In distutils_hack, only add the metadata finder once. In ensure_local…

…_distutils, rely on a context manager for reliable manipulation. Fixes #2958.

_distutils_hack/__init__.py

@@ -3,6 +3,7 @@
 import re
 import importlib
 import warnings
+import contextlib
 
 
 is_pypy = '__pypy__' in sys.builtin_module_names
@@ -52,9 +53,8 @@ def ensure_local_distutils():
     # With the DistutilsMetaFinder in place,
     # perform an import to cause distutils to be
     # loaded from setuptools._distutils. Ref #2906.
-    add_shim()
-    importlib.import_module('distutils')
-    remove_shim()
+    with shim():
+        importlib.import_module('distutils')
 
     # check that submodules load as expected
     core = importlib.import_module('distutils.core')
@@ -129,6 +129,19 @@ def frame_file_is_setup(frame):
 DISTUTILS_FINDER = DistutilsMetaFinder()
 
 
+def ensure_shim():
+    DISTUTILS_FINDER in sys.meta_path or add_shim()
+
+
+@contextlib.contextmanager
+def shim():
+    add_shim()
+    try:
+        yield
+    finally:
+        remove_shim()
+
+
 def add_shim():
     sys.meta_path.insert(0, DISTUTILS_FINDER)
 

changelog.d/2958.change.rst

@@ -0,0 +1 @@
+In distutils_hack, only add the metadata finder once. In ensure_local_distutils, rely on a context manager for reliable manipulation.

setup.py

@@ -55,7 +55,7 @@ class install_with_pth(install):
         import os
         var = 'SETUPTOOLS_USE_DISTUTILS'
         enabled = os.environ.get(var, 'local') == 'local'
-        enabled and __import__('_distutils_hack').add_shim()
+        enabled and __import__('_distutils_hack').ensure_shim()
         """).lstrip().replace('\n', '; ')
 
     def initialize_options(self):