Comment 1 for bug 1794601

Yeah, I think the tempurl work at https://github.com/openstack/swift/commit/5a4d3bdfc will be a really good guide. This is nice and self-contained; good first-contribution material. The main points we'd look for in a patch would be:

* new config option listed in etc/proxy-server.conf-sample,
* formpost's filter_factory parsing the new option, including it in the call to register_swift_info, and passing it on to the actual filter,
* formpost's filter growing some new parsing so it can differentiate between hash functions and validate any of the configured digests,
* updated docs in the formpost middleware's docstring, and
* unit tests to demonstrate the new functionality.

Might be an opportunity to pull some logic from common/middleware/tempurl.py out to common/utils.py and use it here too, but it's really optional.