless YAML.const_defined?(:Syck)
@@ -1324,6 +1329,8 @@ class Gem::Specification < Gem::BasicSpecification
       end
 
       retry
+    ensure
+      Object.__send__(:remove_const, "YAML") if yaml_set
     end
 
     spec = Gem::Specification.new

test/rubygems/test_gem_specification.rb

@@ -1071,19 +1071,27 @@ dependencies: []
   end
 
   def test_handles_private_null_type
+    yaml_defined = Object.const_defined?("YAML")
+
     path = File.expand_path "data/pry-0.4.7.gemspec.rz", __dir__
 
     data = Marshal.load Gem::Util.inflate(Gem.read_binary(path))
 
     assert_instance_of Gem::Specification, data
+
+    assert_equal(yaml_defined, Object.const_defined?("YAML"))
   end
 
   def test_handles_dependencies_with_syck_requirements_bug
+    yaml_defined = Object.const_defined?("YAML")
+
     path = File.expand_path "data/excon-0.7.7.gemspec.rz", __dir__
 
     data = Marshal.load Gem::Util.inflate(Gem.read_binary(path))
 
     assert_instance_of Gem::Specification, data
+
+    assert_equal(yaml_defined, Object.const_defined?("YAML"))
   end
 
   def test_initialize