The Fetcher type was designed to return a 'str'.
That wasn't a good idea because it implies that every fetched file must
be loaded into memory completely. On certain large yum repos,
decompressed primary XML can be hundreds of MB, and it's not appropriate
to require loading that all into memory at once.
Make it support a file-like object (stream of bytes). Since the SAX
XML parser supports reading from a stream, this makes it possible to
avoid loading everything into memory at once.
A test of repo-autoindex CLI against
/content/dist/rhel/server/7/7Server/x86_64/os showed major
improvement:
- before: ~1200MiB
- after: ~80MiB
Note that achieving the full improvement requires any downstream users
of the library (e.g. exodus-gw) to update their Fetcher implementation
as well, to stop returning a 'str'.
