Gem.open_file_with_lock removed its lock file after releasing the lock.
Since flock locks the file itself rather than its path, a process that
was waiting on the old file could acquire it after it had been removed,
while a newly arriving process created a fresh lock file and locked it
immediately, so both ran at the same time.
Remove the lock file while still holding the lock, and after acquiring
it, check that the locked file is still the one at the lock path,
retrying otherwise.
https://github.com/ruby/rubygems/commit/9a93c235e7
Richard Janita
3541b9e034