No output while ResolveLib find matches for root requirements

[McSinyx]

Environment

• pip version: 92f9e77 (almost current master)
• Python version: irrelevant
• OS: irrelevant

Description

Do you think the same reasoning could explain a catastrophic degradation of pip wheel -r requirements.txt --no-deps --use-feature=2020-resolver on python 2, with all wheels in cache ? I'm examining a case right now that seems to take forever (> 3 hours) on python 2, while similar cases work fine on python 3.

Originally posted by @sbidoul in #8664 (comment)

Currently, when a series of requirement specifiers is given to pip, ResolveLib adds each of them and call PipProvider.find_matches upon them. Only until all of the requirements are added, pip starts print the progress (e.g. Collecting …), so it's unclear what is happening during the visual hang, especially if the requirement list is long (cc GH-9065). This shouldn't be a performance issue if the index pages are cached, so I think this has more to do with user experience.

Expected behavior

pip says something about what it's doing, e.g. like when PIP_RESOLVER_DEBUG=1.

How to Reproduce

0. Get a long requirements.txt (if I have your permission, @sbidoul, I'll upload yours here, or you can upload them yourselves with potentially private information stripped away)
1. Run pip wheel -r requirements.txt and notice how quickly it gets to the progress output (and cancel the command)
2. Run pip wheel -r requirements.txt --use-feature=2020-resolver and compare
3. Run PIP_RESOLVER_DEBUG=1 pip wheel -r requirements.txt --use-feature=2020-resolver

As a side note, I haven't run it for very long for the Python 2 requirements set so probably the 3h hang is due to another bug. I'll keep looking into it and see if there's anything else.

[sbidoul]

Thanks to @xavfernandez 's #9078 this is indeed "merely" a UX issue now.

This shouldn't be a performance issue if the index pages are cached, so I think this has more to do with user experience.

Regarding caching of index pages, see also #9082.