class ActiveStorage::Downloader

def download(key, file)
  file.binmode
  service.download(key) { |chunk| file.write(chunk) }
  file.flush
  file.rewind
end

def initialize(service)
  @service = service
end

def open(key, checksum: nil, verify: true, name: "ActiveStorage-", tmpdir: nil)
  open_tempfile(name, tmpdir) do |file|
    download key, file
    verify_integrity_of(file, checksum: checksum) if verify
    yield file
  end
end

def open_tempfile(name, tmpdir = nil)
  file = Tempfile.open(name, tmpdir)
  begin
    yield file
  ensure
    file.close!
  end
end

def verify_integrity_of(file, checksum:)
  unless OpenSSL::Digest::MD5.file(file).base64digest == checksum
    raise ActiveStorage::IntegrityError
  end
end