pip cannot prepare more than one requirement for the same project

[pfmoore]

An InstallRequirement has a source_dir attribute, which is where the requirement will be built. The code here computes the name of that build directory, using the name of the requirement/project.

When preparing, the code confirms that the build directory isn't already in use here.

This causes a problem if pip ever needs to prepare more than one version of a particular project (foo-1.0 and foo-2.0, as both of these will use the same build directory). However, this is exactly what the new resolver needs to do in order to compute metadata for multiple candidates.

This is probably fixable by using randomised build directory names, or something similar. @pradyunsg @uranusjr - any thoughts on this?

[pfmoore]

Update: Aggressive caching of Candidate objects in the new resolver avoids this issue. I'm not 100% clear why, but it looks like this isn't a blocker for the new resolver.

[uranusjr]

I have a gut feeling this might come back and bit us somehow. But it’s not unfixable (e.g. maybe change the directory name to the link’s hash), and we’ll have a better idea how to deal with it when we have the models implemented. Let’s keep this in mind…