Storage#write_file is not thread-safe.

The bug comes out when multiple threads are trying to write data into one index file.
When such situation occurs, one thread moves tmp_file to new_file by FileUtils.mv. At the same time second thread tries to move temporary file to new_file too, but temporary file not exists already, so method fails with Errno::ENOENT: No such file or directory.

You can reproduce it with concurrent creating indexed model instances, something like this:

threads = []
5.times do
  5.times do

    threads << Thread.new { IndexedModel.create! }

  end
end

threads.each &:join

One possible fix is to create temporary file with uniq name. It is possible to create it via Tempfile class or manually, as in pull request.
With this fix, FileUtils.mv will move only temporary file from the current thread.