dependency resolver doesn't work in 21.2.4 but works in 21.1.3

[bholland]

Description

I have a complex pip installation with a bunch of packages. I use pip to ensure that all of the packages have correct versions so I install everything in one line. This worked fine in 21.1.3 but gives me errors when I use 21.2.4. I can elaborate more if this isn't a know issue. I am okay emailing the pip command to a developer for testing but I don't think I can post it here.

Expected behavior

No response

pip version

21.2.4

Python version

3.9.6

OS

Ubuntu 20.04

How to Reproduce

1. run a complex pip command
2. See results like:
   Ctrl + C to do so. To improve how pip performs, tell us what happened here: https://pip.pypa.io/surveys/backtracking
   Using cached websocket_client-1.1.0-py2.py3-none-any.whl (68 kB)
   Downloading websocket_client-1.0.1-py2.py3-none-any.whl (68 kB)
   Downloading websocket_client-1.0.0-py2.py3-none-any.whl (68 kB)
   Downloading websocket_client-0.59.0-py2.py3-none-any.whl (67 kB)
   Downloading websocket_client-0.58.0-py2.py3-none-any.whl (61 kB)
   INFO: pip is looking at multiple versions of websocket-client to determine which version is compatible with other requirements. This could take a while.
   Using cached websocket_client-0.57.0-py2.py3-none-any.whl (200 kB)
   Using cached websocket_client-0.56.0-py2.py3-none-any.whl (200 kB)
   Downloading websocket_client-0.55.0-py2.py3-none-any.whl (200 kB)
   Downloading websocket_client-0.54.0-py2.py3-none-any.whl (200 kB)
   Downloading websocket_client-0.53.0-py2.py3-none-any.whl (198 kB)
   INFO: This is taking longer than usual. You might need to provide the dependency resolver with stricter constraints to reduce runtime. If you want to abort this run, you can press Ctrl + C to do so. To improve how pip performs, tell us what happened here: https://pip.pypa.io/surveys/backtracking
   Downloading websocket_client-0.52.0-py2.py3-none-any.whl (198 kB)

In the above example, websocket-client==1.2.1 satisfies my virtual environment. I know this because this is the version that gets installed using pip 21.1.3.

Output

No response

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[uranusjr]

Merging into #10201.

[notatallshaw]

FYI "run a complex pip command" isn't unfortunately a reproducible step. I have been experimenting with different ways pip could approach this problem but I have shockingly little reproducible examples. If you could provide your specific case I will 1) Add it to my tests, and 2) Potentially advise how you can fix your specific issue.