Summary

The apt-key definition provides dir, name and filename.

In the manifest/keyring.pp (apt::keyring) the destination file on the filesystem is generated as $file = "${dir}/${filename}".

In the manifest/source.pp (apt::source) the destination file is generated with the follwing code

$_list_keyring = if $_key['dir'] and $_key['filename'] {
  "${_key['dir']}${_key['filename']}"
} elsif $_key['filename'] {
  "/etc/apt/keyrings/${_key['filename']}"
} elsif $_key['dir'] {
  "${_key['dir']}${_key['name']}"
} else {
  "/etc/apt/keyrings/${_key['name']}"
}

If the $_list_keyring is generated from dir and filename or name, then there is no / separator between dir and filename. The key::dir: of the apt::source has to contains the / otherwise incorrect filename will be generated. This PR just create final full filename by joining directory name with filename using directory separator slash /.