add2virtualenv logic breaks builds of packages with modern build backend

[dfn-certling]

The logic in add2virtualenv of adding paths to the front of sys.path breaks installing packages in the virtualenv that use a PEP517 build. In preparing the build environment pip tries to determine the necessary sys.path using a "sitecustomize.py" written in pip's _internal/build_env.py, which gets confused with paths not being added to the end of sys.path. The result is that the lib-dynload path of the Python environment gets removed from sys.path and pip install fails with a traceback like this

# pip install .
Processing /Devel/foo
 Installing build dependencies ... done
 Getting requirements to build wheel ... done
ERROR: Exception:
Traceback (most recent call last):
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/cli/base_command.py", line 180, in exc_logging_wrapper
 status = run_func(*args)
 ^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/cli/req_command.py", line 245, in wrapper
 return func(self, options, args)
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/commands/install.py", line 377, in run
 requirement_set = resolver.resolve(
 ^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/resolution/resolvelib/resolver.py", line 76, in resolve
 collected = self.factory.collect_root_requirements(root_reqs)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/resolution/resolvelib/factory.py", line 534, in collect_root_requirements
 reqs = list(
 ^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/resolution/resolvelib/factory.py", line 490, in _make_requirements_from_install_req
 cand = self._make_base_candidate_from_link(
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/resolution/resolvelib/factory.py", line 228, in _make_base_candidate_from_link
 self._link_candidate_cache[link] = LinkCandidate(
 ^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/resolution/resolvelib/candidates.py", line 290, in __init__
 super().__init__(
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/resolution/resolvelib/candidates.py", line 156, in __init__
 self.dist = self._prepare()
 ^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/resolution/resolvelib/candidates.py", line 222, in _prepare
 dist = self._prepare_distribution()
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/resolution/resolvelib/candidates.py", line 301, in _prepare_distribution
 return preparer.prepare_linked_requirement(self._ireq, parallel_builds=True)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/operations/prepare.py", line 525, in prepare_linked_requirement
 return self._prepare_linked_requirement(req, parallel_builds)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/operations/prepare.py", line 640, in _prepare_linked_requirement
 dist = _get_prepared_distribution(
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/operations/prepare.py", line 71, in _get_prepared_distribution
 abstract_dist.prepare_distribution_metadata(
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/distributions/sdist.py", line 54, in prepare_distribution_metadata
 self._install_build_reqs(finder)
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/distributions/sdist.py", line 124, in _install_build_reqs
 build_reqs = self._get_build_requires_wheel()
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/distributions/sdist.py", line 101, in _get_build_requires_wheel
 return backend.get_requires_for_build_wheel()
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_internal/utils/misc.py", line 745, in get_requires_for_build_wheel
 return super().get_requires_for_build_wheel(config_settings=cs)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_impl.py", line 166, in get_requires_for_build_wheel
 return self._call_hook('get_requires_for_build_wheel', {
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_impl.py", line 321, in _call_hook
 raise BackendUnavailable(data.get('traceback', ''))
pip._vendor.pyproject_hooks._impl.BackendUnavailable: Traceback (most recent call last):
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 77, in _build_backend
 obj = import_module(mod_path)
 ^^^^^^^^^^^^^^^^^^^^^^^
 File "/usr/lib64/python3.11/importlib/__init__.py", line 126, in import_module
 return _bootstrap._gcd_import(name[level:], package, level)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "<frozen importlib._bootstrap>", line 1204, in _gcd_import
 File "<frozen importlib._bootstrap>", line 1176, in _find_and_load
 File "<frozen importlib._bootstrap>", line 1126, in _find_and_load_unlocked
 File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
 File "<frozen importlib._bootstrap>", line 1204, in _gcd_import
 File "<frozen importlib._bootstrap>", line 1176, in _find_and_load
 File "<frozen importlib._bootstrap>", line 1147, in _find_and_load_unlocked
 File "<frozen importlib._bootstrap>", line 690, in _load_unlocked
 File "<frozen importlib._bootstrap_external>", line 940, in exec_module
 File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
 File "/tmp/pip-build-env-pitma8s7/overlay/lib/python3.11/site-packages/setuptools/__init__.py", line 8, in <module>
 import _distutils_hack.override # noqa: F401
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/_distutils_hack/override.py", line 1, in <module>
 __import__('_distutils_hack').do_override()
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/_distutils_hack/__init__.py", line 70, in do_override
 ensure_local_distutils()
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/_distutils_hack/__init__.py", line 53, in ensure_local_distutils
 importlib.import_module('distutils')
 File "/usr/lib64/python3.11/importlib/__init__.py", line 126, in import_module
 return _bootstrap._gcd_import(name[level:], package, level)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/_distutils_hack/__init__.py", line 90, in find_spec
 return method()
 ^^^^^^^^
 File "/root/.virtualenvs/foo/lib/python3.11/site-packages/_distutils_hack/__init__.py", line 97, in spec_for_distutils
 import importlib.abc
 File "/usr/lib64/python3.11/importlib/abc.py", line 19, in <module>
 from .resources.abc import ResourceReader, Traversable, TraversableResources
 File "/usr/lib64/python3.11/importlib/resources/__init__.py", line 3, in <module>
 from ._common import (
 File "/usr/lib64/python3.11/importlib/resources/_common.py", line 3, in <module>
 import tempfile
 File "/usr/lib64/python3.11/tempfile.py", line 45, in <module>
 from random import Random as _Random
 File "/usr/lib64/python3.11/random.py", line 49, in <module>
 from math import log as _log, exp as _exp, pi as _pi, e as _e, ceil as _ceil
ModuleNotFoundError: No module named 'math'

This can be reproduced with a fresh virtualenv that contains a couple - in this setup it was three, but in other environments two were enough - paths added via add2virtualenv

mkvirtualenv foo
mkdir dummy1 dummy2 dummy3
add2virtualenv dummy1 dummy2 dummy3

and then installing a basically empty package with a minimal pyproject.toml

echo "[project]
name = 'bar'
version = '1.0.0'

[build-system]
requires = ['setuptools']
build-backend = 'setuptools.build_meta'" > pyproject.toml
pip install .

[dfn-certling]

Version numbers.

# pip show virtualenvwrapper
Name: virtualenvwrapper
Version: 6.1.0
Summary: 
Home-page: 
Author: 
Author-email: Doug Hellmann <doug@doughellmann.com>, Jason Myers <jason@mailthemyers.com>
License: MIT
Location: /usr/lib/python3.11/site-packages
Requires: stevedore, virtualenv, virtualenv-clone
Required-by: 

# pip --version
pip 22.3.1 from /usr/lib/python3.11/site-packages/pip (python 3.11)