Gem::RemoteFetcher#atomic_copy

# File tmp/rubies/ruby-4.1.0-preview1/lib/rubygems/remote_fetcher.rb, line 370
def atomic_copy(source_path, destination_path)
  File.open(source_path, "rb") do |source|
    # FileUtils.cp passed the source mode to File.open, so it only reached a
    # file being created and the umask still applied to it. The writer
    # already carries over the mode of a file it replaces.
    mode = source.stat.mode & 0o777 & ~File.umask
    replacing = File.exist?(destination_path)

    Gem::AtomicFileWriter.open(destination_path) do |io|
      IO.copy_stream(source, io)

      begin
        io.chmod(mode) unless replacing
      rescue Errno::EPERM, Errno::EACCES
        # the filesystem does not carry permissions
      end
    end
  end
end