Resolver spends too much time calculating "weights" to parents

Description

When running something like pip install -c requirements.txt <package_name> sometimes pip starts checking unnecessary versions.

I know that version checking is unnecessary because requirements.txt contains all the packages with exact versions.

Everything works as expected with legacy resolver.

Expected behaviour

Since all of the versions are already specified, just get them.

pip version

21.2.4

Python version

3.8.8

OS

Ubuntu

How to Reproduce

Unfortunately, I can’t provide a reproducible example since it only happens with a package on internal pip index.

The offending package does have the version specified in requirements.txt and a different version is installed when the command is run.

Output

It says something like:

INFO: pip is looking at multiple versions of kiwisolver to determine which version is compatible with other requirements. This could take a while.

But I already have kiwisolver==1.3.1 in my requirements.txt.

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[pradyunsg]

It’s not the dependency resolution that is slow here – I’ve demonstrated that above. As @uranusjr noted, the problem in this case is the logic that’s computing the “order to install packages” (get_installation_order -> get_topological_weights). That logic is being ridiculously slow, likely due to the very edge-heavy/dense dependency graph that this situation has; and possibly due to a state management bug in that logic.

[uranusjr]

From the report, resolution time isn’t the actual issue though, it’s after the resolution where we sort packages for installation.