Extract workaround for pytest-dev/pytest-xdist#376 as a fixture. Invoke the...

Extract workaround for pytest-dev/pytest-xdist#376 as a fixture. Invoke the repair at the session level and only when xdist is present.

setuptools/tests/fixtures.py

+import contextlib
+import sys
 import shutil
 
 import pytest
@@ -39,3 +41,20 @@ def tmp_src(request, tmp_path):
     tmp_src_path = tmp_path / 'src'
     shutil.copytree(request.config.rootdir, tmp_src_path)
     return tmp_src_path
+
+
+@pytest.fixture(autouse=True, scope="session")
+def workaround_xdist_376(request):
+    """
+    Workaround pytest-dev/pytest-xdist#376
+
+    ``pytest-xdist`` tends to inject '' into ``sys.path``,
+    which may break certain isolation expectations.
+    Remove the entry so the import
+    machinery behaves the same irrespective of xdist.
+    """
+    if not request.config.pluginmanager.has_plugin('xdist'):
+        return
+
+    with contextlib.suppress(ValueError):
+        sys.path.remove('')

setuptools/tests/test_build_meta.py

 import os
 import shutil
-import sys
 import tarfile
 import importlib
 from concurrent import futures
-from contextlib import suppress
 
 import pytest
 
@@ -46,15 +44,6 @@ class BuildBackendCaller(BuildBackendBase):
 
     def __call__(self, name, *args, **kw):
         """Handles aribrary function invocations on the build backend."""
-        with suppress(ValueError):
-            # NOTE: pytest-xdist tends to inject '' into `sys.path` which
-            # NOTE: may break certain isolation expectations. To address
-            # NOTE: this, we remove this entry from there so the import
-            # NOTE: machinery behaves the same as in the default
-            # NOTE: sequential mode.
-            # Ref: https://github.com/pytest-dev/pytest-xdist/issues/376
-            sys.path.remove('')
-
         os.chdir(self.cwd)
         os.environ.update(self.env)
         mod = importlib.import_module(self.backend_name)