cleanup isn't concurrent-safe

[ptarjan]

Expected behavior

I can run many rubocop processes at the same time and they all finish.

Actual behavior

If two rubocop processes finish running at the same time, one of them can clean up the cache directory, and the other throws with:

No such file or directory @ rb_file_s_mtime - /cache/rubocop_cache/99c3ac6602c51029823c15c9106045a23955950d/rubocop_cache/02639da205e170093bd901c9c3d16645/_/00ab6b745135000518d35d28f2606f01
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/lib/rubocop/result_cache.rb:35:in `mtime'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/lib/rubocop/result_cache.rb:35:in `block in cleanup'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/lib/rubocop/result_cache.rb:35:in `each'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/lib/rubocop/result_cache.rb:35:in `sort_by'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/lib/rubocop/result_cache.rb:35:in `cleanup'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/lib/rubocop/runner.rb:67:in `inspect_files'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/lib/rubocop/runner.rb:35:in `run'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/lib/rubocop/cli.rb:30:in `run'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/bin/rubocop:14:in `block in <top (required)>'
21:15:35 /usr/lib/rbenv/versions/2.1/lib/ruby/2.1.0/benchmark.rb:294:in `realtime'
21:15:35 /src/vendor/bundle/ruby/2.1.0/gems/rubocop-0.40.0/bin/rubocop:13:in `<top (required)>'
21:15:35 /src/vendor/bundle/ruby/2.1.0/bin/rubocop:23:in `load'
21:15:35 /src/vendor/bundle/ruby/2.1.0/bin/rubocop:23:in `<main>'

Steps to reproduce the problem

$ touch /tmp/a.rb
$ for i in `seq 200`; do rubocop /tmp/a.rb & done

One of them should error out int result_cache.rb:35.

RuboCop version

$ rubocop -V
0.40.0 (using Parser 2.3.1.0, running on ruby 2.1.2 x86_64-darwin15.0)

[bbatsov]

Guess we can write some marker file or something to prevent this from happening. Other ideas are welcome as well.

[jonas054]

Another way would be to ignore Errno::ENOENT exceptions we get on the lines

        sorted = files.sort_by { |path| File.mtime(path) }
        remove_files(sorted, dirs, remove_count, verbose)

We know the files were there an instant ago, so they were most likely removed by another rubocop process, which is something we can accept.