Gem.open_file_with_flock

# File tmp/rubies/ruby-4.0.0/lib/rubygems.rb, line 867
def self.open_file_with_flock(path, &block)
  # read-write mode is used rather than read-only in order to support NFS
  mode = IO::RDWR | IO::APPEND | IO::CREAT | IO::BINARY
  mode |= IO::SHARE_DELETE if IO.const_defined?(:SHARE_DELETE)

  File.open(path, mode) do |io|
    begin
      # Try to get a lock without blocking.
      # If we do, the file is locked.
      # Otherwise, explain why we're waiting and get a lock, but block this time.
      if io.flock(File::LOCK_EX | File::LOCK_NB) != 0
        warn "Waiting for another process to let go of lock: #{path}"
        io.flock(File::LOCK_EX)
      end
      io.puts(Process.pid)
    rescue Errno::ENOSYS, Errno::ENOTSUP
    end
    yield io
  end
end