Improve PEP 518 build isolation (#5824)

Handle .pth files, so namespace packages are correctly supported under Python 3.2 and earlier, and more.

docs/html/reference/pip.rst

@@ -158,6 +158,8 @@ appropriately.
   does not support the use of environment markers and extras (only version
   specifiers are respected).
 
+* ``pip<18.1``: build dependencies using .pth files are not properly supported;
+  as a result namespace packages do not work under Python 3.2 and earlier.
 
 Future Developments
 ~~~~~~~~~~~~~~~~~~~

news/5656.bugfix

@@ -0,0 +1 @@
+- Improve PEP 518 build isolation: handle .pth files, so namespace packages are correctly supported under Python 3.2 and earlier.

src/pip/_internal/build_env.py

@@ -4,6 +4,7 @@
 import logging
 import os
 import sys
+import textwrap
 from distutils.sysconfig import get_python_lib
 from sysconfig import get_paths
 
@@ -61,6 +62,15 @@ def __enter__(self):
 
         os.environ['PYTHONNOUSERSITE'] = '1'
 
+        # Ensure .pth files are honored.
+        with open(os.path.join(purelib, 'sitecustomize.py'), 'w') as fp:
+            fp.write(textwrap.dedent(
+                '''
+                import site
+                site.addsitedir({!r})
+                '''
+            ).format(purelib))
+
         return self.path
 
     def __exit__(self, exc_type, exc_val, exc_tb):

tests/data/src/pep518_with_namespace_package-1.0/pyproject.toml

@@ -0,0 +1,6 @@
+[build-system]
+requires = [
+    "setuptools",
+    "wheel",
+    "simple_namespace",
+]

tests/data/src/pep518_with_namespace_package-1.0/setup.cfg

@@ -0,0 +1,2 @@
+[bdist_wheel]
+universal=1

tests/data/src/pep518_with_namespace_package-1.0/setup.py

@@ -0,0 +1,10 @@
+from setuptools import setup
+
+import simple_namespace.module
+
+
+setup(
+    name='pep518_with_namespace_package',
+    version='1.0',
+    py_modules=['pep518_with_namespace_package'],
+)

tests/data/src/simple_namespace/setup.cfg

@@ -0,0 +1,2 @@
+[bdist_wheel]
+universal=1

tests/data/src/simple_namespace/setup.py

@@ -0,0 +1,9 @@
+from setuptools import setup
+
+
+setup(
+    name='simple_namespace',
+    version='1.0',
+    namespace_packages=['simple_namespace'],
+    packages=['simple_namespace.module'],
+)

tests/functional/test_install.py

@@ -100,6 +100,16 @@ def test_pep518_with_extra_and_markers(script, data, common_wheels):
     )
 
 
+def test_pep518_with_namespace_package(script, data, common_wheels):
+    script.pip(
+        'wheel', '--no-index',
+        '-f', common_wheels,
+        '-f', data.find_links,
+        data.src.join("pep518_with_namespace_package-1.0"),
+        use_module=True,
+    )
+
+
 @pytest.mark.timeout(60)
 @pytest.mark.parametrize('command', ('install', 'wheel'))
 @pytest.mark.parametrize('package', ('pep518_forkbomb',