Close stdin immediately when using popen2e

lib/rubygems/ext/builder.rb

@@ -102,7 +102,8 @@ def self.run(command, results, command_name = nil, dir = Dir.pwd, env = {})
       # Set $SOURCE_DATE_EPOCH for the subprocess.
       build_env = { "SOURCE_DATE_EPOCH" => Gem.source_date_epoch_string }.merge(env)
       output, status = begin
-                         Open3.popen2e(build_env, *command, chdir: dir) do |_stdin, stdouterr, wait_thread|
+                         Open3.popen2e(build_env, *command, chdir: dir) do |stdin, stdouterr, wait_thread|
+                           stdin.close
                            output = String.new
                            while line = stdouterr.gets
                              output << line

test/rubygems/test_gem_ext_builder.rb

@@ -106,6 +106,22 @@ def test_custom_make_with_options
     assert_match(/install: OK/, results)
   end
 
+  def test_class_run_closes_stdin
+    results = []
+    check_stdin_script = <<~'RUBY'
+      if IO.select([STDIN], nil, nil, 1)
+        puts "STDIN: #{STDIN.read.inspect}"
+      else
+        puts "NOT_READY"
+      end
+    RUBY
+
+    Gem::Ext::Builder.run([Gem.ruby, "-e", check_stdin_script], results)
+
+    command_output = results.last
+    assert_equal "STDIN: \"\"\n", command_output
+  end
+
   def test_build_extensions
     pend "terminates on mswin" if vc_windows? && ruby_repo?