Hide staleness check behind a feature flag (#451)

* Hide staleness check behind a feature flag

* Set feature flag for staleness check in unit tests

* Set feature flag for staleness check in Python 2 test

nox/virtualenv.py

@@ -33,6 +33,7 @@
     ["PIP_RESPECT_VIRTUALENV", "PIP_REQUIRE_VIRTUALENV", "__PYVENV_LAUNCHER__"]
 )
 _SYSTEM = platform.system()
+_ENABLE_STALENESS_CHECK = "NOX_ENABLE_STALENESS_CHECK" in os.environ
 
 
 class InterpreterNotFound(OSError):
@@ -312,6 +313,8 @@ def __init__(
     def _clean_location(self) -> bool:
         """Deletes any existing virtual environment"""
         if os.path.exists(self.location):
+            if self.reuse_existing and not _ENABLE_STALENESS_CHECK:
+                return False
             if (
                 self.reuse_existing
                 and self._check_reused_environment_type()

tests/test_virtualenv.py

@@ -351,6 +351,15 @@ def test_create_reuse_environment(make_one):
     assert reused
 
 
+@pytest.fixture
+def _enable_staleness_check(monkeypatch):
+    monkeypatch.setattr("nox.virtualenv._ENABLE_STALENESS_CHECK", True)
+
+
+enable_staleness_check = pytest.mark.usefixtures("_enable_staleness_check")
+
+
+@enable_staleness_check
 def test_create_reuse_environment_with_different_interpreter(make_one, monkeypatch):
     venv, location = make_one(reuse_existing=True)
     venv.create()
@@ -367,6 +376,7 @@ def test_create_reuse_environment_with_different_interpreter(make_one, monkeypat
     assert not location.join("marker").check()
 
 
+@enable_staleness_check
 def test_create_reuse_stale_venv_environment(make_one):
     venv, location = make_one(reuse_existing=True)
     venv.create()
@@ -386,6 +396,7 @@ def test_create_reuse_stale_venv_environment(make_one):
     assert not reused
 
 
+@enable_staleness_check
 def test_create_reuse_stale_virtualenv_environment(make_one):
     venv, location = make_one(reuse_existing=True, venv=True)
     venv.create()
@@ -410,6 +421,7 @@ def test_create_reuse_stale_virtualenv_environment(make_one):
     assert not reused
 
 
+@enable_staleness_check
 def test_create_reuse_venv_environment(make_one):
     venv, location = make_one(reuse_existing=True, venv=True)
     venv.create()
@@ -424,6 +436,7 @@ def test_create_reuse_venv_environment(make_one):
     assert reused
 
 
+@enable_staleness_check
 @pytest.mark.skipif(IS_WINDOWS, reason="Avoid 'No pyvenv.cfg file' error on Windows.")
 def test_create_reuse_oldstyle_virtualenv_environment(make_one):
     venv, location = make_one(reuse_existing=True)
@@ -442,6 +455,7 @@ def test_create_reuse_oldstyle_virtualenv_environment(make_one):
     assert reused
 
 
+@enable_staleness_check
 def test_create_reuse_python2_environment(make_one):
     venv, location = make_one(reuse_existing=True, interpreter="2.7")