When pants resolves the constraints file? Does it currently rely on the pip cache for things, or does it have its own system for knowing when things have changed? It seems to take a long time to resolve a lot of the time. We are using our own pipy mirror, which might not be responding with the correct Etag for the artifacts wondering if that could be the cause.

Yeah, it uses Pex and pip's cache, stored at the folder ~/.cache/pants/named_caches etag?

The http header that caches can use to show if anything has changed in the underlying object. Usually grabbed with a

HEAD

request to see if it's worth sending a get. We noticed our mirror wasn't sending out the those headers on requests while

pypi

does so thought it could be the issue.