/usr/lib/ is in ruby-thread-safe 0.3.5-3.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/
/usr/lib/ruby/vendor_ruby/
/usr/lib/ruby/vendor_ruby/thread_safe.rb
/usr/lib/ruby/vendor_ruby/thread_safe/
/usr/lib/ruby/vendor_ruby/thread_safe/cache.rb
/usr/lib/ruby/vendor_ruby/thread_safe/synchronized_cache_backend.rb
/usr/lib/ruby/vendor_ruby/thread_safe/non_concurrent_cache_backend.rb
/usr/lib/ruby/vendor_ruby/thread_safe/mri_cache_backend.rb
/usr/lib/ruby/vendor_ruby/thread_safe/version.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util/
/usr/lib/ruby/vendor_ruby/thread_safe/util/power_of_two_tuple.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util/striped64.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util/xor_shift_random.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util/atomic_reference.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util/volatile.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util/volatile_tuple.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util/adder.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util/cheap_lockable.rb
/usr/lib/ruby/vendor_ruby/thread_safe/atomic_reference_cache_backend.rb
/usr/lib/ruby/vendor_ruby/thread_safe/util.rb
/usr/lib/ruby/vendor_ruby/thread_safe/synchronized_delegator.rb