[INTERNAL] BuildEnvironment.install_requirements cannot be called twice

[pfmoore]

Environment

• pip version: 18.1

Description
The internal API BuildEnvironment.install_requirements does not work correctly when called twice. Because it uses --ignore-installed to allow installing requirements into the build environment that are already present in the system environment, it's possible for a second call to the function to install a requirement that is already present (possibly a different version) resulting in a corrupted build environment.

Expected behavior
Multiple calls to install_requirements should work the same way as multiple pip install invocations.

How to Reproduce
See the discussion in #5743 (in particular the comments starting here).

As this is an internal API, it is not possible to see an issue using pip as a command line app. However, the changes required for PEP 517 need this capability, so the problem has the potential to cause issues when PEP 517 support lands (specifically, if pyproject.toml and the build backend's get_requires_for_build_wheel hook return different versions of the same project).

[pradyunsg]

#5286 (comment)

Given #5841, I'm not sure if that option is still needed.

[benoit-pierre]

Why not?

[pfmoore]

Surely there could be other packages in the sys.executable site-packages? If the site-packages had setuptools installed, for example, then doing env.install_requirements(finder, ['setuptools'], 'xxx') wouldn't install setuptools without --ignore-installed.

[benoit-pierre]

Indeed.

[benoit-pierre]

Anyway, I've a WIP for this, I'll push something by the end of the week.

[pfmoore]

Oh, cool! Thanks - I've been thinking about this but struggling to find a good way of implementing it. Looking forward to seeing your PR :-)

[pfmoore]

Fix now in #5743

[pradyunsg]

Closing since it's in the PR now.

[lock]

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