Close stdin immediately when using popen2e

It's good hygiene to close the stdin pipe as soon as you are done writing to it.

This can happen for example if "ruby extconf.rb" spawns another
process (for example "cargo build", which may spawn arbitrary commands
to fetch credentials) and any of those subprocesses attempt to read
STDIN until it is closed.

We can close it as soon as it's created since we aren't writing to it at all.

Author: rwstauner

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?