Don't load tmpdir and fileutils at require time

[tas50]

Description

lib/mixlib/shellout.rb has required tmpdir since forever, but nothing in the gem ever calls Dir.mktmpdir or Dir.tmpdir:

$ git grep -n 'mktmpdir\|Dir\.tmpdir' lib/
(no hits)

It's not free — tmpdir transitively pulls in fileutils and etc, which is where essentially all of the require-time allocation comes from.

fileutils itself is used, but in exactly one place: set_cgroup, which only runs when the :cgroup option is set and cgroup v2 is mounted. This moves that require to fork_subprocess, gated on cgroup being set.

Worth calling out why it goes in fork_subprocess and not in set_cgroup itself: set_cgroup runs inside the forked child. require takes a global lock, and if the parent forked while another thread held that lock, the child would block on it forever. Loading in the parent keeps it lazy without the fork-safety hazard.

This matters beyond this gem — it's paid by every consumer (chef-infra, ohai, test-kitchen, inspec) on every process start.

Benchmark

# bench_require.rb — cold process, measures just the require
lib = ARGV[0]
$LOAD_PATH.unshift(lib)
o1 = ObjectSpace.count_objects[:TOTAL]
f1 = $LOADED_FEATURES.size
s  = Process.clock_gettime(Process::CLOCK_MONOTONIC)
require "mixlib/shellout"
e  = Process.clock_gettime(Process::CLOCK_MONOTONIC)
printf("%.3f ms  +%d objects  +%d files\n",
       (e - s) * 1000,
       ObjectSpace.count_objects[:TOTAL] - o1,
       $LOADED_FEATURES.size - f1)

30 cold processes each, ruby 4.0.6, arm64-darwin:

	median	min	max	heap objects	files loaded
before	12.96 ms	12.63 ms	14.36 ms	+2148	+7
after	5.39 ms	4.91 ms	6.58 ms	+0	+5

58% faster, and it no longer allocates at require time.

Verification

• bundle exec rspec — 147 examples, 0 failures (unchanged from main)
• bundle exec cookstyle --chefstyle -c .rubocop.yml — no offenses
• Confirmed neither library leaks in after a require, and that a normal run_command works without them:

  $ ruby -Ilib -e 'require "mixlib/shellout"
      raise if $LOADED_FEATURES.any? { |f| f.end_with?("tmpdir.rb", "fileutils.rb") }
      c = Mixlib::ShellOut.new("/bin/echo", "hello"); c.run_command; c.error!'
  

• Confirmed the :cgroup path still loads FileUtils, and that it loads in the parent before the fork.

spec_helper.rb already requires tmpdir explicitly, so the specs never relied on the library leaking it.